Tire size and aero
 

the previous owner of the car put on what seems to be wider tires on the car and they stick out alittle from the wheel well. i wanted to make a rear wheel skirts but if i did the expensive cardboard skirts would gride up against the tires. my question is are the these tires hurting my aero and in turn my mpg?

To answer your question fairly. YES.
One size up will not affect your mileage much but any larger will hurt your fuel mileage. Wider means more area for air to pass by and more road friction.

If stock is 225 wide and you put on 235's on it will be a minimal impact on gas mileage. Put 255's on and you will see a hit with the gas mileage.
Being the tires are extending past the wheel wells they are much too wide.
Tires excessively wide will also hurt the handling of the car also.

In reading the other thread you started you spoke of a larger tires on 15" vs the stock 14" rims. Depending on what tires they put on it could affect you speedo and MPG calculations.
Just to show and example 185/14/65 is stock. The previous owner swapped those out for 195/15/60's. The speedometer could be off as much as 2 to 3 mph.
Speedometer says 65 mph when your really traveling 67-68 mph.
Your traveling farther than your speedometer is showing. Thus it is going to show lower gas mileage then what your really getting. Plus at highway speeds your car is running faster and thus burning more fuel then it would at speedometer registered speed.

That is why many compare their speedometers to GPS units. My car is pretty close to dead on. The speedometer, Scan Gage, and the GPS all match at any given speed.

I would say that wider tires would definitely have a higher rolling resistance than a narrower tire. I don't know about the aero aspect, but I would assume it slightly changes the aero aspect as well.

Jay, you are probably wrong about rolling resistance. Check the link in my sig. All other things being equal, the wider tire will almost certainly have less rolling resistance.

As for aero (and weight), the wider tire would almost certainly be worse.

ALS, why do you say that wider tires would hurt the handling?

I'm not sure I believe more rubber on the road is less resistant if the tires are the proper size for the vehicle and properly inflated.
A taller tire would serve to better fill in the open space in the wheel well and a narrower tire would bring the tire inside the fender and out of the air-flow. If fender skirts are up for consideration, having the tire more to the inside would allow a lower skirt.
On handling I found a narrower tire easier to turn.

My logic is that the narrower tire has less surface area in contact with the road surface for rolling resistance to act upon. The wider tire has more traction and more surface area in contact with the ground. This seems to me to be a larger surface area for rolling resistance to act upon. Of course I haven't been a Physics classroom since the early 90's. I could be wrong. I would suppose that there is some sort of coefficient for rolling resistance that could be broken down to the square inch, and then be multiplied by the size of the tire's contact patch in square inches. I would figure that's why overinflating tires reduces rolling resistance. The tire gets harder (It does not flex as much), and the contact patch gets smaller.

-Jay

You didn't read the link in my sig, did you? :)

Rolling resistance doesn't come from tire surface in contact with the road surface. It comes from the sidewall deformation for the length of the contact patch. Given the same pressure and weight, the contact patch will be the same size; so a wider tire will have a contact patch for less length, deforming less length of sidewall.

More detail is at that link.

No, honestly I did read that, but the tire size calculator did not really say much about tire width. It had everything else though.

Now that I've had a decent night's sleep, I've started thinking about this, and whether or not I'm doing the right thing... When I bought The Beast the dealer had just put a brand new set of P265/75/R16 Cooper Discoverer H/T tires, (Treadwear 520, Traction A, Temperature B). The OE size is 245/75/R16. Do you think I really have anything to gain by changing the tire size?

No matter which size I choose I will most likely buy a high quality Michelin tire, either the XC LT4, or the LTX A/T2. My brother in law manages the service dept in a local dealer and can get me a really good deal on them.


https://www.michelinman.com/action/ti...+Cab+Short+Box

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.
https://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

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

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

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

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.

[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.

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!