'58 GMC Model 150 Info

[doberman_52]

I have a 1958 GMC Model 150 Pickup. I need to find out the gear ratios fthe tranny. I have found a few different ones. It has the 336 V8 with a 4 speed manual trans. I would appreciate the help, Thanks.

[doberman_52]

It is a SM420 4-speed with gear ratios of 1st: 7.05, 2nd: 3.58, 3rd: 1.71, 4th: 1, Reverse:7.05. It has a differential ratio of 4.56, I need to change the ring and pinion gears so that I can drive at 65 - 70 and get decen mpg. I know that it has to be abou 3.11 or something close to that. how can I find out what kind of ring and pinion gears I would need to get

[MagicRat]

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.

[doberman_52]

it will be a bit before i can find that out but I think it might be 28in, the body is 45+ miles away and my mom would shoot me if I just went down there for that, motors in 2 or more cities too, where would I be able to get a rear end for that, I understand that it is a dana/spicer and know verry little about differentialsand the companies that make them, I also cant get the numbers on the tag for a while either, thanks fo the equation for determining rpms