sm465 to th350 swap

[mattshea125]

Hello I have a 1969 K10 shortbed with the SM465 / NP205 transmission transfercase combo. I have recently developed a high pitch bearing failure sound coming through the shifters into the cab. I have a TH350/NP205 that I could swap over to. My questions is if anyone out there has a TH350 with 3.73 gears and 33" tires. The 4 speed that's in the truck now is pretty lunky to drive (less than ideal 2nd and 3rd gear ratios). Will the TH350 1st and 2nd gear ratios get the truck moving a little better? How is the highway? Anyone who has a driveline combination out there I would appreciate to hear your input. Thanks

[57taskforce]

The TH350 should be fine with 33’s and 3:73’s. It won’t be a rocket off the line but shouldn’t be terrible. The gear spacing is a little more tight than the 465, but it’s not going to be as durable as the 465 is either(obviously). The 465’s continually impress me for how much you can beat on them after all these years.

Highway is going to be pretty much exactly the same as the 465 less converter slip in the th350. They both run a 1:1 high gear ratio.

[i82much]

i have a very mild 396, th350, and 3.73’s with 32 inch tires in a 72 k10. it’s just about a perfect balance of low speed acceleration and highway cruise, as good as you can get with only three gears.

[Mike C]

My kid’s Jimmy has a TH350/205 combo with 3.73 and 35” tires. If you do the math, it’s pretty much exactly the same gear ratios as the 3.07 gears and 28” tires the truck came with. It’s a nice driving combo. Motor has a mild cam (204/214 at .050) and we did put a 2000 rpm converter in it. But yours will have a little more gear with the 33” tires. Be prepared for a 5% increase in rpm on the highway because of converter slippage. My Jimmy with SM465/20, 4.10 gears, and 35” tires is almost exactly same rpm as his at 65 even though it has 10% more gear.

[Richard]

My 72 I am currently working on originally had a 350/350 combo with 31" tires and 3.73 gears. Little higher RPM than I liked but drivable. 10 mpg all day long empty or loaded. 33's will drop the rpm a bit should be fine. Look up rpm calculators and input your info, have used the Spicer one many times. Add about 200 rpm to result for non lockout trans and it should be pretty close. As said all non OD transmissions are 1:1 in final gear, so expect about 200 rpm higher.