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Re: '58 GMC Model 150 Info
It depends on rear tire diameter and your ideal RPM you want at highway speed. It's easy to calculate the expected engine RPM at 60 mph.
Measure the height of your tire when it is installed on the truck. A typical car tire, say a 225/70R15 will be approx 28 inches tall.
28 inches x pi (3.1415) = 88 inches.
This means that one tire revolution covers 88 inches. There are 63,360 inches in a mile. Therefore (63,360 divided by 88) = 720 tire rotations per mile.
720 x axle ratio (3.11) = 2239 engine revolutions per mile. .
Therefore, if you have a 28 inch tire and a 3.11 rear end, your engine RPM will be 2239 rpm at 60 mph. This seems like a perfectly reasonable rpm for good mileage.
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