Savings on Fuel - Part Four

In previous parts of this series we’ve looked at buying the most economical car for your needs as well as maintenance and driving techniques to reduce fuel consumption. In this, the final part of the series, we’ll take a look at car modifications that can reduce fuel consumption...

See Fuel Savings - Part One, Savings on Fuel - Part Two and Savings on Fuel - Part Three for previous instalments.

Taller Gearing

A tried and proven method of improving fuel consumption is to give your car taller gearing. Taller gearing results in lower engine speeds which - unless the gearing is stupidly over-done - will reduce fuel comsumption.

The easiest way to achieve taller overall gearing is to install tyres with a slightly larger circumference than standard. For example, if your car has 15 inch wheels factory-fitted with 195/55 tyres, you can step up to 195/60 or 205/55 tyres to reduce engine rpm across each gear ratio. Be sure to check that the larger circumference tyres have adequate clearance and are legal in your area.

Another way to change overall gearing is to change the differential (aka final drive) ratio. In many popular rear-wheel-drive vehicles you might be able to find a taller ratio interchangeable diff from a wrecker. Front wheel drive diff swaps are less common but it is worthwhile checking if the manufacturer of your vehicle released a variety of ratios to suit. In the case of AWD cars, the process is twice as costly and fiddly – you need to change both the front and rear diff ratios.

A gearbox swap is another way to improve fuel economy. Do a bit of research to find whether your car was released with a ‘long legged’ gearbox. For example, if you own a Ford Falcon 6-cylinder it’s likely that a gearbox from a V8 version will offer taller ratios. Just make sure the ‘boxes are interchangeable before handing over any money.

It’s important to realise that installing taller gearing will typically reduce acceleration.

Speedometer Calibration? When you install larger circumference tyres, or a taller ratio differential or gearbox, you will need to alter the speedometer calibration to compensate. Taller ratios will cause the speedometer to display a lower than actual road speed. Speak to your nearest gauge specialists for options (there might be a different speedo pick-up you can use) or you can employ a Digital Speedo Corrector if your vehicle uses an electronic speed sensor.

Reduced Rolling Resistance

When it comes time to replace a worn set of tyres, it is worthwhile investigating low rolling resistance replacements.

A low rolling resistance tyre has a special combination of tread design, tread compound and carcass construction. Your local tyre dealer should be able to suggest low rolling resistance tyres to suit your vehicle. The Michelin Energy range is one of the most widely recognised low rolling resistance tyres currently in production.

And we must take the opportunity to again point out that tyre pressure is a crucial factor in fuel consumption. A tyre that’s under-inflated by 20 percent increases fuel consumption by around 5 percent.

Reduced Aerodynamic Drag

Improving aerodynamic efficiency is not an easy task.

There are many aftermarket body parts on the market but, given most of these haven’t been developed inside a wind tunnel, it is likely that they cause increased aerodynamic drag.

The only company with the budget necessary to access a wind tunnel is the vehicle manufacturer. As a result, the majority of OE add-on body parts are aero efficient. But there are exceptions. We suggest researching your particular vehicle to find if an optional body kit was available and, if so, whether an aerodynamic improvement is claimed. If there are no aerodynamic figures accompanying the body kit, it’s likely that it doesn’t improve efficiency.

Reduced Weight

Reducing weight is another way to improve fuel economy – but it’s an approach that usually comes with trade-offs.

Car manufacturers are very good at keeping vehicle weight to a minimum so it’s likely that any weight reduction steps you take will have a negative effect. Removing sound-deadening increases noise, removing the spare wheel is potentially inconvenient or hazardous, gutting door and bumper bar structures is dangerous and removing the sound system, power windows and cruise control reduces comfort levels.

But there are a few weight saving steps that have no downsides. Be on the lookout for lightweight wheels and body parts. For example, 1994 Subaru Impreza WRX is fitted with an aluminium bonnet while later models have a heavier steel bonnet. The aluminium bonnet is an easy upgrade for late model WRXs.

Engine Breathing Modifications

The air intake and exhaust system of a car can be enhanced to provide improved fuel economy together with boosted performance.

The air intake of most EFI vehicles can be easily improved to deliver less flow restriction and greater efficiency. There are numerous approaches to intake modification and the best approach will depend on your car, your budget and available time. For our comprehensive series covering a variety of intake modifications see Into the Intake - Part 1

The exhaust system is another area whether substantial fuel economy and performance gains can be found. A budget approach to exhaust enhancement is to replace existing cat converters and mufflers with high-flow units. The next best step is a completely new high-flow ‘big bore’ exhaust system from the cat converter back. The ultimate exhaust upgrade is a full-length high-flow exhaust. In a naturally aspirated car you can replace the existing exhaust manifold(s) with aftermarket performance parts and install a completely new exhaust. Turbocharged cars benefit from a high-flow dump pipe from the back of the turbocharger in addition to a free-flow exhaust system.

For more on exhaust system upgrades see Out the Exhaust - Part One

Custom Engine Tune

In some vehicles – especially those fitted with air intake and exhaust mods - it is possible to improve fuel consumption by altering the engine tune.

By running the engine with a leaner air-fuel ratio under load you can achieve a significant reduction in fuel consumption. Some popular vehicles (such as Holden Commodore and Subaru WRX) have dedicated tuning software that enables mechanics to alter the engine’s air-fuel ratio and ignition timing in particular load/rpm zones. Some of these programs also allow you to change the switchover point from closed-loop to open-loop fuelling modes and adjust parameters for ‘lean cruise’ modes. These fine tuning steps can make a tremendous difference to fuel consumption.

For less popular vehicles, the easiest way to tune the engine management system is to use an aftermarket interceptor unit (such as UniChip). These units allow the tuner to tailor air-fuel ratios under load (similar to using a dedicated software program).

For previous parts of this series see Fuel Savings - Part One, Savings on Fuel - Part Two and Savings on Fuel - Part Three