Just wondering how much tire height really effects gear ratio. I was planning on buy some 18" wheels soon, and wanted to see what my 3.73's would be with the taller tire.

Just for an example:
315/35-18

(vs my now 245/50-16)


Anyone got a calculator for this?

I would guess it would effect your 3.73's greatly.

245/50-16=25.67 tire height
315/35-18=26.40 tire height
not much of a change

effective gear ratio(torque to axles) would be like running a 3.65 rearend gear ratio and mph in each gear would increase by 3%.