Rear end and big tires

[Mertz]

I have my 72 C20 running as good as possible. Timing, Qjet, headers, ignition and transmission. It runs great but I need it to pull a trailer 3 to 4 times a year and it has problems with grades. I thought about a rear end gear change but I am not sure what I have in it now and it would cost a bunch of money. I was close to buying a 96 F250 Diesel to pull the trailer but could not bring myself to buy a second truck that would sit most of the year and the money for the Ford could be spent on the Chevy.

First question is about tires. I foumd a good deal on two new Uniroyals at a swap meet for $100 so I put them on the rear. They are 265/75/R16 and have a 32" diameter. The front tires I just bought from Les Schwab are 245/75/R16 and have a 30" diameter. How much gear ratio would I gain by moving the front tires to the back? Could I get the gearing I need by going to an even smaller tire? What was the stock diameter tire that came with the truck?

What was the stock gear ratio on a 72 C20? What type of rear end was used. I know mine is not an Eaton because of the bolt pattern. I haven't been able to get under it to find a number on it and there are not tags.

[raycow]

Originally Posted by Mertz

How much gear ratio would I gain by moving the front tires to the back? Could I get the gearing I need by going to an even smaller tire? What was the stock diameter tire that came with the truck?

What was the stock gear ratio on a 72 C20? What type of rear end was used. I know mine is not an Eaton because of the bolt pattern. I haven't been able to get under it to find a number on it and there are not tags.

Going from a 32" to a 30" tire would be like multiplying the rear end gearing by 1.067. The only way to know what size tire came on the truck originally would be if it is shown on the glove box plate. Base equipment was 8.75-16.5, but larger tires were optional up to 10.00-16.5.

If you don't have an H052 rear (round cover), you probably have a Dana 60 (10 bolt irregular-shaped cover). Some of the Danas (but not all) were 3.54 because you couldn't get anything taller than 4.10 in the H052, but you could also get most of the H052 ratios in the Dana too. There is no need to guess about your gear ratio, because it is easy enough to find out what it is.

Jack one rear wheel off the ground, put the transmission in neutral, and the parking brake off. Put matching marks on the pinion yoke and differential housing. Turn the wheel exactly TWO revolutions and count how many times the driveshaft turns, estimating any fractional turn as closely as possible. That number is your axle ratio.

If the wheel is difficult or impossible to turn, you may have posi. In that case, jack both rear wheels off the ground and turn either of them only ONE revolution. Count the driveshaft turns the same way as for an open rear.

Ray

[Mertz]

It has the irregular diff cover so it must be a Dana or a GM 8.875. I don't have anything on the glove box sticker that tells me tire size or diff number. I will do the lift rotate method and find out the starting ratio. I will see what the two tire sizes are and see what I did to my gear ration by putting the 265/75/R16s on and hopefully get some pulling power back by going to a smaller tire.

Thanks for the help.

[Mertz]

Turns out the rear is a Dana 60 with 4.10 gears