Gear ratios and driveability

Rim diameter has nothing to do with it, the overall diameter of the tire sets the ratio. Now, there aren’t a lot of tire options for 14” rims so I guess that does come into play some.

A 245/60/14 is only 25.6” tall, which is fairly short for a rear tire. With those tires and 3.91’s you have to turn 3,600 to do 70 mph.

If your B-body buddy is running 255/60/15’s, which would be pretty easy, those are 27” tall. He only has to turn 3,400 rpm to do 70 with those. If he has 275/60/15’s, which wouldn’t be shocking, those are 28” tall and would mean he could cruise at 70 at just under 3,300.

Tire diameter makes a big difference. But freeway speeds for long hauls with 3.91’s is asking a lot. 3.55’s would make life a lot easier at freeway cruising speeds.