I just purchased and installed Cooper Zeon LTZ's to replace my crappy Goodyear tires. Original factory size was 275/60/20 as we all know and I put on 285/50/20's. I went to www.1010tires.com/TireSizeCalculator to calculate the difference in size and speed but I am a bit confused. This might be a dumb question but it says "Revs per mile" on the factory tires is 630.4 and my Coopers 666.2. Okay... then it says my actual speed for the factory size is 60mph where as the Coopers is 63.4mph. Last it says the speedo difference is that my Coopers (285/50/20) is 5.678% too fast compared to the original size.

I just want to make sure that I'm reading this right... Does this mean I am actually going a bit faster than my speedo is reading, which is 3.4mph too slow now? I appreciate the help!

 

You're actually going slower than your speedo says I do believe. I just did the calculations on discount tire website and it says:

That if your speedo reads 65 you're actually doing approximately 61.5 mph.

If you went up in tire size instead of down you would actually be going faster than your speedo says.

http://www.discounttiredirect.com/di...foTireMath.jsp

 

cwboy is correct. Your new tires will display 60, but you will be going slower than that, its usually a % slower and not a flat number.

 

I appreciate it guys! A couple people I asked around here were telling me they thought it was the way I originally stated in the thread but I was thinking what you guys told me. Thanks again!

 

The Miata Tire Calc is the oldest and best one I've ever used. It was the first to show it graphically as well as spell out all the differences in speed.

http://www.miata.net/garage/tirecalc.html

 

Thanks John M! I'll check it out.