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Old 07-16-2009, 06:53 AM   #82 (permalink)
CapriRacer
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Quote:
Originally Posted by RobertSmalls View Post
Thanks, CapriRacer, I was hoping for an explanation from our resident Tire Geek.

I feel the tires on the Insight are larger than they need to be. They're rated for 937lbs, and the car weighs <600lbs at each corner when fully loaded.

However, I can't seem to find low RR tires for road vehicles smaller than an Insight but larger than a bicycle. Are there such tires, in the range of 150-600 lbs max load per tire?
One of the lesson from the Ford / Firestone situation a few years ago was that the way load carrying capacity is calculated for a tire needs to be refined downward - the tire needed to be larger. Put another way, there needed to be more unused (reserve) capacity.

This was partially because the average joe doesn't check his inflation pressure regularly, but also because vehicle operating speeds have been gradually been on the rise.

But for rolling resistance, larger is better.

Every tire has a tread and 2 sidewalls. The sidewalls do not consume much of the fuel when rolling. About 90% is the "tread" - the tread itself , the steel belts. and the plies. Because of this tires with larger capacity are every so slightly more efficient - same 2 sidewalls, but a larger tread.

Large capacity tires tend to weigh more - and that affects the accelleration - but in the big scheme of things, how much more fuel is consumed when accellerating larger tires, compared to the amount of fuel consumed when rolling. I tend to think that larger capacity tires would result in overall better fuel economy. But I am sure there is a point beyond which the gains are small arer not worth the effort.

Besides, tire sizing is limited by what can be stuffed under the fender.

So I don't think you should be looking for smaller tires if your goal is better fuel economy.
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