65 C10 Swapped 3 on the Tree to TH400 - Gearing
 

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.

Re: 65 C10 Swapped 3 on the Tree to TH400 - Gearing
 

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

Re: 65 C10 Swapped 3 on the Tree to TH400 - Gearing
 

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

Re: 65 C10 Swapped 3 on the Tree to TH400 - Gearing
 

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

Re: 65 C10 Swapped 3 on the Tree to TH400 - Gearing
 

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.

Re: 65 C10 Swapped 3 on the Tree to TH400 - Gearing
 

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.

Re: 65 C10 Swapped 3 on the Tree to TH400 - Gearing
 

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.

Re: 65 C10 Swapped 3 on the Tree to TH400 - Gearing
 

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

Re: 65 C10 Swapped 3 on the Tree to TH400 - Gearing
 

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).