Fuel prices are going up and up, but your income isn’t. What can you do? Follow the tips in this post, and you should see a saving in your fuel bills. If you're an average driver, you should also benefit from increased safety, relaxation and wear and tear, without much effect on journey time.

Important

When trying out the tips below, don’t forget:

• Always obey your local traffic laws
• Use common sense
• Be considerate to other road users

Tip 1: Avoid acceleration like the plague!

If you did basic physics at school, you may remember the equation F=ma: force equals mass times acceleration. This is Newton’s second law of motion, and holds for all objects moving at significantly less than the speed of light — like your car.

If you want to accelerate, you need to get your engine to apply an additional force to your car over and above what it’s already doing to keep it at a constant speed. Your engine generates this extra force by turning more chemical energy in the fuel into kinetic energy at the wheels — and that costs money! It costs about 3–4p to accelerate a typical car from rest to 45mph at current UK prices (Excel cost calculator for acceleration and steady state driving).

So, you need to avoid acceleration wherever possible. There is a lot you can do towards this goal without dramatically affecting the time it takes to get from A to B. As an added bonus, most of these tips improve your safety and smoothness, and decrease wear and tear on your car.

Keep more distance

Guidelines suggest at least 2 seconds for safety, but you cannot absorb changes in traffic speed at that distance. Try twice that distance — imagine an invisible car in between you and the car in front — and you’ll find you can soak up minor variations in speed without braking and having to accelerate back up to speed.

Look ahead

Use your powers of observation to look as far ahead as you can, anticipating changes in traffic flow. Has a vehicle just pulled onto the road ahead? Traffic lights, roundabout, junction? Coast in gear early which uses no fuel at all in many cars — but don’t take it to ridiculous extremes if you have other vehicles following you.

Think ahead

If you only have a short distance before the next stop, such as a large roundabout with traffic lights on it, don't get much speed up if you’re just going to have to turn that energy into heat with your brakes. Braking throws energy (which is money) away which you could have used to travel further.

HGVs and speed cameras

In the UK, we have a 40mph (about 65km/h) limit for goods vehicles over 7.5 tonnes on single-carriageway roads where cars are limited to 60mph (about 95km/h). Most truck drivers pay scant regard to this limit where safe to do so — but they’ll certainly observe it past speed cameras! So if you’re following a big truck, and you know there's a camera coming up, ease back a bit and try to avoid slowing right down to 40mph.

Tip 2: Accelerate briskly!

Wait… what?!

When accelerating, you can choose to do it gently for a longer time, or quickly for a shorter time. It takes exactly the same amount of energy to get an object from one speed to another, no matter how hard or gently you accelerate. However, you don't care how much energy it takes to move your car about. You’re only interested in how much fuel is used, as fuel is what costs money. So, what’s the cheapest way to generate this extra energy?

Petrol (gasoline) and diesel engines are not very efficient at turning the chemical energy in the fuel to kinetic energy (movement). At best, their efficiency is around 50%, and this varies depending on the engine speed and load. Most engines’ peak efficiency is near the peak torque point. You’ll need to look this up for your own engine, but it’s typically somewhere around 1500–2000rpm for diesels and 2000–5000rpm for petrols, at or near to full throttle. This can be tricky to achieve with an automatic transmission, as it will probably try to kick down to a lower ratio when you open the throttle. Using an economy setting, if provided, will reduce its willingness to kick down.

This variation in efficiency is called the brake specific fuel consumption (BSFC). BSFC charts are like a contour map, showing efficiency in g/kWh for each speed/load point.

Here’s a BSFC chart for a 90PS (horsepower, approximately) diesel VW Beetle (new), modified by TDIMeister to show lines of constant power and posted in this thread on TDIClub:

So, for this engine only, the point of maximum efficiency is at almost full load at 1750rpm, putting out 50PS, using 197g of fuel for each kWh of energy produced. If you were to accelerate more gently, producing just 20PS, you would only be able to achieve 210g/kWh (at around 1250rpm), and thus use about 5% more fuel over the time you’re accelerating.

More difficult to read, and with constant power lines in kW rather than HP, here’s a chart for the Mercedes-Benz M271 I4 petrol engine from this forum thread:

This engine shows a peak efficiency of somewhere between 240 and 250g/kWh at around 2500rpm and 60–70% load, where it’s putting out 40kW (55PS). This is significantly less efficient than the diesel engine above, as you would expect from a more powerful petrol engine.

Unfortunately, BSFC maps are not generally available for each engine. If you want to read more and see more charts, try this article.

What this rather technical diversion means is that if you have to accelerate, accelerating briskly may well be slightly cheaper than accelerating gently: not what you’d expect.

