The Stovebolt Site Forums The Shop Area Engine Shop Need gear ratio info. '51 GMC 1.5 ton

I've found a clean, original forest green, 51 GMC 250 series (1.5 ton), short wheelbase (125"? per oldgmctrucks.com), 228. I'm wondering what diff ratios it might have, it has a huge trucky pumpkin. Also wondering if the 4 speed has the 7.02:1 granny like my old 53 3/4 ton had, or is it lower. Also, roughly how tall were the stock tires on the rig? The rims are large truck size, not 16, 17, or 18" like I kinda remember fooling with on my old '53 3/4 ton.

I haven't added this rig to my harem yet. Just drooling so far.

Sidehill

This is from the '54 GMC shop manual; it may not be correct for '51. :confused:

The 250 is a 1-ton and came with 5.14:1 rear gears. The 280 (1.5 ton) came standard with 5.14:1, and had a 6.17:1 option. 280's have bigger axles.

Sidehill welcome to Stovebolt. You have found a good spot with good people and generally good information (except politics) when you found this place!

With GMC identification, it all starts with the serial number, which can be found on the driver's doorpost ID plate. 250 is a one ton truck, not a 1.5 ton which is a 300 series. (280 is a 1.25 ton)

A '51 one ton shortbed would have a serial number on the Doorpost ID plate that would read 251 22 P _ _ _ _ _ or similar.

Wheelbase ID is the 3rd digit:
1 = 116"
2 = 125.25"
3 = 137"

I have never seen any GMC one tons other than 137" but I know that they are out there...somewhere.

If it's a 3/4 ton the number will be 152 22 P _ _ _ _ . First two digits are the series and rating.

The 3/4 ton stock rear was 4.57:1 and the optional was 5.14:1. The one ton stock rear was 5.14:1. The 3/4 ton rims are 15" split rims as stock, and 17" split rims optional. The One ton has 700-17 tires stock and 700-18 tires optional both on split rims.

The SM420 four speed tranny was the same one that was used for close 25 years so yup, it is the same as your '53 3/4 ton most likely.

Thanks for the info guys.

I'm more of a gearhead than a motorhead. I get all excited about ratios; I guess because I'm a fourwheeler. You both mentioned that a 250 is a 1 ton. The truck has an old stock type "250" chrome emblem and definately has a huge diff. I was thinking 1.5 ton because I've seen a few early 50's Chevy or GMC 1 ton pickups that I think had the same size diff as my '53 3/4 ton. They just look like a really long pickup. The bolt pattern on this rig is also different than my old 3/4. Duals of course and not aftermarket. Seems like the wheels were bigger than 17 or 18" cause I've messed with those before.

It has a pretty short wheelbase(stock. Maybe it's the 1.25 ton. The truck is 150 miles away so I don't know the serial number and wheelbase etc.

Thanks again for the help. I'll be able to get the serial number next week so will know more. I'm sure I'll have a zillion questions if I buy this rig. Sure glad I found this site.

Sidehill

Side: You should get that serial number before you go for the truck so you know what you are buying. The 1/2 ton to 1 ton trucks have the same sheet metal. The 1.5 ton and up are different with the exception of the hood.

If that truck got a replacement hood (off a one ton) then you may be mislead into thinking you know what it is. This is why I say it all starts with the serial number.