Is there some kind of reasonably easy way to calculate different wheel/tire sizes that will fit about the same (I mean, that will be about the same overall diameter or height)? Another way to ask the question: if I have 15 x 7" wheels with 235/75/15's, what size tire on a 16" or 17" rim would be about the same overall size?
Here's a tire size calculator.... http://www.miata.net/garage/tirecalc.html Enter your current size, and then try different sizes. At the bottom, it will tell you how close your speedometer reading will be For your 235/75R15 size, I got a comparable 16" tire in size 265/60R16 It would be 1.2% 'too fast' - when your speedo reads 60, you would actually be going only 59.3 Of course, there's no telling whether a 265/60R16 would actually fit inside your wheel wells.... In a 70-series tire, 235/70R16 is almost a direct match - only 0.2% off. A 65-series tire might be a good compromise. 245/65R16 would be the closest.
The most common way to determine a starting point is to keep the same width tire, go up one inch in rim diameter, and down a full 10% of aspect ration. A 235/75R15 would translate into a 235/65R16 or a 235/55R17.
If you don't have the website handy and just have a calculator, here's what you do. (Width x aspect ratio as a decimal / 12.7) + listed rim size = outside diameter of tire Example: 235 75 R15 235 x .75 / 12.7 = 13.88 13.88 + 15 = 28.88 inches diameter So if you are thinking about a 265 60 R16, applying the same formula gives 28.52 inches diameter. Divide the new diameter by the proper diameter to get the factor for the actual difference in road speed. 28.52 / 28.88 = .988. Multiply .988 by the speed. So at 60 on the speedo you would really be going 59.3 mph. Divide the other way to get the factor for the difference in speedo reading for the same actual road speed. 28.88 / 28.52 = 1.013. Multiply 1.013 by the speed. So to really go 60 the speedo would have to read 60.8 mph.