So if you’re pulling away from the lights onto a clear road, or accelerating down a motorway slip road (on-ramp), don’t be afraid to use a lot of throttle at low to mid revs, but don’t rev it to the limiter.

Of course, if you only have a short distance to travel between stops, accelerating hard then stopping will give a much higher average speed than accelerating gently to the same peak speed and then stopping, and that will cost you more fuel.

Tip 3: Reduce load

Even driving at a constant speed uses fuel, of course. If you coast in neutral (warning: this is illegal in some places, including the UK), your car will gradually decelerate due to a combination of two factors:

• Aerodynamic drag: proportional to your vehicle’s drag coefficient (CdA) multiplied by the square of your speed through the air;
• Rolling resistance and driveline friction: proportional to your speed on the road surface;

To maintain a constant speed, the engine has to provide an constant force to counter that deceleration.

The engine also has to overcome its own drag, caused by its internal friction (metal moving against metal, hopefully with a layer of oil in between) and the power it provides to other components (power steering pump, alternator, water pump, fuel pump, air conditioning compressor etc etc).

To maximize fuel economy, you need to reduce all of these factors as much as possible. Much of the following advice is oft-quoted and common sense.

Aerodynamic drag

Remove roof bars, cycle racks and other protrusions when not in use. Cosmetic spoilers and bodykits often deteriorate drag, ironically making the vehicle slower than stock.

When travelling fast, it may be more efficient to shut windows and use the air conditioning: the aero drag from the open windows increased dramatically as your speed increases. The point where A/C is more efficient will vary for each vehicle, but it appears that there is not a lot in it at fast cruising speeds, and it’s much quieter with the windows closed.

Rolling resistance

Minimizing rolling resistance is quite simple: make sure your car is properly maintained, and that your tyres are correctly inflated. When fitting new tyres, choose lower rolling resistance types if possible — although it is hard to get solid comparative data on different tyres’ performance.

Load and friction

Turn off unnecessary electrical loads. If you drive around with front fog lamps on because you think it looks cool, turn them off (unless your local regulations require them as part of some hare-brained daytime running lamp law; see UK Highway Code rule 236). You wouldn’t leave over 100W of lamps on in your house for no reason, so why do so in your car? It only makes a small difference — two 55W fog lamps will rob your car of about one-fifth of a horsepower — but it adds up over time. At current UK prices, in-car electrical load is much more expensive than mains electricity: see the calculator.

Don’t use the air conditioning if you don’t need to. By all means turn it on if you are hot, or if the car needs de-misting, but consider it something that should be off by default. This may be difficult on modern climate control systems, but hopefully they’ve been designed to maximize efficiency anyway.

Get your car serviced regularly to maintain peak engine efficiency. Change the oil, oil filter, air filter, spark plugs etc at least as often as the manufacturer recommends.

Tip 4: Choose your route carefully

If you have a journey to undertake — particularly a routine commute — plan your route carefully. Your goal is to minimize the amount of fuel you use by careful route choice, not your average fuel consumption over the trip.

For example, I have two feasible routes for my daily commute. Both take the same amount of time, but one is about eight miles longer than the other. The shorter route (obviously) involves lower average speeds, and as it doesn’t involve a lot of stop/start driving, uses significantly less fuel than the long route.

I could find an even shorter route, but it would be down narrow twisty roads, and would involve a lot more accelerating (bad, remember?). It’d almost certainly take longer, too.

Tip 5: Track your consumption

To get a good idea of your fuel consumption, it’s important to track your fuel use. A site like SpritMonitor (German by default but with English option) allows you to keep track of your consumption and cost over time. Never take a single tankful as a reliable data point, as it’s impossible to fill your tank to a consistent level each time.

If your car has a fuel economy meter, use it. The instantaneous meter, if you have that feature, is great for seeing the effect of acceleration, hills and even wind direction — but don’t degrade your concentration on the road by watching it too much. The average meter is good for tracking individual journeys, too, but nothing can match the accuracy of good tracking across multiple tankfuls.

Other tips

There are a few other things you can do, but often, these are not such a good idea:

Turn your engine off when stopped

If you’re certain your car restarts immediately when hot — and mine doesn’t — it’s worth turning the engine off if you’re going to be stationary for more than a few seconds.

Slipstream trucks

Some extreme “hypermilers” in the US practice this to reduce aerodynamic drag. Please be clear: tailgating is really stupid! Don’t do it.

Don’t slow down for corners

Maintaining speed does reduce the amount of costly acceleration you need to do, but you should always put safety first. Think about the broken-down tractor in the road just around the corner that you can’t see — can you stop in time?

Summary

So there you have it: a collection of tips that should reduce your fuel costs, save wear and tear on your car, and give you a more relaxing drive, without slowing you down too much.

Attribution

Header photo is a derivative of this photo by Sandro Menzel. Licencing information.