Quote:
Originally Posted by mada88
Hello everyone,
I used to own a car with 17 inch wheels and in the owners manuel it said that 38 PSI was the best pressure for economy.
I now own a car with 15 inch wheels and I have been over inflating all 4 tyres to 38 psi.
I 38 psi optimum for fuel economy? making the tyres have more rolling resistance with a higher psi?
What is the optimum for a 15 inch tyre?
I saw a myth busters episode and they said 30 psi was best.
Or does it depend on the wheel and tyre size?
Any help is much appreciated thank you
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First, in the US, every car has a vehicle tire placard which lists the original tire size and the specified pressure for that size. I believe other countries use a placard as well - may be even the same one. Since 2008 the placard is supposed to be on the driver's doorframe, but prior to 2008 it could also be found in the glove box, on the fuel filler door or in the trunk (boot).
Second, "optimum" means there is a peak value - and for RR, increased pressure always results in better RR - up until the tire explodes. However, beyond a certain point, the increases in pressure don't result in much improvement in RR.
Third, the maximum pressure listed on the sidewall of a tire is exactly that - a maximum. It is NOT a recommendation. It also is relatively arbitrary - which I explain here:
Barry's Tire Tech - Load Tables
About 1/3 of the way down, I talk about "The notes on page 1-34", which deal will the maximum pressure. Others may disagree, but I think the maximum pressure is not something that tire manufacturers do a lot of thinking about as the load table is what is important and the load tabl;e defines the relationship between pressure and load.
Fourth, while RR improves with inflation pressure, other properties don't. The most obvious is traction which likely peaks, then goes down. This is the result of the shrinking contact patch. The problem here is that this is not well documented. There aren't charts for the literally thousands of different tires out there showing where that peak is. What we have is anecdotes.
Also, tire wear is somewhat tied to inflation pressure, with increasing inflation pressure causing more wear in the center of the tread. Again, this is not well documented - in fact hardly documented at all. All we seem to have is those ubiquitous anecdotes.
Other properties that seem to be adversely affected by increased inflation pressure are groove wander (aka tramlining), puncture resistant, impact resistant, and durability - again, anecdotal.
And lastly, I am NOT a fan of using a lot of inflation pressure, mainly because of the compromise in traction. So I don't recommend using more than 5 psi over the placard pressure. Other people have different opinions.