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raycow · 05-31-2010, 03:02 PM

The TH400 is 1:1 in high gear just like your 4 speed, so your engine rpm won't drop any for the same road speed. If anything, it will increase slightly because of converter slip.

If you still want to go automatic, look at an O/D transmission like a 700R4. If your only desire is to reduce engine speed, a 5 speed with O/D like a NV4500 will be less work to swap. Since neither transmission was offered in 71, you will need an adaptor to bolt it to your transfer case. Adaptors are available from outfits like Advance, Novak, and others.

You may also want to consider an axle swap as an alternative. This could be less work/money than a transmission swap if you find a later 4WD donor truck, as these would typically have taller gearing.

Ray

05-31-2010, 03:02 PM	#3
raycow Registered User Join Date: Mar 2003 Location: San Francisco, CA USA Posts: 2,454	Re: 4 speed to TH400? The TH400 is 1:1 in high gear just like your 4 speed, so your engine rpm won't drop any for the same road speed. If anything, it will increase slightly because of converter slip. If you still want to go automatic, look at an O/D transmission like a 700R4. If your only desire is to reduce engine speed, a 5 speed with O/D like a NV4500 will be less work to swap. Since neither transmission was offered in 71, you will need an adaptor to bolt it to your transfer case. Adaptors are available from outfits like Advance, Novak, and others. You may also want to consider an axle swap as an alternative. This could be less work/money than a transmission swap if you find a later 4WD donor truck, as these would typically have taller gearing. Ray Last edited by raycow; 05-31-2010 at 03:04 PM.