How to Improve Fuel Efficiency
last updated 30/05/2018
The purpose of this article is to explain how you can improve your fuel efficiency and save on fuel costs. A nice by-product is that you save the environment too. There’s heaps of information already on the internet on this topic, but I thought I’d deliver it from a slightly different point of view and focus on a couple of things that in my opinion have the greatest scope to improve your fuel efficiency. I have another page on hilux fuel efficiency. That article focuses on my actual fuel use whilst touring around Australia, and its relationship with speed.
What Efficiency is Achievable?
In Australia the official fuel efficiency figures quoted for vehicles is an Australian Design Rules (ADR) calculated value and is often optimistic. Most people use more fuel than indicated – around 15% more in city cycle and 34% more in country cycle according to a study on in service fuel usage. This is not vehicle manufacturers purposely misleading you. The tests are done independently according to a well defined standard and form a good basis for comparing fuel efficiency between vehicles. The tests are not designed to predict your real world fuel usage. There are too many variables to make that possible. The ADR values are good for making relative comparisons between vehicles. Most people use more than the ADR calculated values.
In city driving conditions, you should be able to use less fuel than that indicated by ADR. City driving should also use less fuel than on highway runs. Why do people generally always use more fuel in city conditions compared to highway? Driving habits is the main answer. This will be explained below. My Hilux has a combined ADR rating of around 8.5L/100km. With a vehicle fully loaded for touring, I can get it down to around 7.9L/100km city driving. That’s with a tonne of gear in it and heaps of crap on the roof. Unloaded and milking it for all she’s worth, I’ve gotten it down to less than 7.0L/100km, and that’s with a steel bullbar, canopy, dual batteries and storage drawers. Six-point-something litres per hundred. Pretty good for a big 4WD. That’s with good traffic conditions and taking driving style to the extreme so isn’t reasonably achievable in the long term, but it does give you an idea about how much better you can do than the ADR figure if you try.
For highway driving, you may be able to achieve around equal to the ADR figures – maybe slightly more, maybe slightly less, depending mainly on cruising speed. Apart from cruising speed, driving habits are less important for highway driving. Fuel consumption is dominated by speed.
Before going into the details of improving fuel economy, let me first give you the best way to save a butt load of fuel. It’s best for the environment, it’s best for your health, it makes you live longer, it makes you happier and it’s best for your wallet. Avoid using your car. A car is a several tonne machine designed to carry several people and their stuff hundreds of kilometers. What a ridiculous tool for the job of carrying one person on short commutes. Ride your bike!
What Consumes Fuel?
In this article, we’ll assume that the energy released from burning fuel gets consumed by the following five factors and look at how to mitigate each one:
– Wind Resistance / Drag. Friction between the air and your vehicle opposes movement.
– Mechanical Friction. This is friction in the engine, friction in the drivetrain, and friction between the tyres and the road.
– Thermodynamic losses within the engine. Heat engines waste energy according to the laws of thermodynamics.
– Engine overheads. This is energy consumed by the alternator, fuel pump, oil pump, water pump, cooling fans, air-conditioner, sucking in air, pushing out exhaust etc.
– Acceleration of mass. This is the energy that increases the speed of your vehicle.
Wind Resistance / Drag
Pushing air out the way of a moving vehicle consumes energy. Wind resistance or drag is proportional to speed squared. For example, doubling speed results in a 4 times increase in drag. So at low speeds, drag is small, but as speed is increased drag escalates quickly and becomes the dominating energy consumer of your vehicle.
To minimize drag:
– Drive slowly. This can make a huge difference. At high speeds, wind resistance goes through the roof and your engine drinks fuel. Highway driving is where this can make the biggest difference as there is greater scope to reduce speed. Significant savings can be made by cruising at a slower speed. For example, my fully loaded Hilux, with all the extra stuff on the roof, consumes around 9.0L/100km at 90km/h but this increases to 14.0L/100km at 120km/h. See Hilux Fuel Efficiency article for more details.
– Minimize what you carry outside the vehicle. This might be stuff on the roof, in the tray, hanging on your bullbar or hanging out the window. Anything outside the car will contribute to additional wind resistance and therefore fuel consumption.
