8 inch wide for 10.5 tire , and 10inch for 11.5 and 12.5 tire.
This is normal numbers for this kind of wrinkel wall tire.
You know this I supose :
26x4.10/28=3.81 If you have 4.10 and install a bigger tire...
So, your gear ratio is approx 3.80
Ps, nice engine bay : )