Seff

Am I correct in calculating that with a M22 and a 1:2.73 rear, doing 75 in 4th gear is going to take about ~4400 RPM? If this is so, I'll have to look for a 5 or even 6 speed tranny.

jensenracing77

5 speed or 6 speed won't matter i don't think. if i am thinking right only the last gear in each will be overdrive. i don't think that the 5th gears in a 6 speed is overdrive but i may be wrong.

joe_padavano

It's impossible to answer this question without knowing the tire size. Assuming you have a 26" tall tire, fourth is 1:1 (as it is for any non-OD GM transmission) and with a final drive of 2.73:1, at 75 MPH your engine will be turning about 2650 RPM. I have no idea where you got 4400 RPM.

Now, if you meant a final drive of 3.73:1, then the engine is turning about 3600 RPM at 75 MPH. With 4.33 gears, you'll be turning 4200 RPM.

Seff

Ah, that's what I did wrong - for some reason I managed to decide that my tire was 15 inches tall. That'll teach me to do math late at night. :P

According to this, some 5 speed trannys have overdrive (less than 1.0, yes?), while none of the 4 speed trannys get below 1.0.

That was what I was basing my assumptions upon. Glad that it isn't actually like that. :P Thanks for the replies.

joe_padavano

All five speed transmissions that I'm aware of have OD fifth. I think nearly all six speed transmissions have double OD (fifth and sixth).

Seff

Ah, suspected so, thanks.

Semi-related questions - when looking for transmissions for my 1972 Cutlass Supreme, I can make most GM transmissions bolt up to my 350 either directly or with an adapter plate like this one, yes? Am I going to need my torque converter with the manual transmission? When looking for a pedal, will any/most do, or do I need an A-body or even a Cutlass-specific pedal assembly?

TRW

Joe, I think the Richmond Street 5 speed has a 1:1 5th gear. They just came out with the new Super Street 5 speed that does have OD.

Tom

Rickman48

My son had some Ford POS trans - 4 speed and 4th was OD!

joe_padavano

Yeah, I KNEW you guys would bring these up.

Please note my very carefully worded response in my first post:


Note that Chrysler also had a four speed where the top gear was OD. This was actually used in Chevy trucks for a while. Both the Chrysler and Ford OD 4spd transmissions were stop-gap responses to the first oil crisis in the 1970s. It was easy to change one gear ratio to improve gas mileage. Both of these transmissions actually put the OD gears in the THIRD gear location, so third was the 1:1 former fourth gears and the OD fourth was the former third gears. The tab on the shift arm was flipped to keep the new third and fourth in the same location in the H pattern.