I'm looking at some 245/75/16s which is approximately a 30.5 in tire.

(They don't make a 16in rim/tire combo on the low end of 31in, and I can't run something close to 32s)

I have 3.55 gear ratio and I'm not sure which would be more accurate for my speedo and odometer, the 32 tooth (for 31s) or 33 tooth gear (for 30s).

Let me know!

 

Well either you have it just over callibrated(31" one) or just under callibrated(30" one). Id think you would want to be over callibrated, then you'll be driving just under the speed limit when you match the pin with the posted speed limit. Although your speedometer probably isn't off by very much anyway. Maybe 1-2%

 

I think your over thinking it a little, most 31" tires are not 31", most are 30.1-30.7" same goes for 32's and 33's ect, hardly ever do they actually measure the size they are listed at. some 35's measure a 33.5" tire...

I would go with the 32 tooth for 31's, better to be a little slower then it says then faster like stated above. the difference won't translate to much on the speedo.

 

Thanks! I was just using a conversion calculator to find the size and it was 30.5. I'll definitely get the 32 teeth gear when I get my tires.

Thanks for the clarification everyone!