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Our '67 is needing some transmission overhauling. I'm trying to figure out before I start if I'd be better off with a different transmission though. The drivetrain is not original and as I understand it, the trans and rear end aren't even really compatible with each other. Its got 3.70 gears and an M-20 four speed. Our interstates are now speed limit 80 and I'd like to have something in the car that isn't pushing the RPMs quite so high. Would a simple gear change be enough or do I need to think about a 5 speed? Would switching to an M-21 help? I need some educating here. Thanks in advance!
Pluses and minuses on both sides. It's easy to spend $4000 for a 5-speed conversion. A rear change/rebuild $1000. How mush time do you spend on/at 80mph? Dennis
For a 80 mph speed limit you will probably want to get overdrive trans and change rear axle ratio. A .70 OD will get you to approx 2.8-3k if want to drop RPM more you will need a different rear gear. There calculators to help with gear ratios with both trans and diff. IMO I would look for OD trans first.
All the above information is good.
For sure, to drop your Interstate RPM, SOME KIND of change will be necessary.
Your M20 (wide ratio) will work with a 3.08 rear. And yes, taking off from a stop will be slightly more sluggish than what you are accustomed to. A 5sp with a somewhat lower 1st gear will allow you to retain a snappy take off with the 3.70, and a 5sp with an OD will allow you to keep your 3.70 rear and will noticeably drop your Interstate RPM.
I have an M20 and a 3.08 rear in 4 of my cars and I'm quite happy with the combination. BUUUUUUUUUUT, also, all of my engines are in the 400cube range, thus they have plenty of torque to turn a 3.08 rear from a dead stop with no effort. My 56 Vette, 23 T-bucket and 51 Chevy turn right at 3000 at 76mph. There is no tach in the Chevelle, but it has the same drive train.
Personally, if I were in your shoes, I'd opt for an OD 5sp -------------------- or maybe even a 6sp, but keep the 3.70 rear.
5 spd youll love it
Car will feel quicker and cruise the interstate all day
Although not a vette, did a T56 conversion on an older car
Used a hurst superboot, short shifter drilled out a muncie 4 spd ball, finally yellowing some. Noone can tell its not a 4 spd.
to do it again 5 spd would be plenty;best upgrade youll ever do!
DZ is correct 3.08 is miserable with the 4 spd
Cost-wise, what DZ said. A 3.08 or even a 3.36 rear gear will be a huge advantage over the 3.70 you now have and will work perfectly with the M-20. M-21's have a taller first gear, and are meant for use with 3.70-4.11 rear gears. A 5 speed is a huge improvement, but will cost you around 4 grand for the upgrade. If it were my car, I'd slip a 3.08 third member in and be done with it. A simple one-day bolt in deal and you're off driving the car.
my 66 l-79 has a wide ratio and 3.08 very user friendly on highway
around town I don't have a problem with take off, comes down to how much do you use on highway as opposed to in town use.
I would just change rearend gears to 3.08 and drive it for a while and see if you can live with that combo.
I went the 5 speed retaining a 3:73 posi. First gear is a 2.68:1 ratio. That indexes to 10.00
The 5th gear ratio is .68. Mine's the Keisler SS700 Legend tranny. Been in the '5 going on 4 years now. No issues.
Here's pics from our 2013 Route 66 trip.
Taching @ 62 mph
Taching @ 80 mph
This is not a cheap conversion but definitely one for those who drive their car a lot. Tripping is a blast. You're just not spinning the motor to stay with traffic.
With my setup, solid lifter 327 , '70 LT1 cam, lifters, intake, she's very responsive (love the jump). Running around town, I can slip into 5th gear 42-45 mph. She just drives right out.
I also went the 5 speed rout witchy '66 L79. I have 3.70 rear end and had the M20 Muncie which ran at about 3000+ rpms at 75 mph. After installing the Kiesler 5 speed about 8 years ago and with the same 3.70 rear, it runs about 2200 rpms at 75mph. And with the lower 1st gear, you can really have some fun with it. But as other posters have stated, this is a more expensive option than changing the rear end gears.
