So I was followed home last night and told I was going too fast. My speedometer was anywhere from 5-7mph off from the person whom was following me. The Jeep came w/ 17" wheels when I bought it and they have 235/55 17 tires on them. I am getting some new tires soon, for some Jeep turbine 15's... 235/75 15. Will it really make that big of a difference?

 

no

 

Outside diameter is what matters. That's the 235 number. The rim size won't make any difference.

 

Who knows if the one fallowing you has a correct reading on there speedo?

 

Yeah, I'm aware of that. I know the current tires measure somewhere in the ballpark of 27" and the 235/75 15 variety is just shy of 29".

 

I do! It is pretty accurate, according to various sources that I have relied on; GPS being a few of them. My wife's car utilizes factory tire specs and it's pretty spot on. It is definitely more accurate than mine.

 

The 235 is not, in itself, the diameter. In metric tire measurement the first number is the tread width in mm. The second number is a percent of that width pertaining to the height of the sidewall. Thus, the rim size does matter in calculating tire size in metric. Example:
235/75/15

235mm wide tread
(ONE) sidewall height is 75% of 235mm tall
*SO* the tire diameter is 2(.75x235)mm+15 inches tall. Ends up being 28.9" if you convert for 25.4mm per inch.

So as you can see, if it were a 16" rim, your tire would be 1" larger in diameter, with the same 235/75 designation.

I believe a 225/75/15 tire was stock for many XJ's. When I installed my 33"s I calculated against that size to get a ratio so I can approximate my speed. Tested with a GPS and my approximations were spot on.

 

On 15s I went for 235/70 the Speedo was almost perfect against the GPS.