– If you are not concerned about safety, slipstream trucks. This is also called drafting. Big road trains are best. They leave a large low pressure region in their wake which reduces drag of the following vehicle. However it’s a very dangerous practice. Your whole field of view is obscured by the truck. You can’t see anything ahead. You could sideswipe an oncoming car or cyclist or other obstacle since you could drift outside of the truck’s wheel tracks and you can’t see what’s coming. Something could fall off the truck and slam you right in the face. You’ll probably die. Even if none of that happens you’ll be breathing in the truck’s toxic exhaust for long periods of time and probably get cancer.
– Avoid external modifications to the vehicle. Anything from bullbars to canopies to roof racks all negatively impact fuel economy.
– Keep the outside of the car clean. This contributes to a small but measurable improvement in fuel consumption (check mythbusters results on their golf ball effect episode).
– Slow down up hills. This offsets the extra fuel used to get up the hill with less wind resistance. You can then use the down hill to re-accelerate the vehicle.
Anything moving has friction losses. On your car the main friction losses are in your engine, through your drivetrain and between your tyres and the road. To reduce mechanical friction:
– Check your tyre pressures regularly, keep the pressures up, and adjust them according to load. More load means you need more pressure. Ensure your pressure is, at minimum, equal to that shown on your vehicle’s tyre placard. I always keep my tyres at a slightly higher pressure, and increase it further according to load (always ensure you stay below the maximum pressure specified on the tyre). Tyre pressure has many compromises. For more information check out the article on tyre pressure. If you’re not sure what the best pressure is for your requirements, then use the tyre placard details as a guide and check with your tyre shop.
– Get regular wheel alignments. If your wheels aren’t aligned they are fighting against each other, causing increased friction as well as increased tyre wear. Avoid hitting bumps or curbs at speed to reduce misalignment.
– Avoid chunky or aggressive looking tyres. They develop more rolling resistance. There are a few tyres specifically designed to minimize rolling resistance, but they aren’t widely available. It may be an option to pursue.
– Minimize engine RPM. Less revs mean less friction in your engine. Use the highest gear possible and drive slower. Engine rpm should never exceed the rpm where maximum torque is achieved.
– Short shift when accelerating. This means shifting to higher gears earlier, thus minimizing RPM during acceleration.
– Keep your lubricant servicing up to date and use the correct lubricants according to your manufacturer’s specification. This includes engine oil, gearbox oil, differential oil, greasing of bearings and universal joints. Some 4WD vehicles also have a separate transfer case with it’s own oil. How often should you service your car? Click here for my explanation. In my opinion, follow the manufacturer guidelines.
– Purchase vehicles with a manual transmission. Automatic transmissions have higher drivetrain losses. Quoted figures by manufacturers are usually for combined city / highway cycle so the difference between manual and automatic may not be much. But if you’re doing lots of city driving the difference grows. Low rpm at low speeds and constant slippage in the torque converter in an auto gearbox causes poorer fuel economy. Also an automatic gearbox is bad for driving style. You’re more likely to waste fuel accelerating and braking all the time in traffic rather than holding a steady speed with a manual gearbox. I’ve found I can easily beat the manufacturer’s specified fuel consumption in a manual but not so easily with an automatic.
This is how efficiently an engine converts heat energy into kinetic energy and is governed by an engine’s design and the laws of thermodynamics. It’s basically fixed for a given engine. In my opinion, after market fuel savers, performance chips, gas injection, magnets and any other fuel saving device don’t work. If there existed cheap fuel saving devices that worked, they would be exploited by vehicle manufacturers. The manufacturer isn’t going to throw away fuel for nothing if it can be saved cheaply. Gas injection is a special case, where it does actually change the combustion process within the engine. It is claimed that it increases the burn rate which yields improved efficiency. I doubt it’s true. Burn rate and peak combustion pressure are timed to provide optimized energy extraction according to the engine’s design. Faster burn rate doesn’t necessarily mean more efficient. Actually sharp spikes in combustion pressure can be less efficient. Claimed efficiency gains from gas injection would likely be from the extra energy released from burning the gas. It is no different to injecting more fuel. Apart from buying a different engine, smaller engine (lower overheads), or an engine that runs at higher compression (eg diesel), you’re stuck with these few options:
– Run your motor at it’s most efficient RPM. This is at around maximum torque, when compression is at its maximum. Check your vehicle’s manual for the rpm that yields maximum torque. However it’s not that simple – this is in conflict with minimizing mechanical friction as identified in the friction section above. Most efficient RPM is a compromise between thermodynamic efficiency and friction losses. Under light loads, friction dominates and minimizing engine RPM is more effective at reducing fuel consumption. With higher loads, thermodynamic losses become more significant so it may be more efficient to operate at higher RPM closer to the maximum torque RPM. This may be the case when towing a heavy load or going up a long hill. Under these circumstances you may be better off using a lower gear. However, most of the time, minimizing RPM yields the lowest fuel consumption.
