Rear end gears for 72 c20

[darush01]

I have a 72 c20 350 4spd its geared low i know that much but im not sure what it has in it and i cant find the tag, anyone know how to figure thus out without opening it up? I can tell you that in 4th gear wide open its topped out around 55 mph. Im curious it really doesnt matter ill probably chang the gears anyways Any help is appreciated
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[Longhorn Man]

if it topped out at 55, then you have MAJOR problems. Even with the 4.56 gears, it should be good for over 100 MPH.
4.56 is what you should have if memory serves. It'll be either that or 4.10.

[bobs409]

My 71 C20 with 350/4 speed came with the 4.10 rear gear. What size tires are you using? I have tall tires on mine and it sails along nicely at high speeds and fairly low RPMs. In fact, it loves the big highways. I never tried to get it up to top speed but it certainly would go 100+

Maybe you have the 4.56 and using short tires too?

There is a way to tell ratio. Lilft tires off ground. If posi, just put a chalk mark on driveshaft and one on tire. Turn tire one revolution and count how many times the driveshaft turns.

If non posi, same procedure but you have to turn the tire twice.

[LilD]

Maybe its not the rear end. I looked at/test drove a 71 GMC when I was truck hunting and the owner had replaced the throttle cable with one out of a newer truck that was just short enough to not let the throttle open up enough to get past 45 mph.

[darush01]

Here is what i know for sure its all original except hoses and belts at least any replacements were years ago, my dad says everything seems nirmal to him and hes had a few dozen of these trucks over the years, could it be the gearing in the trans causing it, oh and im running the stock size tires only change is the guy i got it from put one peice rims on it
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[darush01]

Let me rephrase this a little itll probably go 70 or 80 but i have only gotten it to 55 because it seems like its wound up too high, i dont have a tach yet so i just go by ear and i dont have mobey for a new motor so its a little scary to try and push it
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