rear end ratio question

[Harley1650]

My 63 has a 454 with a Muncie (M20 I think). My rear end is way too low for my liking. It sucks on the freeway and sucks the gas!. Theres a stock rear end on Craigslist from a 66' SWB 6 cylinder 3 speed. He says its a 12 bolt. The guy said he thinks it should be a 373 gear due to the 6 cyl 3 speed. Can anyone confirm this? I'm actually looking for a taller gear, maybe a 273

[Chevy Fleetside]

You will have to open the cover and count teeth and do some math. The ring gear should be have some # punched on it and then just count the teeth on the pinion gear and divide. Like the rear end I bought. The ring gear as 43 teeth and the pinon has 14. Divide those # and you get 3.07428571428571428571 and so on. Basically 3.08 gears which will be high enough depending on how tall your tires are. 2.73 gears usually run in cars but they have smaller tires.

[Narxoleptic Customs]

In my rear is a 3.90 set. It was a inline 6/ 3 spd truck...
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[Captainfab]

I've seen some 6 cyl trucks with 4.10's. I agree on pulling the cover and have a look. IMO with a Muncie 4 speed's relatively high 1st gear, I wouldn't go mich higher than 3.42's, preferably 3.73's. If you go to a higher gear, your rpms will decrease, but it will also be more difficult on take off, and you will be slipping the clutch more to get going. Of course this also depends on the diameter of the rear tires you are running.

[Harley1650]

Thats a good point

[66 lwb cs]

Another method to sort of eye ball what you have it the pumkin is to jack up the rear end and release the E brake make a chalk mark on the drive shaft and on the pumking then spin wheel one rotation while counting the number of times the mark on the drive shaft goes around past the mark on the pumkin. It won't be exact but as you can guess just over 4 rotations would be 4:10 and just under 4 rotations would be 3:90-3:73 and so on. If you're just looking for a base line with out pulling the cover and all that it's a good way to generally figure out if you have something steep in there like 5:14's or something lower like 2:73's without the mess of dealing with the diff cover. Hope this helps even during your search for a replacement this can be used when someone doesn't know what's in the diff they're selling off Craigs or something like that. Best of luck.

[SIXOSLED]

That does work I did that to one of my old trucks. Except I marked my tire too. I had 3:73 gears, 3 3/4 revolutions of the driveshaft to 1 revolution of the tire.

[Harley1650]

That sounds good. I will do that so at least I'll have an idea what I have. Thanks

[mikie 1954]

MY truck is a 3/4 ton gmc and I had 353 gears put in it, it has a 350 sbc and sm420. It is alot more driveable now and has no problem starting off.If he has a 454 his should handle taller gears with on problem.

[tatanka]

hello
j have the same problem with a 350 engine th350 box the engine is running too fast to bridge what to put as original ratio? chevrolet c10 1966

[GMR-PERFORMANCE]

you could have the best of both worlds and drop in a TKO 600 trans. I run the .64 OD version.

3.90 rear gear great low end acceleration and still cruise the hwy at 2300 rpms.

[cruznfb]

just tryed this trick with my truck ....only got a little over 2 full turns. running 350/350 trans 12 bolt......truck to slow on the freeway with high rpm. i guess i need a rebuild or a new rear end. thanks for the info.

[61_FL_Apache]

Pull the cover. You will find number stamped on the ring gear and the year it was made for additional info. I tried the tire rotation vs. driveshaft rotation and came up with a different number. I had a 3.08.

[Captainfab]

A 3.07 gear change would be a good choice in the original 12 bolt differential. Also a larger diameter rear tire will help if your current tires are somewhat small.

Originally Posted by tatanka

hello
j have the same problem with a 350 engine th350 box the engine is running too fast to bridge what to put as original ratio? chevrolet c10 1966

[Captainfab]

Did you do this with both wheels off the ground, with an open differential? If so, then you need to turn the tire 2 turns to find the correct gear ratio. It sounds to me like you probably have 4.10 gears.

Originally Posted by cruznfb

just tryed this trick with my truck ....only got a little over 2 full turns. running 350/350 trans 12 bolt......truck to slow on the freeway with high rpm. i guess i need a rebuild or a new rear end. thanks for the info.

[Harley1650]

Well, a nice little tag on my rear end says its a 3.54. Now I'm really bummed because I'm thinking the Muncie 4th gear isnt right for my liking. Do you think I'd gain anything going to a 3.08 or should I concern myself with possibly a transmission

[Captainfab]

I think you would gain more of a rpm drop going to an OD trans. However an OD trans that will hold up behind a big block isn't going to be cheap. A 3.07 gear change would likely be less $$, but I don't know how it would be with the tall 1st gear of a Muncie.

[Harley1650]

Im actually considering switching to a 700R4. The 454 is essentially stock so its not big horsepower

[cruznfb]

yep....your right. I have 4:10 gears and just got a quote for $1000 for gear change and rebuild. sounds a little high but will still check for other options.
was told to go to 3:08 gears??? does this sound right? Any other ideas to do this cheaper??

[cruznfb]

question for captainfab...if i bought a whole rear end with non posi 3:08 gears 12 bolt....can i take the everything and just switch into my rear end housing. carrage and all ....add new bearing of corse.. ??

[Captainfab]

Yes, as long as both differentials are truck 12 bolts, and one of them is not a '63 or '64 truck 12 bolt. The '63 and '64 12 bolts originally had 17 spline axles. The '65 and up had 30 spline. But the gear pattern will still have to be checked to be sure it is correct. Swapping used gears is a gamble. Sometimes they setup okay, other times they tend to howl. What about changing the complete rear end?

Originally Posted by cruznfb

question for captainfab...if i bought a whole rear end with non posi 3:08 gears 12 bolt....can i take the everything and just switch into my rear end housing. carrage and all ....add new bearing of corse.. ??

[cruznfb]

captainfab....yes i thought about the entire rear end but... i had a drive line made to fit and brakes are all new. the axle is 5 on 5 and all my air ride is welded to it. i guess its still a learning cruve for me. i did find a early 80 rear end with 3:08 gear set up but not sure if it a truck rear end. i just wanted to change out the entire center.or i might have to save to get it done in a shop. thanks for the info.

[Lauppster]

I switched my 3:73 out to a 3:08 posi. Bought all the parts at www.drivetrain.com; they were very helpful and gave a discount for a "complete" package. Found a local guy to assemble and set the new gears for a few bucks. Still cost $800 but that included almost $400 for the posi unit. For what it's worth.
The truck is not on the road yet so I can not comment on the gear ratio and driveablity yet. I did not worry about the ratio for starting from a stop because the 3 speed 1st ratio was so bad with the 3:73 you had to shift at 10mph; not even possible to get across the intersection without shifting.

[cruznfb]

wow... thats good to know..thanks for the info...my auto 350 shifts fast also. still ok around town, but wasting lots of gas...cant wait till i someday get new gears. i have to weigh out all my information.