Does anybody know how to calculate the gear ratio changes with the change in tire size. Example:a 245/45/17 and a 3:55 gear. to 205/50/15 what would that change to gear ratio tooo???

 

Not really sure, but what about using the tire size calculators out there? They will show you the variation in sizes and what you're speedometer error would be. If you were traveling an indicated 70 mph but were actually moving at 72 mph, your error would be 2.75%. Wouldn't your overall gear ratio be off 2.75% as well? Anyone people good with theory out there know if this might work?

 

http://www.4lo.com/4LoCalc.htm

Never thought I would be giving this site out on Maxima.org... this is what I used when putting different tires on my truck and when making myself a conversion sheet for it after they were on (comes in handy when the cop is right behind you looking to pull you over just because 75% of your truck was built at home - kudos to the cops that just give me a thumbs up though!).