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.

[stevieturbo]

For a few weeks my van needed a steering rack and was scrubbing the inside of the front tyres. Clearly not an efficient setup.

I do around 1500-2000 miles most weeks, and even after it was fixed, made no difference at all to economy, and I do keep a close eye on it for obvious reasons

So I'd find it hard to believe that some of these fancy tyres that are supposed to improve fuel efficiency actually do anything either.

A downside of running odd pressures, would be the tyres would wear out faster, which is an added expense.

[PaulKemp]

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.

[stevieturbo]

Driving style will have more impact on economy than anything else outside of engine tuning for economy.

Finding the best cruising speeds and gears/throttle etc for any given speed and maintaining them will yield the best results.

And certainly as far as my van is concerned, I am convinced Texaco diesel is the best

[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.

[SuperchargedVR6]

PaulKemp said:

You need a repeatable test for comparison.

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....

[Huff]

I'd guess the fact that tyres generally last 10000miles + with all that scrubbing from toe settings, and generally appear evenly worn when replaced suggests 'toe' is an utterly negigible parasitic.

I've tried simlar experiments as Puma over the years with my old BMW e34 540, which likes a drink. Running the fronts slightly higher pressure than book (at 3.0 not 2.4bar, the full load/high speed recommendation but with no commensurate increase in rear pressure of 3.2) markedly improves turn-in and steering feel - implying the recommended setting is a bit soft presumably for comfort. It also makes not a jot of difference on long motorway runs.

[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.

[Nick1point9]

Any improvement in fuel economy would surely be offset by the increased cost of tyres due to premature wear in the centre of the tread?

I looked into this as part of my university final year project; tyre pressure has very small effect on improving rolling resistance, the benefit comes from lower vertical loading. It's mainly because the contact patch is designed not to balloon like on a bicycle, so the actual amount of tyre in contact with the road virtually doesn't change.

[JonnyVTEC]

Nick1point9 said:

Any improvement in fuel economy would surely be offset by the increased cost of tyres due to premature wear in the centre of the tread?

Do you have cross-ply tyres or something?

[StuartGGray]

As an aside, I've been using All season tyres on my car. V70 with 15"
195/65 tyres. I've been running them at the recommended 29 psi for a
while, then on the recommendation of a car building friend, stared to
run them at 38 psi. The max is someting like 41 for high speed or
toowing etc.
Instead of getting about 400 miles tops out of the 70 litre tank, I'm
getting 520+ out of the tank. No difference in comfort or tyres wear or
noise. Used to squeal a bit on roundabouts when very warm, but quieter
now.

(I am driving like a granny with the pumped up tyres)

[Nick1point9]

JonnyVTEC said:

Nick1point9 said:

Any improvement in fuel economy would surely be offset by the increased cost of tyres due to premature wear in the centre of the tread?

Do you have cross-ply tyres or something?

Nope, over-inflated radials wear the centre of the tread more so than the rest.

I simply meant that say the centre of the tyre was 1mm lower than the rest of the central 3/4 (not especially uneven wear) then you'd lose over 15% of the life of the tyre for next to no fuel economy benefit. Depending on the car >15% increase in the cost of the tyres could be more significant than the fuel saving.

[JonnyVTEC]

Did you read that in the Haynes manual?

[miln0039]

I went up to do a Car Limits Day with Andy Walsh at North Weald. He insists you pump your tyres up the the maximum rating as given by the tyre manufacturer to avoid killing your sidewalls during high G cornering. I therefore popped 52 psi in my Z4 Coupe (up front around 35 I usually use).

I couldn't be bothered to reduce the pressures afterwards as there was a queue at Shell. Anyway, I did calculate my MPG (brimmed not OBC) for the 120 mile motorway journey - similar conditions and the difference was:

There at 35 psi - 34.2mpg
Home at 52 psi - 35.1mpg

So a little difference, but hardly material. I guess I had supposed it might do a bit more...

[Simon says]

stevieturbo said:

Driving style will have more impact on economy than anything else outside of engine tuning for economy.

Finding the best cruising speeds and gears/throttle etc for any given speed and maintaining them will yield the best results.

And certainly as far as my van is concerned, I am convinced Texaco diesel is the best

Totally agree with this + driver traffic anticipation pays off for fuel economy and long term wear & tear on the mechanicals

[Nick1point9]

JonnyVTEC said:

Did you read that in the Haynes manual?

Nope, read it in the 4 years of running/working in tyre shops.

[StephenM44]

My Volvo S80 has an 'eco' setting on the tyre pressure page of the manual, normal pressures for <100mph and 3 people is 32f30r but eco is 38f38r, I run 36f34r as it handles better like that. There is a different on the motorway in mpg between the soft and eco pressures but it's not big, maybe 2mpg and that's over a whole tank and 500-600 or so miles of 80mph driving. Tyres are 225/50-17 and I can tell a difference if one of them is a bit soft but the car weighs almost 1800 Kg.

[Nick1point9]

Simon says said:

stevieturbo said:

Driving style will have more impact on economy than anything else outside of engine tuning for economy.

Finding the best cruising speeds and gears/throttle etc for any given speed and maintaining them will yield the best results.

And certainly as far as my van is concerned, I am convinced Texaco diesel is the best

Totally agree with this + driver traffic anticipation pays off for fuel economy and long term wear & tear on the mechanicals

There's a place on my drive home where I lift off over a mile away from a junction (very conveniently it goes from hatched to solid lines between lanes at about 1.1 miles from the junction), simply because there is a slight downhill gradient and almost never any other cars. I go from 60-30 in that mile and don't use a drop of diesel.

[Munter]

StephenM44 said:

My Volvo S80 has an 'eco' setting on the tyre pressure page of the manual, normal pressures for <100mph and 3 people is 32f30r but eco is 38f38r, I run 36f34r as it handles better like that. There is a different on the motorway in mpg between the soft and eco pressures but it's not big, maybe 2mpg and that's over a whole tank and 500-600 or so miles of 80mph driving. Tyres are 225/50-17 and I can tell a difference if one of them is a bit soft but the car weighs almost 1800 Kg.

That sounds similar to my experiance. If I run the Avensis at the recommended pressures I get about 52mpg doing my standard trip to Taunton and back. Run them at the >100mph pressure and I can get 54mpg on the same run. Everytime it has a service they drop the pressures and I forget until I notice the mpg has dropped.

[VitesseEFI]

I have an elderly Audi A6 2.5TDI (straight 5), which is a pretty fuel-efficient old beast anyway. However, I do find that if I raise the tyre pressures from the "official" unladen settings of 34psi to the fully laden settings of 38 psi this will gain 2 - 3 mpg. This increase doesn't seem to have an effect on the roadholding (though she ain't no sports car at any time!), however, going much above these pressures does start to affect grip, especially in the wet.

I do also notice wind direction can make a quite startling difference. As I quite regularly travel from east Somerset to Cornwall, heading south west into a frequently south westerly wind and then reverse the journey later in the day, I can often see 5 - 6mpg difference between outward and return journeys on the trip computer. The record is over 15mpg difference - but it was a very windy day and the return average over approx 140 miles with a strong tail-wind (acc to the trip computer) was 65mpg, a figure never seen before or since!

Cheers

Nick