I need your help with gear ratio . I have a 69 skylark with a 455 ,a m21 trans and a 12 bolt rear end. Have no idea where to start with gearing. Does anyone have close to the same setup that went through this already? I need your help thank you
How are you planning on using the car? Highway Street Street/strip Strip. And how modified is the 455? We run automatics, but 3.42 has been a good overall performance gear.
Yeah I was thinking around mid 3s. The engine is moderatly modified. I will be using the car for mostly street but want it to be good for strip once and a while. I'm bad at figuring out what I need for gears
Try a 3.08 or 3.23 ratio. 3.42 yields to many revs for the highway use. The low end torque of the 455 will still give you plenty of low end acceleration.
I am thinking 3.23 for my convertible build. We run 4.10 in the race car, but will be going back to 3.42. Based on what you are planning, the 3.08 or 3.23 would be nice.
Do not to figure tire size diameter into your equation... It will help teak the ratio. 12 bolt chevy ratios are as follows: 2.56 2.73 (2 series carrier) 3.08 3.31 3.42 3.55 3.73 3.90 (3 series carrier) GM 3.90 was 4 series but they are very rare even to find (3.73 aftermarket can be a on a 4 series carrier too if needed ) 4.10 4.56 4.30 4.88 (4 series carrier) aftermarket make the 4.10 4.56 4.88 in 3 series also Not 3.23 in the 12 bolt chevy rear and 3.31 usually will find only used. So 3.42 sounds like the plan for all around performance and budget. Jim
3.08 posi in my GS 350. Nice on the highway, but will still lay a couple of black marks on take off :3gears:
I went thru this several years ago with a 68 GTO that I wanted to be tamer for street use. Came stock with an M21 and 3.55 rear--good for stoplight racing, but lousy on road trips. At 60MPH I was taching around 3000 RPM. My plan was switch to a M20 (wide ratio) tranny and a rear slightly above or below 3.00. Bought the M20 and still had not found the rear when I had to sell the car, but I'm pretty sure I had the right plan. Just to show how times change, in 1972 I had another 68 GTO with a 3.90 rear and close ratio tranny as my everyday driver and never thought about how high it tached at highway speed. John :beer
i'm only running a 3.08 ratio w/ 28in tiures which makes my ratio something like a 2.94..with my lil 350 I still have great accelleration and can cruise at 75mph like I want! so being a bigblock manual car I think you'll do just as well with a 3.08 or 3.23
fine 3.31 then lol :bla: whgat tire size you running? 245-60/295-50 =26.6/26.58 255-60-15 = 27 275-60 = 27.99 what speed to you tend to cruise on the hiway I geared mine to be just under 3k rpms(2856) at 75mph...I didnt calculate converter slippage though http://www.ringpinion.com/Calc_RPM.aspx
The Muncie M21 is a close ratio with a 2.2 first gear.... With a rear ratio of 3.08 or even 3.23 you'll be slipping the clutch a lot to get the car moving. That first gear would be happier with a 3.42 rear behind it. Like John (Nailhead) said, A Muncie M20 with a lower 2.56 first gear would be happy with a 3.08 rear: 2.56 x 3.08= 7.88 overall first gear ratio. For that same overall ratio a M21 would need a 3.58 rear ratio.
Looks like iwill be running around 3.20s-3.30s. This car was so hacked when I got it I still need to figure out coils and tires. What size tires did these come with and what size looks good. I would like a slight rake with stock springs. I have so many questions. I want good stance , acceleration and traction. I know there's other forums to ask for help in but you guys seem to be clues into these bucks. I need lots of help!!! Thanks for any advice
Oh and why would they ever make a tranny like the m21 I think it's everything I don't want lol close gears, long first, I can rip through the gears but very tough to start off from a stop. It's like you can't start off easy with it. I almost smell my clutch burning off everytime lol.
m20 3.73 rear gear with 28" tire, can't get the smile off my face! I agree with Nialhead, people worry these days about too many revs on the highway i think. having a slight "rake" in a 4 speed can be a bad thing for traction, acceleration, and longevity of parts
What causes the bad wear on parts . I have I think load coils in the rear and I'm ripping them out , horrible ride . I'm going with stock coils . But would it be bad to put taller tires in the rear to get the rake? Maybe I'll just scrap the rake idea lol
