I just did mine with a 3.42 rear and 2.74 first gear(200R-4)and it is 9.37.

I bet that will work out nice. Automatics would be less forgiving because you can't use the clutch as a 'fudge factor'.

Don't forget that you also need to swap out the cluster shaft.

Thanks Kurt. Sounds like you would have to swap out all the gears.... And then you have an M20! Still can't wait to try it (7.106 resultant) out!!! I'm getting the idea that power and torque will overcome the problems outlined with going below 9 in the earlier presented equation, that was wrote to the tune of Chevrolet engines. It will be a pleasure to put it to the test!

Actually the torque converter makes the automatic more forgiving. Instead of just allowing the engine speed to be in a more favourable range (like slipping the clutch would), it also multiplies the torque from the engine. Depending on the stall speed, the usual amount is around 2.4 to 1. As the rest of the drivetrain catches up to the engine, that number diminishes until it is almost 1:1, the rpm that it happens at again depending on the stall speed.

My '69 with a switch-pitch TH400 and a 2.56 rear gear was traction limited when in high stall (3000 rpm). If you do the numbers, you get 2.48 x 2.52 = 6.25; then x 2.4 = 15! No problem blazing the tires! Sort of like dumping the clutch but easier on the rest of the drivetrain and the effect takes several seconds to drop to 1:1. By about 30 mph my set-up was basically at 1:1 and allowing the engine rpm to climb normally. It acts like another gear below first which is why autos work well at the strip.

This thread ties into the torque converter thread as they both deal with selecting the right parts to match driving demands and expectations. Either way can require a few experiments before arriving at the ideal combo to suit the owner.

Good info... What I was thinking when I said 'forgiving' was the adjustibility of the driver's foot on the clutch.

Yeah, I've done some "adjusting" with my left foot a time or three! The switch-pitch allowed choosing either 1800 or 3000 rpm as the stall speed, a little like slipping the clutch but not as adjustable!

Nope. The 2, 3, 4 gears are the same between M20 and M21. Only the input and the cluster are unique.

Year 1966-1974 Type: M20 Rings: Two Ratio: 2.52 / 1.88 / 1.46 / 1.00 / 3.11
Year 1963-1974 Type: M21 Rings: One Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Year 1967-1974 Type: M22 Rings: None Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27

i have a 3:08 rear gear and im going with a m-20 transmission and a pontiac 400 stroked out. the guy im buying my trans from told me it would be a killer on the streets. he is no spring chicken to building cars either so i like jim am interested in seeing just how well th equation works for this!! the rear gear


2.52 x 3:08 = 7.7616

it will be a posi so im not too worried about off the line power!!

Jim,
That's gear ratios, not gears. The gears are the same - contact any trans rebuilder or look in the parts manual. The input (1st) changes the ratios of the other gears.

It's good to learn.

this should be added to the tech fame club as it gives some info on choosing the proper gear size and ratio's for the rear axle and transmission. it will give newbies a chance to figure which rear gear to use with their trans if they so choose to go non stock!!!