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Final drive ratio

what would be the rpm difference at 70mph with a 3.23 versus a 3.73 on similar vehicles with same tire sizes?

If the vehicles are IDENTICAL except for axle ratio, just divide the two ratios. Otherwise, no way to know without knowing the transmission ratios.

Looks like an 11.5% difference in RPM at any speed. These sound like common Ford RWD ratios…

I have it at 13.5% reduction 3.73(.135)=.503 3.73-.503=3.227

These are common GM rear ratios.