Tyre pressures and mpg

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Crawfster

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It's been discussed a few times here and is obvious enough, so I thought I'd do a small experiment with my new A2 1.4 SE TDI, Open Sky. As a keen racing cyclist, the link between speed/energy input and tyre pressures is very clear - more pressure is more speed or less effort....but only up to an optimum value, where "hard-wheel" discomfort and the risk of punctures becomes disproportionately greater than the speed/effort gains.

I'd been running my A2 at 30 psi on all 4 tyres, and have increased this to the "full-load" maximum of 38 psi in 2psi increments with each new tank of diesel. What I find, in very broad terms, is that for every extra 2 psi, I'm getting back an extra 1-1.5 mpg - at 30 psi, I was getting around 56 mpg, at 38 psi I'm getting around 62 mpg. Obviously, it's very different to keep all the other variables constant - although I've tried fairly hard (only one passenger (ie the driver) in all tests etc - but as a rule of thumb, it seems like higher pressures have a definite effect. Quite whether there are any downsides of running on these higher pressuires, I have yet to discover....
 
Since you refer to your car as new, I assume it has very low mileage at the moment. So one obvious interacting variable is the engine loosening up. Might be interesting to try the experiment in reverse now, by reducing the pressures, and see what you get.
 
Viffer

Was aware of the "engine maturity" issue before starting the "experiment" so I waited until I had 4000 miles on the clock - which previous thread have indicated is probably the distance at which no further "maturity" benefits to mpg are seen. I could, perhaps, reverse the test, but I'm not sure I wish to willingly reduce my mpg now!!!!
 
I didn't realise this was so effective but I was told by an experienced fleet car driver to do this. It increases tyre wear though so if you pay for your tyres there will be an optimum ballance. As I don't pay for mine I will try this and report back.

Phil
 
Is it likely to work in reverse, so that if you ran lower pressures you'd use more fuel, but save tires? Just thinking about my Punto's front tires - the originals lasted 20k, and now 10k later the replacements (same manufacturer) don't seem to be lasting quite as long (probably about 17-18k). The first 9.5k the car did wasn't by me, so they could have been low, thus saving me money! Course, the rear's seem to be fine, and I reckon the originals will see 50k! Just as well as it takes skinny ones that they stopped making in quantity once low-profiles were the rage - thus costing even more. So if it's not a new car (hopefully the A2 I dream of) then some nice alloys...
 
I immagine there will be an optimum pressure for minimising wear but it is probably dependant on driving style, road surface, temperature etc. Obviously with 0 psi your tyres would not last long.

Phil
 
The problem with increased tyre pressure is that the tyre wears in the middle, so doesn't last as long as the middle is bald with the outside edges OK.
Reduced tyre pressure has the opposite effect - worn on the edges but OK in the middle.
It's best to stay within the manufacturers suggest tyre pressure ranges.
I tried running at full load pressures - I think the consumption improved from around 36 to 38 mpg - this could also be because I have only covered 2500 miles, so the engine is still loosening up.

I have now put the pressures back to standard - the ride is slightly improved.

MartinW - Dolphin Grey 1.4SE
 
I don't have the figures yet but I was achieving around 44mpg consistently and I have checked my tyre pressures and they were around 28psi. I have upped the pressures to 38psi and bearing in mind I have a 2002 petrol I have travelled 325 miles and still not heard the bong which I think will be my best ever figures. The car has done 16000 miles so the engine should have bedded in.

On the downside the ride is noticably harsher.

I bet Audi get the fuel data on tyres at full pressure.

Phil
 
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