Our '67 is needing some transmission overhauling. I'm trying to figure out before I start if I'd be better off with a different transmission though. The drivetrain is not original and as I understand it, the trans and rear end aren't even really compatible with each other. Its got 3.70 gears and an M-20 four speed. Our interstates are now speed limit 80 and I'd like to have something in the car that isn't pushing the RPMs quite so high. Would a simple gear change be enough or do I need to think about a 5 speed? Would switching to an M-21 help? I need some educating here. Thanks in advance!
I am with Warrenmj ihad the m20 w 70 series tires i turned 3300 at 75 now w the t600 5 spd 2300 at 75 low around town sport gearing w overdrive for hwy . I think its ideal .
the OP stated M20 needs repairs. Even if he swaps out 3rd member as assembly, which is how I would do it. It could be saved or sold it off set costs. I would not take a good performance rear diff apart to install highway gear set. He still needs to fix trans; which you never know for sure what you got until it's apart. I still would go with OD trans 1st.
the OP stated M20 needs repairs. Even if he swaps out 3rd member as assembly, which is how I would do it. It could be saved or sold it off set costs. I would not take a good performance rear diff apart to install highway gear set. He still needs to fix trans; which you never know for sure what you got until it's apart. I still would go with OD trans 1st.
Tire and wheel size also play a part in all of this. Here is a link to an RPM calculator that we have on our website. This will let you know exactly what kind of what RPM's you'll be turning at any given speed (you select) based on the transmission that you choose, your tire/wheel size, and gear ratio. Let us know if you have any questions.
In your shoes I would get a five speed OD. I had a Corvette with 3.08 gears and a 327 and it was fine, but you will still feel like you need one more gear when you get up to highway speeds. Cruising at 65-70 mph I was able to get about 20mpg (LA to San Diego), and with a 6500rpm red line you do get to hit 60mph in first gear with 3.08 gears.
But you are used to the 3.70 that you have now, and I think it will feel like you lost all your bottom end torque if you swapped to a 3.08 and kept the 4 speed.
As someone mentioned above, you want the index to be about 10.0 in first gear, and 2000-2500 rpm at highway speeds as well. The only way to get this is an OD transmission. More money, but this is what I would (will) do.
I am in the same boat, deciding what to install, I have a 5 speed on the shelf with 2.95 first gear and .64 overdrive 5th gear. The car currently has 3.70 rear gears and an M21. I have a 3.36 extra set of gears and still deciding what will work best. I think you just need to decide what RPM you will be happy with at 70 MPH and how much power you wish to have for stop lights and plug in the parts that get you there.
Personally I don't wish to give up any acceleration but I really want to be comfortable cruising at 75 MPH on our modern highway without spinning 3,000 + RPM to do it.
I am in the same boat, deciding what to install, I have a 5 speed on the shelf with 2.95 first gear and .64 overdrive 5th gear. The car currently has 3.70 rear gears and an M21. I have a 3.36 extra set of gears and still deciding what will work best. I think you just need to decide what RPM you will be happy with at 70 MPH and how much power you wish to have for stop lights and plug in the parts that get you there.
Personally I don't wish to give up any acceleration but I really want to be comfortable cruising at 75 MPH on our modern highway without spinning 3,000 + RPM to do it.
Sounds like the McLeod 5 speed you have there. With 3.70 gears, you will hit 40mph at about 6,000 rpm, and cruise at 70mph at about 2,200 rpm. Going down to 3.36 gets you close to the ideal 10 (9.912 for first gear) total gear in 1st and will let you get to 45 mph at 6K rpm, and 70 mph will be closer to 2,000 rpm.
Not a huge difference, and not enough to warrant taking apart the rear end. But if you needed to get into the rear gears anyhow, you might want the 3.36 gears. You will have more gear in 1st that you do now with the Muncie (likely a 9.3 ratio assuming 2.52:1 1st gear), and way more overdrive on the highway.
06-22-2016, 01:57 PM
question about gear ratios and transmissions
Dennis
