What gear ratio ?

[Frkypunk70]

Just wondering if someone knows what gear ratio would be in my truck? Or what was the most common for that year. It's a 1970 chevy cst. It came with the 350/350 turbo. Any info would be appreciated as I'm thinking of putting a 700r4 into it. It seems like it needs another gear on the highway. No clue what rpm it's running at, as I have no tach. Thanks hoping for the experts to chime in.
Posted via Mobile Device

[Frkypunk70]

By the way it seems to be a posi judging by the two black strips it leaves in front of the house.
Posted via Mobile Device

[Longhorn Man]

you left out the most important part... what size is your truck?
C/10 would be a 3.07
C/20 would be a 4.10
This is assuming there is no optional gear ratio or modifications done.

[cdowns]

2quarts of rearend lube and a cover gasket will solve any doubt about it

[Longhorn Man]

agreed one million percent!
Nothing like trusting 20 to 40 year old oil to keep your bearings rolling smooth huh?

[Frkypunk70]

Its a C10. I was just thinking the must be a common ratio they ran in these.

[jakespade]

Fry, are you the original owner? Also what I think cdowns is saying is the crack open your pumpkin and take a look for yourself. You really cant trust nothing has ever been done to you truck. Here's an example.... friend of mine bought what he thought was a California special fastback 68 mustang from a real estate sale. Turned out he actually got a 67 gt500. The paint was striped as was some of the body work due to a little driver door damage and it being prepped for a new paint job. He almost sold it until he ran the numbers...
Posted via Mobile Device

[Frkypunk70]

I will have to pop the pumpkin cover, but just wanted to know what it came with. Also was wondering if there was any way to tell what it is without tearing it apart only because as of right now I'm just contimplating doing the swap but don't want to lose my lower end. Or buy a transmission that I can't use. Just basically collecting info. Thanks though guys.
Posted via Mobile Device

[Wrenchbender Ret]

Ive been running a 3:08 rear gear with a 700R4 in my 70 for several years. It has a stock 350 engine. It works great You can cruise the high. & hardly hear the engine & you can drop it down a notch if you are pulling hard.

[Longhorn Man]

a 3.07 gear and a 700R4 is generally a bad combo in these rigs with old school engines. There's always the exception, but generally the MPG drops when you drop the RPMs that far.

[chevy71c10]

Without having to open the diff, you could jack up the rear and put a mark on the driveshaft and one on the tire. Rotate the tire one revolution and count the number of turns the driveshaft turns will give you the ratio. Just over three turns of the shaft will be a 3.07, three and three quarters will be 3.73, etc...Of course the trans will need to be in neutral so be sure to properly support/secure truck.

[Longhorn Man]

and that way you don't have to replace the 20 to 40 year old oil that isn't doing a whole lot to keep things cool and lubed

[Frkypunk70]

I was under the impression there is a tab on the cover that had info on the rear. Yes... you guys are right I'll just pop the damn cover off. Might as well replace it with a chrome one while I'm at it. I was under there welding my 4" exhaust cut outs in recently and it looks like its in need of some preventive maintenance. So thanks for the help. By the way should I run synthetic gear oil or is it just not worth the money?

[Longhorn Man]

regular oil will do, but synthetic is a lot better though. If the funds are there I'd say go for the better oils.
I've heard they had tags, but I have yet to see one in person.

[cdowns]

from the factory posi rears had a plastic tag on the fill hole bolt

[Frkypunk70]

I'll check the fill hole for the plastic (you guys have the strangest knowledge) I just went down my "mom and pops" local auto parts store and they will have a chrome cover and gasket kit in the morning for me. It was nice getting compliments on the truck too.

[capev86]

a 3.07 gear and a 700R4 is generally a bad combo in these rigs with old school engines. There's always the exception, but generally the MPG drops when you drop the RPMs that far.

agreed! my dad swapped the rear from a 3.08 to a 3.73 in his 87 burb c10 using the axle from his 84 burb c10. soft gear might have been ok down south. but up here in new england the slightest hill had the truck struggling. "couldn't get out of its own way" was traded for "yes there is actually is a v8 under the hood". fuel economy stayed right at 18mpg.

he now has a 99 burb c10 w/ 3.42 rear and better aerodynamics. the 3.42 is a good compromise and works good with the newer generation engine that has more power. mileage is around 22mpg. remember that trucks weigh a lot more than cars and the tall tires lower the rpm. my 72 burb c20 (307/3OT) turns 2900rpm at 60 w/ a 4.10 rear whereas my sister's old merc topaz (2.3/3 auto) would turn 3k w/ 3.22 gears. a stout rear with a good overdrive ratio is like having your cake and eating it too!

straight 3.07 = 3.07
3.07 x .7 = 2.149
3.42 x .7 = 2.39
3.73 x .7 = 2.61
4.10 x .7 = 2.87

regular oil will do, but synthetic is a lot better though. If the funds are there I'd say go for the better oils.

again...hit the nail on the head! i spent $18 a quart to have the very best (Royal Purple) synthetic gear oil and manual trans fluid in my burb. saving a little by running mobil 1 high mileage synthetic in the engine because it gets changed frequently anyway.

[Jacfourteen]

Originally Posted by Longhorn Man

I've heard they had tags, but I have yet to see one in person.

Mine had two tags on it. One had the gear ratio on it and the other said "use limited slip additive" or something along those lines. they were made out of brass I think. I have pictures around here somewhere.

Edit:
here is the pic of the ratio tag, not the greatest pic but you can read 41-9 4.56 which denotes 4.56 rear gears

[Frkypunk70]

Originally Posted by capev86

agreed! my dad swapped the rear from a 3.08 to a 3.73 in his 87 burb c10 using the axle from his 84 burb c10. soft gear might have been ok down south. but up here in new england the slightest hill had the truck struggling. "couldn't get out of its own way" was traded for "yes there is actually is a v8 under the hood". fuel economy stayed right at 18mpg.

he now has a 99 burb c10 w/ 3.42 rear and better aerodynamics. the 3.42 is a good compromise and works good with the newer generation engine that has more power. mileage is around 22mpg. remember that trucks weigh a lot more than cars and the tall tires lower the rpm. my 72 burb c20 (307/3OT) turns 2900rpm at 60 w/ a 4.10 rear whereas my sister's old merc topaz (2.3/3 auto) would turn 3k w/ 3.22 gears. a stout rear with a good overdrive ratio is like having your cake and eating it too!

straight 3.07 = 3.07


Tire size!!! I can't believe I didn't think of that. I'm running 235/60/15's all the way round. I might just need to go to a taller rear tire so the truck doesn't dive on the highway. So simple but easily overlooked. Thank you for the insight.
3.07 x .7 = 2.149
3.42 x .7 = 2.39
3.73 x .7 = 2.61
4.10 x .7 = 2.87



again...hit the nail on the head! i spent $18 a quart to have the very best synthetic gear oil and manual trans fluid in my burb. saving a little by running mobil 1 high mileage synthetic in the engine because it gets changed frequently anyway.

Posted via Mobile Device