– Avoid large diameter tyres. Bigger tyres offset the overall gearing which was chosen by the manufacturer to optimise fuel efficiency.
– Ensure your air intake, ignition system and fuel delivery system are working optimally. This means checking and maintaining the air filter, air monitoring instruments, oxygen sensors, exhaust sensors, spark plugs, ignition coils, engine tuning, fuel injectors and fuel monitoring instruments.
– Buy smaller engined cars. Cars these days have oversized engines to give good performance. It means that most of the time the engines are under-utilized and are operating inefficiently, wasting fuel. Even the smallest, least powerful modern vehicles are ridiculously fast, relative to the job they need to do. There is no need for large engines.
– Avoid short trips with a cold engine. An engine’s efficiency is poor when not at operating temperature. I have no good data, but based on trials using my trip computer, short cold trips can use a massive 80% more fuel compared to longer trips at operating temperature. Combustion efficiency is low when the engine is cold. Then there’s increased losses from engine, gearbox and diff oils that are too thick. Additionally short trips involve a disproportionate amount of idle time / low speed travel. Walk or use a pushbike for very short trips. Consolidate multiple errands into a single trip. When you need to use a car put the reason on a list and wait until several items are on that list before using the vehicle.
There’s not many things you can do to reduce engine overheads that actually work. Turning off electrical loads will make so little difference it will be hardly noticeable Perhaps keeping your engine properly serviced and lubed according to the same idea of reducing mechanical friction above will help minimize engine overheads. Otherwise:
– Avoid Idling. Park the car and get off your ass instead of using the drive through. Stop the engine if stopped in heavy traffic.
– Turn off your air-conditioning. Toughen up. Drive around in your undies and no shirt to keep cool. Take a spray bottle with you and spray yourself with water when you get hot. Open your windows. Apart from when driving above around 100km/h, it is more economical to open windows compared to running the air-conditioning. Most of the time (all of the time for city driving), opening windows is more efficient.
– Performance or low restriction air filters don’t make much difference. There isn’t much energy consumed pulling air into an engine. If there was then vehicle manufacturers would look at ways to reduce it, for example through low restriction air filters. I’ve done a test on one of my old cars completely removing the air filter. There was no noticeable difference to fuel efficiency, but it did make the engine sound louder. There is evidence that suggests manufacturer supplied paper filters are the most efficient at removing contaminants over the life of the filter. Other filters compromise filtering ability, reduce engine life, and insignificantly improve performance / fuel efficiency. Have a search on the net to see some of the studies.
– Increasing exhaust size is commonly thought to improve efficiency. Actually a larger exhaust reduces fuel efficiency because it makes the engine less efficient over the bottom half of its rpm range which is where it typically operates. Exhaust size is a compromise between flow velocity and resistance to flow. Bigger isn’t necessarily better. A smaller exhaust is more efficient under low exhaust volume (low RPM). A bigger exhaust is more efficient under high exhaust volume (higher RPM). Exhaust flow comes out in pulses as each exhaust valve opens. A smaller exhaust diameter increases flow velocity so the the gas has more inertia and the constant stop / start of exhaust flow is smoothed out. This means the engine doesn’t need to work as hard to re-accelerate the exhaust gas every time an exhaust valve opens. Also a smaller exhaust pipe means less mass of gas retained within it, again allowing the engine to work easier as it isn’t accelerating as much gas on each exhaust stroke. However a smaller pipe has a greater resistance to flow in the usual sense and will generate more back pressure at higher exhaust volumes. The manufacturer picks a size that, according to their design, is the best compromise. A smaller exhaust is better to save fuel because driving to save fuel also means minimizing rpm. Maintaining exhaust size whilst reducing resistance to flow will improve efficiency. This can be done by using low restriction catalytic converters and mufflers / silencers, however the gains are almost imperceivable and the increase in noise is significant. Vehicle manufacturers aren’t going to restrict exhaust flow unnecessarily if there is a practical and legal way to improve it without compromising emissions and noise. Completely removing the exhaust system provides the most efficient solution, but this is very noisy and illegal. See Exhaust Pipes: Is Bigger Better for more details.
