What gear ratio on 1975 c20 pickup??

[TheSwede]

Anyone know what gear ratios on 75 chevy c20 truck (8lug)

[hiway]

3:73 or 4:10 are common. Pull the cover and have a look.

[TheSwede]

Originally Posted by hiway

3:73 or 4:10 are common. Pull the cover and have a look.

Does a chevy van with 8lug have the same gear ratios?

im gonna put a mercedes turbodiesel 6 cylinder in it so i want to keep the rpm as low as possible.

the mercedes donor car has 2:65

[hiway]

It might since it wasn't really meant for towing like the truck was. Worth a shot. I personally haven't seen a 3/4 ton or heavier rear with "highway" type gearing (taller than 3:73).

[Pyrotechnic]

You can also pick a speed and look at how many RPM's you are turning. Find out the rev's per mile rating of your rear tires and you should be able to figure out what gears you have with a little math. For an automatic, speeds over 50 are best since the converter slip isn't much by then.

[TheSwede]

Originally Posted by Pyrotechnic

You can also pick a speed and look at how many RPM's you are turning. Find out the rev's per mile rating of your rear tires and you should be able to figure out what gears you have with a little math. For an automatic, speeds over 50 are best since the converter slip isn't much by then.

im using the mercedes trans so i dont konw if its works on that.

[factorystock]

Standard ratio for V8 c20 is 3.73, but there were other optional ratios of 4.10 and 4.56 and a highway ratio of 3.21

[jeffspower]

[QUOTE im gonna put a mercedes turbodiesel 6 cylinder in it [/QUOTE]

please
do a build thread on it with lots of pics !

[PhilaTruck]

This is how I found out I am blessed with a 2.73:1

Open differentials: Secure the car from rolling forwards or backwards and place the car in neutral. Raise ONE SIDE of the car off the ground (left in this example) and place jackstands under the car to safely support it. If you just did steps 1-3 as a test, note that this is different from step 2, requiring you to change your setup.

Place a mark on the tire at bottom-dead-center. Crawl under the car and place a mark on the driveshaft. You'll be counting how many times the driveshaft turns in this test, so place the mark at a convenient place that you can call "zero".

Have a friend turn the left wheel for 20 rotations. Have them count to themselves while you count driveshaft rotations. When they are completed, divide the number of driveshaft revolutions by 10. For instance, 30 turns is a 3.00 differential ratio. 37 and a third turns is a 3.73 ratio. If you know the factory-available ratios, this will put you close enough to quickly determine which ratio you have. If you don't know the factory options, then do a larger number of wheel rotations (say, 100) and divide by 50 for a little more accuracy on that last decimal place. You're done! You can put the car back on the ground now.

"Posi" differentials: Secure the car from rolling forwards or backwards and place the car in neutral. Raise the rear end of the car so both rear tires are off the ground. Place jackstands under the car to safely support it. If you just did steps 1-3 as a test, note that this is different from step 2 as you now need to be in neutral.

This procedure relies on the fact that your left and right wheels (thanks to the posi) will rotate the exact same number of turns as each other. Place a mark on the tire at bottom-dead-center. Crawl under the car and place a mark the driveshaft. You'll be counting how many times the driveshaft turns in this test, so place the mark at a convenient place that you can call "zero".

Have a friend turn the left wheel for 10 rotations. Have them count to themselves while you count driveshaft rotations. When they are completed, divide the number of driveshaft revolutions by 10. For instance, 30 turns is a 3.00 differential ratio. 37 and a third turns is a 3.73 ratio. If you know the factory-available ratios, this will put you close enough to quickly determine which ratio you have. If you don't know the factory options, then do a larger number of wheel rotations (say, 50) and divide by 50. You're done! You can put the car back on the ground now.

Copied from: http://www-unix.oit.umass.edu/~tcroy...fferential.htm

BTW: That web site just happens to be devoted to building racing Volvos.

[TheSwede]

Originally Posted by jeffspower

[QUOTE im gonna put a mercedes turbodiesel 6 cylinder in it

please
do a build thread on it with lots of pics ![/QUOTE]



i will! as soon as i start the projekt!

and thanks philatruck!
nice to be able to check the gear ratio without opening the axle.

[MylilBowTie]

The ratio is also stamped on the ring gear. You need to remove the rear diff cover. Clean the old gear oil out and clean up the ring gear. Should be stamped with something like 41:12 (3.42) or so. Clean up the mating surfaces install the rear diff gasket tighten all the bolts. Refill with new gear oil and add posi addative if you have have a posi carrier. Cost is pretty low about 20. You get to find out if it has any type of traction control. See what gear ratio is it. Plus get some differintail matinance that usually get negected to ever get done.

[PhilaTruck]

I popped the cover off of mine after I bought it, assuming there was 20 y.o. oil in it. It was. Smelled like something you can't say on TV.

[MylilBowTie]

Yeah gear oil is smelly but I don't see why everyone doesn't change it regularly. Guessing the smell and not getting the cover to seal? I haven't had a sealing problem yet. But I use a gasket and don't hose it down with silicone.

[porpaintt]

Hey Swede,Get me the 3 letters stamped in the housing,topright tube within 4' of pumpkin,looking at the truck from the rear.