What did you "measure at 3 different places"? The diameter of the tire or the ride height of the car?

The "40" and "50" are "aspect ratios"..... the sidewall height of the tire divided by the section (maximum width) width of the tire. A 275/40 means the fattest part of the tire is 275mm (275/25.4 = 10.83 inches) wide. Since the aspect ratio is "40" the sidewall of the tire is 0.40 X 10.83 = 4.33" high.

To compare the 245/50-16 to the 275/40-17:

Section width:

245 / 25.4 = 9.65 ------------------- 275 / 25.4 = 10.83"

Sidewall height:

0.50 X 9.65 = 4.82" ------------------ 0.40 X 10.83 = 4.33"

Overall diameter:

16.0 + 4.82 + 4.82 = 25.65" ------- 17.0 + 4.33 + 4.33 = 25.66"

With new tires with similar tread depths, the outside diameters are virtually identical. Comparing worn tires to new tires could account for an 0.63" difference in diameter. If you put a new 285/40-17 in place of a badly worn 245/50-16, its conceivable that the difference in outside diameter of the tire could be 1". But, even that 1" diameter increase would only raise the car off the ground by 1/2", and it would fill in the gap between the wheel and the tire by a similar 1/2".

You can't accurately measure the diameter of a tire by using a tape measure across the tire diameter. With regard to actual diameter off the car, you would have to use a level on the top surface of the tread, and measure the height from the ground, or wrap a string around the tire and divide the resulting length of the string by 3.1416

To get the effective "rolling diameter".... the affect the tire would have on the speedometer/odometer.... you have to mark the tire and the ground, roll the car forward until the tire mark is again on the ground, then measure the difference between the marks on the ground and divide by 3.1416.

To get an idea of the affect of wheel weight and tire width on rear wheel HP, go to the WS6.com site. He dyno'd his WS6 with 275/40-17's on the stock 17x9 wheels. Then he had the stock wheels widened to 17x11, and ran 315/35-17 tires. He lost 3.6rwHP/3.5rwTQ.

 

As for ZR1s with 275/40/17

they are much noticeable heavier then 235/55/16s( I know I swapped my wheels yesterday and had ot carry them up and down stairs )

I lost a like 2-3/10ths at the dragstrip with them. But they handle alot better.


As for guys saying they affected your rid height? did you measure your car after you jacked it up. Mine usually takes a few a drive or two to hlep the suspsension settle back down.

 

I measured the ride hight of the car. Also my old tires were worn out and these I put on were new, and I measured it after I had it jacked up so maybe I should have check it after I drove it few times, but I know that it did increase the ride hight and it does not corner as well.

 

I just like pretending to be a moderator, I don't like be a snitch.