Differential Gearing Question
 

I will be putting a crate SBC 350 4 bolt main into my truck...bolted to that will be a rebuilt Muncie M20 wide ratio 4 Speed. First gear is 2.56:1.

Because the truck originally came with an inline 6 cylinder engine, the rear end has 4.11 gears in it. I know the 4.11 has to go, so I am looking to replace the gears in the rear end and hope to get a happy medium between half-way decent take-off power and highway drivability.

So here is my question:

What gearing will give me the best drivability in town and on the Interstate? I am looking at 3.43, 3.23, and was also looking at 3.08.

I have been told that 3.08 may make the truck too sluggish taking off from a stop in town. Either way I do not want to be 'that guy' who holds-up traffic in town or on the interstate. I would like to be able to do 70 - 75 mph without the truck screaming at high RPMs.

Thoughts?

Re: Differential Gearing Question
 

3.43 is a nice all around ratio without overdrive.

With overdrive I bump up to 3.73.

K

Re: Differential Gearing Question
 

I drove my 71 chevelle with 3.31s and an m20 for a while, and it was OK. I won't go any numerically lower than 3.23. Another option if you want a 4 speed would be a 2.88 1st gear ratio super t10. You could run a 3.08 in the rear and first would be similar to the m20 and a 3.42 rear.

Just my .02

Good luck

Here is a link with t10 ratios
https://www.chevelles.com/threads/su...-ratios.23335/

Re: Differential Gearing Question
 

You don't have to ditch the 4.10 before you do the swap. Swap it first, drive it some and see what you think. 29" tall tires and the 4.10 are pretty nice for 55 mph and down driving. Once you get some miles on the rig then think about how it performs vs. what you want.

Re: Differential Gearing Question
 

I believe the M20 has 1.00 final gear. What size tires are you running? With a 29” tire in the rear and a 3.55 gear, you would be running around 2050 RPM @ 50mph. The 3.55 would be a happy median. I think you’ll have a dog on your hands with a 3.08 gear.

Re: Differential Gearing Question
 

I swapped a 4:10 rear behind a 250-6, for a 350 SBC and 3:08 gears.

I did an entire rear end swap from a salvage yard because I know entirely zip about setting up lash.

Just how fast do you plan on cruising through town?

A 350 CI Small Block Chevy built with torque in mind won't be blocking traffic if you don't want it to. :rolleyes:

Re: Differential Gearing Question
 

I have a TH350 and failed to picture you trying to start up a steep hill in town after being forced to stop in the middle of it. :o

...well you shouldn't be listening to the baloney I spew out on this forum anyway. :rolleyes:

Re: Differential Gearing Question
 

I did not see where you mentioned your Tire Size / Height. There are several calculators online similar to the one in the link below that can give you a ball park idea how any changes you make will effect the drivability.

https://www.summitracing.com/newsand...tio-calculator

Re: Differential Gearing Question
 

Thanks, I just don't want my RPMs high on the highway. No overdrive.

Re: Differential Gearing Question
 

Thanks! I already have the M20, so I want to use it for a while. Trying to stay somewhat period correct for the truck.

Re: Differential Gearing Question
 

In town I just want to keep up with traffic and not be an annoyance because I am geared too low.

Re: Differential Gearing Question
 

Yes, final gear (4th) is 1.00... I don't know off hand the tire height, but will check this morning. I simply got the radials that fit the 15" wheels that were close to the original bias plys.

Re: Differential Gearing Question
 

Thanks!

Re: Differential Gearing Question
 

My tire size is a 235 75 15 @ 28.25" height. Figured out the RPMs @70 MPH will be approximately 2675 if using a 3.23 rear end gears.

I think I can live with that.

Re: Differential Gearing Question
 

Which crate 350 engine are you planning to run? I ask because the torque curve will be the determining factor.

My dad has the 290hp crate engine with a COMP Thumpr cam, mated to an M20 in his 56 BelAir. When he first installed it, he had a 9" rear with 3.00:1 gears. It was terribly sluggish, and would slip the clutch on inclines. He ended up installing 3.70 gears, which are perfect. Cruise rpms are tolerable at 70. Also, it only has 27" tires(255/50/17).

Are you certain your current rear has 4.11 gears? I ask because I always thought my truck(originally a 250/3ott) had 4.11s, but were actually 3.73.

I honestly wouldn't go any higher than 3.42. Remember that a manual transmission spins at a different rpm than automatics because the clutch is engaged directly. There's no torque converter to use up extra rpms.

Re: Differential Gearing Question
 

I know you stated you want to stay with your 4 speed, but if you are not driving the truck, beating on it a ton, you might look into the 833(?) 4 speed that 4th gear is overdrive, and leave the 410 gears.

You didn't say how much time you will be driving at highway speeds.

Re: Differential Gearing Question
 

The first gear may not be deep enough for the trans you have combined with your tire height.

I put a T10 with 3.23 gears in a car with 255/60r15's, and my first gear was close to yours. It was a bit hard to get it rolling compared to where it should of been. In my opinion. It originally had a 4 speed saginaw with roughly a 3.0 first gear.

I would drive it with the 4.10's for a start, and then make changes.

Re: Differential Gearing Question
 

This is likely where I will get the engine:
https://www.powertrainproducts.net/P...iABEgLCpfD_BwE

Re: Differential Gearing Question
 

This truck will be my daily driver. I will spend probably 35% of my time on the highway.

Re: Differential Gearing Question
 

Thanks, good advice. Given how easy it is to get to the differential, I may go this route to see how it performs.

Re: Differential Gearing Question
 

I can understand that, I ran the 4:10 rear for a while after replacing the 6 with an 8, and while the truck was a stump puller, I got tired of the dirty looks from little old ladies passing me up on the freeway. :o

I'd still try the 4:10 at first anyway just to see what you think.