235/75 R15 = 28.87 tall / 90.67" circumference

265/75 R16 = 31.64 tall / 99.37" circumference

1 mile = 63,360 inches

235/75 R15 = 698.8 revs per mile * 3.55 = 2,480 driveshaft revs per mile
265/75 R16 = 637.6 revs per mile * 3.21 = 2,046 driveshaft revs per mile

difference: 17.5% reduction in driveshaft for same distance

Based on http://www.drivinithome.com/mopar-speedo-gear-chart/ , as the effective ratio gets taller (driveshaft count goes down), gear count gets smaller

Going from a 34 gear to 40 gear was the wrong direction; I think you'll actually want a 28 gear (34 stock * 0.825)

 

Thank you very much for your help and now I understand how to calculate something like this.

You guys are a great forum here and great guys

Peter