Rear Gear Ratio Decisions

I have seen a lot of questions about what gear ratio a person should run. A lot of times they are thinking about the final out come. For instance, what will my RPM @ 60 be? A lot of these people don’t event think about the transmission and how it plays an effect as well as tires.

Firebirds with manual transmissions plays a vital role on what is optimal as a rear gear ratio. For a street driven car, for optimal performance you want your first gear final drive ratio (trans gear x rear end ratio) to be between 9 and 10. Anything over 10 and the car will be less desirable for street application and more for strip in my opinion. Any thing less then 9 you will have to slip clutch.

So with a 68 M20 Muncie, your 1st gear is a 2.52 ratio. With a 3.36 rear gear ratio that puts the final drive at 8.46. It is close to that magical number of 9. Now what helps with this magic number is torque. The more torque you have can help with that number.

Just like a torque converter increases torque and it allows a automatic transmission to run a higher gear. With a Pontiac 400 you get more torque then with less say a Chevy 350. You can get away with a little below that magic number. You can bump the throttle to get the RPMs up thos increasing the torque to allow you to pull off easier.

But when you ask about if would be alright to run a 2.73 rear gear, that would make your final drive ratio 6.87. Not to optimal is it and well below that magic number of 9. Taking off from a stop with that ratio will be way to hard on you clutch. Again your looking for that magic number of 9.

I have a 3.36 with a final ratio of 8.46. My engine produces 445 Ft -Lbs torque and on a hill I have to slip the clutch some. I think I would rather have a 3.55. My RPMs will go up from 2650 to around 2774 in 4th @ 60 but I it would help with the take off because my final drive ratio would go from 8.46 to 8.94.

Now has a mucie M22Z with a first gear of 2.98. Man wouldn’t that be nice having a steeper 1-2 gear with my 3.36. That would make my final drive ratio 10. IIt would be a great off the line. That first gear would be perfect with a 3.08 rear too. That would put your final drive ratio at 9.17. Still better then my 2.52 first gear final drive ratio but would be around 2200 rpms @ 60.

To calculate what you final RPMs of your 4th gear ratio with is a 1:1. You can use a RPM calculator like this one

3.36 rear gear and 25.8 tire size @ 65 =  2,844.28

3.55 rear gear and 25.8 tire size @ 65 = 3,005.12

It is tuff to find a middle ground with the M20 Muncie but everyone seems to think 3.36 is about as efficient as one can get with out changing the transmission.


Here are some references:

To calculate RPMs with a known rear gear and tire size – Rear gear calculator – Tire height calculator


5 thoughts on “Rear Gear Ratio Decisions

  1. I did the same type of calculations after installing a super T10 in my ’68 Firebird convertible. The unit has a 2.61 first gear ratio and with the stock 2.56 rear end it was a chore to launch. After putting in a 3.23 rear it is much easier to drive but I wish it rev as high at 70mph. The calc turns out as 8.43. I would probably enjoy having a 3.08 better or change to a 5 speed

  2. I have a 3.55 with M-20 in my 68 conv. It came with a 3.36. Body is on rotisserie and 95% ready for paint. Engine (100% stock rebuild). Will be back in two weeks. I hope my decision to upgrade to 3.55 was a good on. Fortunately I did find another 68 rear end that I had rebuilt – it was original 3.55 Posi. Worst comes to worst i will rebuild 3.36 original. Hope to be on the road by June. I will let you know then if it was a good decision.

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