Acceleration of Mass
Ok here is the silver bullet. The miracle cure. Well there’s no miracle cure, but along with driving slower to reduce drag, the tips in this section are the most effective in reducing fuel consumption. One in particular deserves to be singled out: Stop accelerating towards a situation where you will need to brake. Drive with the goal of minimizing brake usage and you will use less fuel than the ADR rating. You can save maybe around 30% in fuel costs by following the points in this section.
Driving in the city is at slower speeds compared to highway. So your wind resistance is less, your mechanical friction is less and your engine rpm is lower. Why does city driving use more fuel then? The answer is the energy consumed accelerating the vehicle after having to slow down. This really chews the juice. To minimize the energy consumed accelerating your vehicle, follow these points:
– Stop accelerating towards a situation where you will need to brake. This is tantamount to throwing money in the fire. Change your driving habits so that braking is minimized. Braking turns your hard earned cash you just spent accelerating your vehicle into waste heat. Why spend money heating your brakes and wearing out your vehicle? Anticipate conditions. Look ahead. Plan your speed. Slow down gently. Slow down under the natural rate that your vehicle slows when your foot is off the accelerator pedal. Get your foot off the accelerator pedal early enough so that you hardly have to brake. This includes when you are approaching slow moving traffic, red traffic lights, roundabouts, corners and intersections. If you are driving in stop / start traffic with repeated acceleration and braking cycles, do not stop / start. Try to pick a speed where you no longer have to stop / start. Try to judge the average speed of the stop / start traffic so that you can drive at a constant speed. Make it a challenge to try to drive all the way through the stop / start traffic without using the brakes. Stop accelerating to the rear end of vehicles and slamming on the brakes. This saves you heaps of fuel and reduces the risk of accidents, including the risk of you ramming up the rear end of those in front, and those behind you ramming your rear end. Apart from saving fuel, it also saves time. The time it takes for you to work to pay for the fuel you have unnecessarily burnt. The time it takes for you to work to pay for the unnecessary wear and tear that you’ve inflicted on the vehicle. If you drive impatiently, accelerating hard and slamming on the brakes, it’s only a matter of time until you cause an accident, either in front or behind you. In that case changing driving habits would have saved you a huge amount of time.
– Do not drive aggressively. Avoid becoming angry or impatient. Various studies have proven that driving aggressively increases fuel consumption significantly. It’s been verified by academic institutions as well as on an episode of Mythbusters. On the episode it was found that driving stressed / agitated increased fuel consumption by a whopping 68%!
– Smooth out your speed profile. Minimise speed variability. This means trying to travel at a more consistent speed. Avoid acceleration and deceleration. Travel at the traffic’s average speed rather than constantly speeding up and slowing down with traffic.
– Avoid the fast lane. The fast lane is full of people who think a continuous cycle of speeding up and slowing down is a good idea. This makes it harder for you to smooth out your speed profile, since you’ll be influenced by the wildly erratic speeds of the vehicles ahead and behind you.
– Generally accelerate slowly and drive slowly to minimize the amount of kinetic energy you waste during any unanticipated slow downs.
– You should be annoyed every time you touch the brakes. Every time you use the brakes you should think to yourself “Dam I’m an idiot I’ve just thrown away money due to my own poor driving. I accelerated unnecessarily which caused me to unnecessarily use the brakes and waste fuel”. You drove badly. You could have anticipated the conditions better. Build an emotional case to help you remember to do a better job next time.
