TH350 transmission

[Just a Dad]

New to this group. Just bought an 84 K20 long bed with a 350 and recently rebuilt TH350 transmission. We will be driving this in 65-70 MPH speed zones. It really screams above 60MPH. What is a good recommended gear ratio to lower RPM’s and not lose a ton of bottom end. Thanks for any input.
Won’t be towing much at all. Just a jon boat (less than 1,500 lbs).

 

[Ricko1966]

Need to check actual rpm with a tach or confirm what gears and tires you currently have. Might look at taller tires 1st. Or O.D. transmission. Remember with a K you are looking at 2 sets of gears and all necessary extras,as well as set up labor on 2 if you can't do it yourself.

 

[Just a Dad]

Need to check actual rpm with a tach or confirm what gears and tires you currently have. Might look at taller tires 1st. Or O.D. transmission. Remember with a K you are looking at 2 sets of gears and all necessary extras,as well as set up labor on 2 if you can't do it yourself.

I’m not sure of actual RPM but I know what high RPM sounds like. It currently has 33” tires, was told it had 35’s before. I believe this truck was an original OD transmission because it has an OD indicator on the dash gear indicator. No tach on the truck. I know this is a can of worms, just looking for the best starting point. Don’t really want to go away from a fully rebuilt trans but if it’s cheaper to go another route I will.

 

[Ricko1966]

I’m not sure of actual RPM but I know what high RPM sounds like. It currently has 33” tires, was told it had 35’s before. I believe this truck was an original OD transmission because it has an OD indicator on the dash gear indicator. No tach on the truck. I know this is a can of worms, just looking for the best starting point. Don’t really want to go away from a fully rebuilt trans but if it’s cheaper to go another route I will.

A tach is cheap put one on even temporary.What sounds high to you may not seem high to us,numbers work better than,sounds high,numbers we can approximate what you have. Tell us your rpms at 60mph that will tell us a lot, from there we can figure out how much difference taller tires,different gears,O.D. transmission will make. Something to consider a Th350 has a 1st gear of 2.48 a 700r4 has a first gear of 3.07 . Th350 has top gear of 1 to 1 700r4 has top gear of .73. So a 700r4 will have lots more gear in 1st especially if you start running highway gears. Find out what rpm you have at 60mph now, we'll do some math. Putting 35s back on it would probably be a 10 percent drop in rpm.

 

[ali_c20]

Welcome from Austria. Post some pics.

Here are rpm calculators which gives you ball park numbers for the rpm.

https://spicerparts.com/calculators/engine-rpm-calculator#more

https://purperformance.com/p-29669-rpm-calculator.html

 

[Grit dog]

The age old question. Trans or gears?
No right or wrong answer but find out what you got for gears and what rpm’s you’re at.
Then cost vs intended use.
Getting both front and rear re-geared at a shop is roughly the same cost as a decent trans not including install.
But you could recoup some on the 350 trans especially if you have the paperwork and could pre sell it with a test drive.
Just things to consider.

 

[75gmck25]

Checking it with a tach is the best start. These trucks echo like tin cans, so noise from the engine is amplified. When I replaced cab bushings I also found that my cab was resting down on the transfer case, so all the drivetrain noise was transmitted directly up to the cab.

My K25 has about 31” tires and 4.10 gears, and it runs about 2800-3000 rpm at 60 mph. Sounds pretty loud at 65 mph, but doesn’t cause any problems.

Also crawl under there and find out what transfer case you have. Swapping to an overdrive transmission can get harder if you have an unusual combination and need the correct adapters.