93zder

I'm going to 17"/18" F/R rims and tires soon and am wondering:

How much will my speedo be out if i run a 35/18" tire on the rear as opposed to a 30/18 "tire?

i mean will the difference between the 30 and 35 ratio make that much of a difference?

thanks

SSCamaro99_3

The 30 and 35 number are sidewall height as a percentage of treadwidth. As long as you maintain a near stock tire height at all corners, you should have no issue.

245/50/16 = (24.5/2.54)*.5 = (4.823*2)+16 = 25.646 height
275/40/17 = (27.5/2.54)*.4 = (4.33*2)+17 = 25.66 height

In the above example the sidewall height is about a .5 inch smaller to make up for the increase of 1 inch in whell diameter.

93zder

Thanks!

My calculations show then that a 305-35-18 tire will result in a tire height of 26.4". meaning that it will be about 3% off the stock 25.6 height.

So therefore my speedo would be off by 3% as well?

Injuneer

Depends on what tires came stock on your car. Check the tire info label on the diver's door.

The numbers quoted in the post above are for the V8 models with the Z-rated tire option (245/50-16) and the SS/WS6 coupe option (275/40-17). But the base V8 cars got 235/55-16 tires (26.18").

And yes, if you change the tire diameter by 3%, you will change the speedometer and odometer by 3%. With a 26.4" tire compared to the 25.6x" tires the speedometer will read 3% lower than actual speed, and the odometer will only add 970 miles for each 1,000 miles driven.

That assumes you didn't reprogram the PCM with the new tire diameter.

93zder

Mine came with the 245-50-16 but i now have 275-40-17 so i would be off by about 2-3% going to a 305-35-18 on the rear - i'm not really worried about it. I bet some speedos come from the factory that are off as well.

96 Z 28

I run 275/35/18s out back, and that's a .271% Difference from the 245/50/16 Sizing.

93zder

So if I was to run a 295-30-18 which is at 25.1 - I would be about 2% lower?

Injuneer

Most maufacturers quote the 295/30-18 as a 25.0" tire. Some measure it at 24.9", others at 25.2". The calculated diameter is 24.96", putting the tire 2.6% smaller than the optional 245/50-16 (or 275/40-17) tires, and 4.6% smaller than the stock 235/55-16. Speedo will indicate that percentage higher than true speed. Odometer will rack up that percentage extra miles for each mile driven.

The thinner the sidewall, the harsher the ride, and the greater the chance of rim damage from potholes. The smaller diameter will make the already too large stock fender gap even larger.

93zder

I was thinking the same thing. I think I will try to fit the 305-35 in there and hope I don't have to work the inner fender too much.

93zder

I've got my wheels and tires mounted on the rear. i did some BFH and i'm now left with about 1/2" space from the closest point the tire is to the inner fender. Its at the front of the inner fender - about an inch of area. The rest is good to go.

How much space is safe to have without danger - is this 1/2" enough?

thanks