Tyre pressure vs fuel economy

[Pumaracing]

I've been undertaking a little test on my Focus this last week. My normal book recommended tyre pressure is about 30/32 psi and my last three tanks of fuel gave almost identical mpg at about 34 mpg. I decided to try pumping the tyres up to 40 psi hoping for an improvement in mpg but everything I read online where people had tried this seemed to indicate it made little difference.

I wrote a couple of spreadsheets to calculate tyre rolling resistance and the resulting potential fuel economy which also indicated that any prospective improvement in mpg would be minimal. Maybe 0.5 mpg at best. In the end I found that tyre grip was considerably reduced, especially at the rear end which stepped out a couple of times under hard cornering and mpg actually went down to 32.4 mpg although over such a short mileage that it couldn't be considered statistically significant. However it was clearly not an obvious improvement.

Sadly it seems that anything other than stock tyre pressures (apart from very low pressure of course) make little or no difference to rolling resistance or mpg.

However it still strikes me that higher pressures "should" reduce drag. Reading some of the online literature about bicycle tyres shows that higher pressures can actually increase drag. The stiffer tyre doesn't conform to road irregularities as well as the more supple tyre and drag actually goes up.

Food for thought anyway.

[Pumaracing]

PaulKemp said:

In the dim recesses of my mind I remember some articles on fuel efficiency detailing a couple in Anerica (30 years or more ago) who did all they could to achieve incredible mpg ( for the day) they reduced the weight of their car, adopted an economic driving style (this probably made the most difference) converted the car to 3 wheels and importantly for this forum thread used extremely narrow tyres pumped up to 100psi.
I take a few points from this,
you need more than one thing to make a noticeable difference.
You need a repeatable test for comparison.
I would think it would be very difficult to drive exactly the same way as proved by the rear stepping out.

Tyres that are designed for very high pressure such as bicycle tyres can achieve much lower rolling resistance coefficients than car tyres which are primarily designed for grip and certainly you could see improvements in mpg using them provided you didn't want to corner very fast. Wiki has some data on bicycle vs car tyre RR coeffs.

My main area of interest was to see if anything could be done to improve the tyres you have on your car anyway and it seems not. Obviously very low pressures cause considerable drag but by the time pressures are up to the stock recommendation going any higher makes little difference and may even make things worse.

The other factor that plays into economy is that tyre drag, especially at higher speeds, is not a large part of total drag. At 70 mph my Focus requires approximately 29 flywheel bhp to drive it. Of this only 8 bhp is from tyre drag and 21 bhp from aero drag. Even if I could reduce tyre drag by 5% which is unlikely it calculates out as only a 0.5 mpg improvement.

At lower speeds tyre drag becomes a bigger factor. At 40 mph it takes 8.6 bhp to drive the car and 4.7 bhp of this is tyre drag. However the 2 litre engine is now operating on such a low throttle setting and is therefore so fuel inefficient that mpg is no better than at higher speeds. Only a smaller less powerful engine operating closer to its best BSFC setting would materially affect this.

My Focus has the very annoying trait of it being impossible to greatly alter its mpg. I can drive long trips at 60, 70 or even 80 speedo mph (about 5 mph less true speed) and economy stays exactly the same at about 37 mpg. Pottering about on A roads at 40/50 mph trying not to slow for corners or use any more throttle than necessary barely nets me 35 mpg. With smaller engined cars I've owned in the past I could get vastly better mpg by driving slowly and carefully but the Focus just trades off lower speed and bhp requirement for less efficient cylinder burning and worse BSFC. That tells me of course I need a diesel engine not a petrol one but I like the Focus too much to swap it.

Anyway at least I can say I tried the experiment and won't be repeating it. I've gone back to 32 psi.

[Pumaracing]

SuperchargedVR6 said:

Indeed. I would expect even a stronger head wind on the next day's testing to have an overall impact on mpg as the engine will be under a bit more load, therefore burn more fuel.

What about running no toe angle at all so all 4 tyres aren't 'scrubbing' to track the car straight? Would that work? It'd probably drive all over the shop, but for experiment's sake....

Tyres aren't supposed to scrub in the first place and the whole point of static toe settings is to get the tyres parallel when they're rolling.

RWD cars tend to push the undriven front wheels slightly into toe out so the static setting is usually a little toe in to compensate. FWD cars tend to pull the driven front tyres slightly into toe in so the static setting is usually a little toe out to compensate.

Rear tyres on live axle or beam axle suspensions are obviously parallel because there's no choice but occasionally independent rear suspensions might use a little rear toe setting to achieve some specific desired handling balance.