65 C10 Swapped 3 on the Tree to TH400 - Gearing

[65Oldred]

Hello,

Awhile back we dropped in a "new" V8. While we had the truck apart, we replaced the 3 speed because it appeared to be shot (and it was) with a rebuilt TH400.

The gearing must be off. It drives like it could pull out a tree stump but seems constrained at highway speeds. (I believe the truck has the original rear end.)

Is the combination of a TH400 and gearing in the differential a problem?

Would going to a TH350 or 700R4 solve this problem?

Thanks.

[Wikid61]

The rear end has steep gears. A 700r would help. The 400/350 turbo has the same 1:1 final in drive.

[NVS-C10]

Originally Posted by 65Oldred

Hello,

Awhile back we dropped in a "new" V8. While we had the truck apart, we replaced the 3 speed because it appeared to be shot (and it was) with a rebuilt TH400.

The gearing must be off. It drives like it could pull out a tree stump but seems constrained at highway speeds. (I believe the truck has the original rear end.)

Is the combination of a TH400 and gearing in the differential a problem?

Would going to a TH350 or 700R4 solve this problem?

Thanks.

200-4R or 700R4.. the 200 will be closest in size to what you have now; should mount right up.

[dmb]

the 400 is a very good transmission, I would change the rear end gear ratio to a 3.07 to 1

[SCOTI]

Originally Posted by 65Oldred

Hello,

Awhile back we dropped in a "new" V8. While we had the truck apart, we replaced the 3 speed because it appeared to be shot (and it was) with a rebuilt TH400.

The gearing must be off. It drives like it could pull out a tree stump but seems constrained at highway speeds. (I believe the truck has the original rear end.)

Is the combination of a TH400 and gearing in the differential a problem?

Would going to a TH350 or 700R4 solve this problem?

Thanks.

There's no difference in final drive gearing between the original 3spd manual trans & a TH400 since they're both 1:1 final drive ratio. You either need an OD transmission or to re-gear the rear-end to change the rpms @ highway speeds.

The OD transmission provides better gearing from a standing start & @ highway speeds vs just re-gearing. The OD trans swap also requires more financial investment.

[MikeN]

Depending on your torque converter, you'll have varying amounts of converter slippage, so RPM's might be a tad higher than with your 3-speed manual, but not too much.

I put a TH400 in my 64, and went to 4.11 gears for drag racing. My rear tires (drag radials) are pretty tall, so I'm satisfied with how it drives around town. I wouldn't get out on the highway geared like this though.

[AcampoDave]

Originally Posted by dmb

the 400 is a very good transmission, I would change the rear end gear ratio to a 3.07 to 1

X2. You've already got the rebuilt 400. It's strong and capable as is. Spending even more on an overdrive trans wouldn't make me happy in your position. If your motor is stout, a 307 rear would be a great all around choice. You can roll 65 mph under 2500 rpm and still fry the tire on an empty pickup....I'm not really a fan of most overdrive automatics anyway. They just never seem to hold up well on my friends trucks. I've heard they can work well, but I've never seen it happen. I guess I'm old school. I love the Turbo 350's and 400's for their simplicity and reliability.

[AcampoDave]

This is a good read https://www.holley.com/blog/post/eve...transmissions/

[65Oldred]

Thank you folks for the good advice. I think I will go the route of re-gearing the rear end (3.07). The engine is a 409. I'm just looking to cruise around and occasionally haul lumber or landscaping material.

The truck is up in the Portland, OR area. Any one know of a shop that could help with the gear change out? My brother has a speed shop but he's perpetually busy and he's got a couple of kids now (and he loves to have fun with them).