Need help choosing rear end ratio for my 1977 c20 454 4 speed

[bigblockfletch]

I have a 1977 C20 with a 454 and a 4 speed manual transmission. Currently has a 14 bolt rear with 4:56 gears and its pretty frustrating not being able to go over 55 mph. Im wondering what gear ratio i should be working with if i want the truck to get closer to 70 mph and still be able to tow. i have a 14 bolt with 3:23 gears (says the guy that sold it)and im wondering if that would make the truck too much of a slug. Thanks for any advice

[LTZ C20]

Welcome. I have a C20 also, mine is a Camper Special, it has a 14 bolt with 4:10 gears and positraction.

I have 365hp/430ftlbs TBI 350 and auto 4 speed trans, I can cruise at 60 mph and the tach reads 1500. But I also have a lockup torque converter. With the manual you have no converter but you do still have 4 speeds. I'd bet you will probably be more satisfied with 4:10 gears.

Just me and the truck takes off and runs like a rocket, kinda hard to spin the tires, just grabs the ground and goes, when towing I have plenty of muscle with the 4:10s to get something heaving moving without being that really slow guy with the old truck that's takes a week to get to 55 mph.

My friend has a K20 with 350, auto 3 speed turbo 400 and 3.73 gears, his truck also gets up and goes great and plenty of muscle when he tows. He's got a 19 ft bass boat he pulls with no problem, gets up to speed easily.

[Jason S]

With a 454, SM465 and 4.56's, have you considered getting into the house moving business?

The 4.10:1 ratio would be better. But with no overdrive, the engine would still be wrapped up pretty high at 70 mph.   

I'd lean toward 3.73's to keep rpm's down at highway speeds and still be able to tow. 

Have you pulled the cover on the spare 14-bolt to verify the ratio?

[zieg85]

I have a 3.21 gear in my 1985 with a 454 4 speed and love it.  Never a problem, even getting 8K lbs. moving.  L is a usable gear.  Keeping it under 60 14-15 mpg, just under 16 on time.  I would love to find a 3.21 gear set for my extended cab.

[bigblockfletch]

I have a 3.21 gear in my 1985 with a 454 4 speed and love it.  Never a problem, even getting 8K lbs. moving.  L is a usable gear.  Keeping it under 60 14-15 mpg, just under 16 on time.  I would love to find a 3.21 gear set for my extended cab.

thats what i was curious about, considering that L is virtually useless at the moment. is your 454 stock?

[bigblockfletch]

With a 454, SM465 and 4.56's, have you considered getting into the house moving business?

The 4.10:1 ratio would be better. But with no overdrive, the engine would still be wrapped up pretty high at 70 mph.   

I'd lean toward 3.73's to keep rpm's down at highway speeds and still be able to tow. 

Have you pulled the cover on the spare 14-bolt to verify the ratio?

the truck will move a house no problem. How can i tell what the ratio is when i pull the cover?

[zieg85]

Bone stock, still move a house with no problem. 

[Dr_Snooz]

My white truck also has a 4.56 and it's a stump-puller for sure. Assuming you can put the power down, of course. My other truck has what I assume to be 4.10 and it's much better, though I could go lower and not be sad.

Not sure what your plans are for the truck, but if you plan to put bigger tires on at some point, that will also have the same effect as putting in a higher ratio diff. Just factor that in to your calculations. You don't want to put in a 3.42 and then put on 36" tires and expect to pull your 5th wheel to Arizona.

I determine gear ratios by counting teeth on the pinion and ring gear. 14-bolt ring gears are usually 41 teeth, so ratio is determined by the pinion teeth.

If you have a tach, you can measure the diameter of your tires and determine what your RPMs will be at various speeds with different diff ratios using a calculator like this: http://www.advanced-ev.com/Calculators/TireSize/

[roundhouse]

There should be numbers stamped into the ring gear
Xx/xx. 
Will be ring and pinion teeth.  But count them anyway
Use a paint marker  fingernail polish or something to designate the starting tooth
One tooth can make a difference

What size tire you running ?
If it's 2wd a NV4500 would be a pretty easy swap and get OD



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