– Don’t carry around unnecessary weight in your car. If there’s stuff in the car that doesn’t need to be there for the trip you are undertaking, then take it out. Imagine you are riding a push bike. In that case you’d definitely jettison any unnecessary weight. Extra weight is a huge burden when pedaling under human power. However pushing your foot harder on the accelerator pedal is very easy, not much of a burden on you physically, but it is a burden to the environment and your back pocket.
– Avoid larger diameter tyres, which are not only heavier in mass but also have much higher rotational inertia. This results in an equivalent weight penalty much more than the extra weight of the tyre.
– Buy lighter cars. Cars these days are unnecessarily heavy, packed with features for the sake of pandering to fashion and trends. They are bulky and bloated with weight. Seek out lighter cars if you want to save fuel.
More Advanced Options
There’s a community of people called hypermilers who go to extreme measures to save even tiny amounts of fuel. It’s an interesting hobby where, if you want to, you can really push the limits on what can be achieved. Most of the methods are aimed at improving aerodynamics. Not only is it a great hobby, it will also make you rich. I have not personally tried any of these initiatives but some of the things hypermilers do include:
– Fix flat panels to the underbody of the car. This reduces aerodynamic drag.
– Fix flat panels over the rear wheels (called wheel skirts or fender skirts). This reduces aerodynamic drag. You can’t do the front wheels since they need to be able to steer, unless you create skirts that can swing out of the way to accommodate the turning wheels.
– Cover your wheel rims with flat panels or “moon caps”. This reduces aerodynamic drag.
– Fold in your external mirrors or remove them completely. This reduces aerodynamic drag.
– Remove the air-conditioner system completely. This reduces weight and mechanical losses from turning the pulley.
– Remove anything else that you can live without. Floor mats, sound deadening, seats, interior panels, electric window motors, whatever. This reduces weight.
– Remove the power-steering pump and replace the steering system with one designed without power assist. This reduces weight and eliminates the engine power that the power steering system uses.
– Tape up any gaps in the body of the vehicle. This includes gaps between body panels, around light fittings, etc. This reduces aerodynamic drag.
– Pulse and glide. This involves controlling the vehicle’s speed between an upper and lower limit by accelerating rapidly and then shutting off the engine. This improves thermodynamic efficiency by operating the engine at a higher load. It also reduces engine overheads since the engine is switched off for a large portion of the time. Note coasting with the engine off is illegal in most jurisdictions and can also be dangerous. Power steering will be lost and the steering will become slow and heavy. Power brakes will be lost (vacuum assist is usually still good for a couple of braking cycles with the engine off). You could accidentally lock the steering wheel and run off the road or into traffic.
– Install a kill switch and push button start so you can easily shut down and restart the engine at traffic lights. This will also help with pulse and glide by eliminating the risk of locking the steering wheel as well as reducing distraction / workload related to fiddling with the keys.
– Install a boat tail / kammback. This is a streamlined aerodynamic aid fitted to the rear of a vehicle. It reduces the low pressure system at the rear of the vehicle and significantly improves aerodynamics.
– Block out gaps in front radiator air intakes. This reduces aerodynamic drag at the cost of reducing the radiator’s cooling capability. Make the blocking panel removable so you can take it out on hot days.
– Remove the alternator. This saves weight, reduces mechanical losses and eliminates the engine power that would go into charging the battery and running electrical loads. You won’t be able to run any electrical stuff (fans, headlights, etc) and you’ll need a way to keep your battery charged. Perhaps a solar panel on the dash board (don’t put it outside, too much wind resistance). You won’t be able to drive at night.
Becoming a Fuel Miser
I take great pleasure in seeing my trip computer report record low fuel consumption. I think about the extra dollars in my pocket topping up my beer fund and allowing me to enjoy longer and more frequent camping missions. I think about the good that I’m doing for the environment, minimizing carbon emissions and reducing my consumption of finite resources. It’s also an interesting and satisfying challenge to see you low you can go.
Driving slower and avoiding the brakes are the main techniques that will save you the most fuel. You may find driving this way annoying or unpleasurable. It’s a balance between what driving style you enjoy, how much you’re prepared to pay for it, and how much fuel you want to save. For me there are other things I want to spend my money on, so I try to minimize fuel consumption as much as I can. One day I think I’ll even pursue some of the hypermiling techniques.