fuel mileage and tire size

[darcane]

Originally Posted by CapriRacer

Sidewalls play very little role in RR, but tread compound - both amount and properties - plays a HUGE role.

Hrm....

I thought the primary reason big rigs used Super Singles was that they have fewer sidewalls (two instead of the four of the duals they replaced) to flex and this gave you your reduced rolling resistance. Is this not true?

Mike

[darcane]

Originally Posted by CapriRacer

OK, Based on what you've told us, you just may be at the limit of the transmission's gearing. What you want to do is keep it from shifting as much as possible. So going up in overall diameter is not a good option.

Your truck came with 15" P metric tires. Those have a tire sizing structure that looks like this:

P235/75R15.

You've got Flotation sized tires on there now. Those have a sizing structure that looks this this:

29X8.50R15LT.

The other common sizing system is LT metric - and they look like this:

LT235/75R15.

You do NOT want LT metrics or Flotation sized tires on your truck. Those tires were designed for more rugged usage. They will not be built out of materials that are good for RR.

You'll want to use as big a 15" tire in a P metric you can find. Unfortunately, your truck came stock with 6" wide rims and that limits you to P235/75R15. There are only a few LRR tires listed on Tire Rack in that size. The problem here is "LRR compared to what". I do see some tires that might be OE on something - Goodyear Wrangler SR-A for example - and those are likely LRR even though Tire Rack doesn't list them as such.

On my Silverado, I was considering LT tires for my upcoming tire replacement since they would allow for me to run higher tire pressures. Are you saying that they will have worse rolling resistance in spite of the higher pressures? Some appear to have a good tread pattern for highway use.

Mike

[CapriRacer]

Originally Posted by darcane

Hrm....I thought the primary reason big rigs used Super Singles was that they have fewer sidewalls (two instead of the four of the duals they replaced) to flex and this gave you your reduced rolling resistance. Is this not true?

Mike

There are 3 reasons why big rigs use super singles:

1) Overall they weigh less (16 tire and rim assemblies and attaching hardware compared to 8!) That means the rigs can carry slightly more cargo.

2) The cost of a super single is less than for a pair of duals (don't forget the cost of the wheels!)

3) The rolling resistance is less for a super single than for a pair of duals. Yes, there are 2 fewer sidewalls. but a super single is physically smaller than a pair of duals - and therefore more efficient. In some respects it's confirmation that bigger = better, particularly considering that we are talking a completely different type of tire.

But more importantly, completely eliminating 2 sidewalls is quite different than talking about the effect sidewall changes in a tire have. The changes that can be made to - say - the tread compound have huge affects on RR compared to what can be done to the sidewall.

[CapriRacer]

Originally Posted by darcane

On my Silverado, I was considering LT tires for my upcoming tire replacement since they would allow for me to run higher tire pressures. Are you saying that they will have worse rolling resistance in spite of the higher pressures? Some appear to have a good tread pattern for highway use.

Mike

To facilitate this discussion I am going to assume your truck came with P metric tires. If you replace those tires with the same sized LT metric tires, and make the appropriate inflation pressure changes so that the load carrying capacities are the same, then the LT metric is pretty much guarranteed to have worst RR.

But if we factor in an increase in inflation pressure - which would be on the order of a 30 psi increase, then it becomes a bunch less clear. I don't think there is enough information to reasonably quantify it one way or the other - and my best guess is that it would be a toss-up.

The net effect is that going from a P metric to an LT metric at higher inflation pressure probably isn't going to net enough change compared to what can be done staying with a P metric and the changes that are possible with that combination.

[Execut1ve]

I just went down and checked the tires. Currently one of the tires on the truck is a P (size is in the format P235/75R15) and the other 3 are just plain 235/75R15. I just had a flat recently and the exact model and tread pattern I have on the truck is no longer made, so I had to get a different tread pattern but the same model. Into what category do the 235/75R15's fall?

[comptiger5000]

It depends on the vehicle and gearing. My Jeep is notably overgeared, so going from stock 225/70R16 tires (28.4 inches tall) to my current 245/70R16 tires (29.5 inches tall) actually helped mileage very slightly on the highway, with no noticeable difference around town (yes, the speedo is corrected for the new tires). The gearing improvement was enough to offset the weight penalty, as well as the worse aero from wider tires and the Jeep sitting a 1/2" higher off the ground.

[CapriRacer]

Originally Posted by Execut1ve

I just went down and checked the tires. Currently one of the tires on the truck is a P (size is in the format P235/75R15) and the other 3 are just plain 235/75R15. I just had a flat recently and the exact model and tread pattern I have on the truck is no longer made, so I had to get a different tread pattern but the same model. Into what category do the 235/75R15's fall?

If there aren't a letters in front of, or behind, the tire size, then those are what some call "Hard metric" - and that can be further divided into Japanese or Euro. For practical purposes Hard metric and P metric are the same.