Tire size question

[JRAY2K]

Hello. I'm sure there is somewhere out there that explains all this, but I can't seem to find it. Trying to figure out the equation for tire sizes. Example: (just throwing random numbers).. Lets say I have a 16" rim that runs 245-50R16 and I buy a 17" rim but want the overall diameter to be the same. How do I figure the number? I know the middle number is the sidewall, so do I just subtract 1" equivalent from that?? Feel free to respond or post links to places that explain it. Thanks!

[CapriRacer]

Google "Tire Calculator"

Here's my favorite:

http://www.1010tires.com/Tools/Tire-Size-Calculator

Please note: Since tires come in discreet sizes, there is rarely an exact fit. This becomes a "hunt and peck" kind of thing.

[jdmccright]

I like that site's calculator as well. You're right that you won't get an exact match, but you can minimize the amount of error by keeping the difference to less than 1%.

Or if you know you're speedo/odo is out of whack, you can resize appropriately to restore accuracy. This can be done with any GPS that shows your instant ground speed. Alternatively, you can use the highway mile markers over a long stretch and compare to your odo reading.

If your speedo is reading higher than actual, your rolling circumference is too small and you should consider a new wheel/tire combo that is slightly larger. A 2 mph indicated difference when traveling at 60 mph ground speed is 62/60= 3.3% difference.

Hope this helps!