Quote:
Originally Posted by lasitter
Generally, larger tires mean lower fuel economy....
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Actually, No! Larger tires generally give BETTER fuel economy for 2 reasons:
1) The rolling resistance is lower - all other things being equal. This is a pretty small difference, but it is there.
2) The larger diameter tires is like reducing the axle ratio. You get more distance per engine revolution.
But the real problem is that things are hardly ever equal. When you change to 31" tires, you are also changing the type of tire - P type to LT type - and LT's are typically not designed for fuel economy.
Also, there can be more difference in fuel economy between different tires (meaning make and model) than the difference between different tire sizes. Careful selection of the exact make and model can pay dividends.
And lastly, LRR is a relative term, not an absolute one. LRR means better fuel economy compared to other tires with similar wear and traction characteristics.
Put another way, a LRR tire doesn't necessarily have a low rolling resistance. It's just lower than similar tires. It is quite possible for a tire NOT labeled LRR to have a better RR level.
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Quote:
Originally Posted by Joggernot
Consider 6-ply tires (4 on tread and two on sides). They can run at 80 psi. If there is an LRR 6-ply tire, all the better.
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Ah ...... Mmmmmm ........ not exactly .......
Most tires are built with a 2+2 construction. That is, 2 body plies (typically polyester) and 2 steel belts. When listed on the tire's sidewall, this will come out as:
Tread - 2 steel plies and 2 polyester plies
Sidewall - 2 polyester plies.
Please note the 2 polyester plies are the same ply, just listed differently. That tire would NOT have 6 plies.
Sometimes a nylon cap ply is added over the belts. Sometimes 2 nylon cap plies. These would be listed under "Tread" on the sidewall stamping.
The 2+2 construction can be used in a 35 psi max tire or a 95 psi max tire. It's a little complicated about how many plies a tire needs - mostly because the cords can be made out of many things - including steel cable. But you will find plenty of examples of tires NOT built with a 2+2 construction.