Why a car uses less fuel after chiptuning (and when it does not)

It sounds contradictory: an engine that delivers more power and uses less fuel at the same time. Yet with a properly calibrated Stage 1 it is the rule rather than the exception, as long as you understand where the gain comes from and what you have to do yourself. In this article: the physics in plain language, the differences between petrol and diesel, the cases where it does not work and a worked example with today's fuel price.

An engine is most efficient when it does not have to work hard

Every combustion engine has a zone where it is most efficient: usually at low to medium revs under a decent load. Outside that zone a larger share of the fuel is lost to friction, pumping losses and heat. Two things decide where your engine lives all day: the gear you pick and how much torque the engine can deliver at that engine speed.

A stock engine often has little torque to spare at low revs. To merge onto the motorway or climb a hill you shift down, the revs rise and consumption goes up. After a Stage 1 there is clearly more torque available in that same low rev range. You stay in a higher gear, revs stay low and the engine works less hard at the same speed.

• Fewer downshifts. Every time you go down a gear the engine turns hundreds of rpm faster for the same speed, with more friction and pumping losses.
• Up to speed sooner. Accelerating always costs extra fuel; the shorter that phase, the smaller the loss.
• Cruising at lower revs. On the motorway a tuned car sits in top gear where the stock car often needed one gear lower.

In the graph, do not look at the peak but at the zone between 1500 and 3000 rpm. That is where you drive in town and on the motorway. That is exactly where a Stage 1 adds the most torque, and therefore exactly where the saving is.

Petrol and diesel: different mechanism, same effect

Turbo petrol. A petrol engine controls its power with the throttle valve. At small throttle openings that valve is almost closed and the engine has to draw air against a vacuum: pumping losses. More torque at low revs means you need less throttle and fewer revs for the same gear, so fewer of those losses. On top of that we calibrate injection and ignition to the actual boost, so the engine needs less enrichment in the part-load range.

Turbo diesel. A diesel has no throttle valve and controls its power with the amount of fuel injected. Here the gain comes mainly from torque: a 2.0 diesel with 340 Nm stock and 410 Nm after Stage 1 pulls a caravan or a fully loaded van one gear higher, at lower revs, and therefore uses less. Injection timing at part load can also be slightly more efficient, within the limits of soot formation and exhaust gas temperature.

What is realistic?

• Relaxed driver, mostly motorway or country roads: around 5 to 10 percent less fuel. This is the scenario where the gain is largest.
• Mixed use, town and country: around 3 to 7 percent. In town you often drive in low gears and extra torque helps less.
• Vans and towing vehicles: often at the top of that range, because the stock engine works almost constantly at its limit.

These figures are indicative. They depend on the engine, the gearbox, the tyres, the weather and above all on you.

When it does not work

• Short trips with a cold engine. For the first kilometres every engine runs rich and with extra friction. Software changes nothing there.
• Mostly city traffic. At 30 and 50 km/h in second or third gear there is little to gain from extra torque.
• An engine that is not healthy. A clogged particulate filter, a leaking intake or worn spark plugs cost more fuel than a tune can ever save. That is why we check beforehand.
• Spirited driving. See the box above. More power is a possibility, not a guarantee.

Work it out with today's price

The saving in litres is one thing; what it costs at the pump is what you feel. Below, the current Dutch national fuel price is filled in automatically. Choose petrol or diesel, slide to your kilometres and consumption, and see what a relaxed driving style saves per year.

How to measure it honestly

1. Reset the trip computer and drive your normal routes for a few weeks before the tune. Note the average.
2. Do the same after the tune, in the same season and with the same tyres and tyre pressure. Winter weather alone costs 10 to 15 percent.
3. Only compare after at least two full tanks. The trip computer itself is off by a few percent.

In short

A car does not use less fuel because there is "more power", but because there is more torque where you use it every day: low in the rev range. That means fewer shifts, shorter acceleration phases and a higher gear. A sensibly calibrated Stage 1 saves a relaxed driver around 5 to 10 percent, a spirited driver nothing, and an unhealthy engine needs maintenance first.

See under chiptuning what we expect for your engine, read about diesel chiptuning or ask your question via contact.