Tire size and aero

[Jay2TheRescue]

Originally Posted by theholycow

Ah, I see the problem. I failed to be specific enough...I should have said to read the "tire width" canned post, but you went to the size calculator.
http://www.gassavers.org/showthread.php?t=7713
That's the in-depth analyzation of rolling resistance.

The tires that you have now are taller as well as wider than the OE size. They probably gain FE for taller gearing and less RR, but they probably lose a little due to aero concerns. If your speedometer and odometer weren't adjusted, your mileage calculations and speed are slightly inaccurate.

What pressure are you running in them? I may have already asked you that.

I'm running them at 45 psi, and they have worn perfectly even. No excessive wear in the center of the tread in relation to the rest of the tread. At 60 mph reading on the speedo I'll get 60 to 62 mph on my GPS, so I feel that's about as accurate as any vehicle on the road, probably more accurate than many I've seen.

-Jay

[theholycow]

That's definitely as accurate as you can get.

I have been running 80psi on my truck for ~125,000 miles and also get no center wear. I have allowed the rears to settle down to 70psi because it gets just a little skittery in low traction conditions when the bed is empty.

[Jay2TheRescue]

Originally Posted by theholycow

That's definitely as accurate as you can get.

I have been running 80psi on my truck for ~125,000 miles and also get no center wear. I have allowed the rears to settle down to 70psi because it gets just a little skittery in low traction conditions when the bed is empty.

I've had the rear end slide out on a few rare occasions when making a right turn from a stop sign in the rain. I just figure its because my tires are nearing the end of their useful life though.

-Jay

[theholycow]

Yeah, that particular traction loss is normal. After all, it IS a truck.

I'm not entirely sure that 70psi in the rears gives me any better result than 80psi, but where I noticed it most is turning (not from a stop, at speed) onto an uphill highway entrance ramp and getting on the gas quickly. I enjoy getting a little sideways occasonally, so I really haven't made an effort to test and see what PSI would be best for it. I just haven't bothered to inflate when I check them and they're 70. Considering the light weight of the empty bed, I doubt the 10psi makes much difference in FE anyway.

[Jay2TheRescue]

Originally Posted by theholycow

Yeah, that particular traction loss is normal. After all, it IS a truck.

I'm not entirely sure that 70psi in the rears gives me any better result than 80psi, but where I noticed it most is turning (not from a stop, at speed) onto an uphill highway entrance ramp and getting on the gas quickly. I enjoy getting a little sideways occasonally, so I really haven't made an effort to test and see what PSI would be best for it. I just haven't bothered to inflate when I check them and they're 70. Considering the light weight of the empty bed, I doubt the 10psi makes much difference in FE anyway.

Yeah, it is a truck, but it handles better than the Buick. That 4wd pickup can hang onto a cloverleaf ramp at 55 mph I have to slow the Buick down to 40 or 45 for the same corner. I do think that I need to replace the front springs in that car though, the front is really low.

-Jay

[goofy1]

when do u guys inflate your tires? when they're cold or warm? and also i'm confused by the psi. the oem tires are marked for a max of 28(according to the toyota sticker in the glovebox. but i checked these tire(though they are taller and wider) they were set by walmart at 38psi. should i keep them at that? and also some have said to run the rear tires at lower psi then the front. does that really work? if so how much lower?

[Jay2TheRescue]

Originally Posted by goofy1

when do u guys inflate your tires? when they're cold or warm? and also i'm confused by the psi. the oem tires are marked for a max of 28(according to the toyota sticker in the glovebox. but i checked these tire(though they are taller and wider) they were set by walmart at 38psi. should i keep them at that? and also some have said to run the rear tires at lower psi then the front. does that really work? if so how much lower?

I always fill my tires warm. This way when I fill the tire it does not go beyond what I set it at. If you filled your tires cold to 30 psi, and then drove down I-95 at 70 mph for an hour in the middle of summer you could have several more pounds of pressure in the tire at the end of your run. By checking my tires hot I know what my max pressure is.

-Jay

[theholycow]

There is a link in my sig about tire pressure.

I run mine at the maximum pressure that's marked on the side of the tire. That's the rating for the maximum pressure that they're willing to say the tire will safely hold. The tire won't fail at higher pressure, but I don't exceed it just in case I ever have any kind of tire failure and an accident results. In such an event, there could be a question of legal liability.

The pressure that Toyota marked in the glovebox is the minimum pressure for the weight of your car, just enough to prevent the tire from failing.

[theholycow]

[QUOTE=Jay2TheRescue;106747By checking my tires hot I know what my max pressure is.[/QUOTE]

The maximum pressure marked on the tire often says "cold". Either way, that's the maximum cold number. Maximum rating warm is somewhat higher.

[richochet]

The tyre contact patch on the road is always stationary, relative to the road, no matter at what speed the vehicle is travelling (except if the wheels are spinning, or skidding - when they can be stationary - or still rotating if the vehicle is sliding sideways). That means that usually, energy loss is due to sidewall flexing, but there has to be some "give" in tread and sidewall just in front of and just behind the transverse contact line.

Anyway, this is an aerodynamics discussion, so, it the tyre contact point is stationary and the vehicle moving (normal rolling) the top of the tyre must be moving at twice vehicle speed; it also has tread on it so keep it out of the airstream!