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Old 01-11-2011, 09:45 AM   #16 (permalink)
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Originally Posted by stovie View Post
Check out this explination on tires and there psi to weight figures HowStuffWorks "How Tires Work"
it says u can guess pretty close to the weight of your vehicle just by looking at the area of your contact patch and yes i understand that your not going to be exact or anything with that calculation but i think you would get pretty dang close. I don't think that friction with the road is all RR is about but if you increase the tire pressure not only are u decreasing the contact with the road but the amount of deforming the tire does therefore less energy converted to heat in the tire now if you could get class D tires with 80Psi max and with the low rolling resistance material then you'll have really good FE gains. i got the idea from watching mythbusters the other night they took tires that had a max psi of 35 and increased it to 40 and got a 6.5% increase in FE and thats 2 points better then the best LRR tire i've seen so far but if you find a better one then let me know ok.(and yes i know i'm not goin psi to psi ok but it steal works)
Sorry, it's an old wife's tale that a tire's contact pressure and inflation pressure are somehow related in a linear way. Here's a link that is a lot more detailed on the subject:

Fact or Fiction? Tire contact patch and air pressure.

As you can see, the contact pressure and the inflation pressure don't seem to be very close at all.

And as far as Mythbusters is concern. I saw that episode, too. They measured the fuel economy at steady state - and the effects on inflation pressure are a lot more easy to measure - which is why they did it.

But let's start off with this as a basis:

Bruce Lambillotte of Smithers Scientific Services is reporting a 60% difference between the best tire and the worst tire in 2 different sizes - pages 11 and 12.

That is HUGE - and much bigger than any test results I have seen for inflation pressure increases. Don't get me wrong - I am not saying inflation pressure increases don't improve fuel economy, but compared to what the differences can be between tires, changes in tire size are small, and changes in inflation pressure are next, but differences in tires is the largest of them all.

And you have to be careful when you change tire types. P metric tires are constructed quite different than LT metric tires - and as a general rule, LT metric tires - even with their 80 psi pressure - can be completely out classed by an efficient P metric.
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