Need professional advice on gear ratios
Thread Starter
Le Mans Master
Joined: May 2006
Posts: 5,820
Likes: 414
From: Pittsburgh suburbs Pa.
Need professional advice on gear ratios
My 68 has an M20 4 speed with 336 gears in the rear and on the interstates it seems like it needs to shift to a higher gear. I want to take the stock rearend out and put it in storage and replace it with a rear with higher gears. I'm looking more for gas economy on the highway and I realize it will also make 1st 2nd and 3rd higher and I'm looking for the right rear ratio that will still have reasonable power for starting out in 1st gear and less engine reving in 4th. Has anyone experimented with this ? I would be interested in hearing what combinations you are running if you are running a 4 speed and don't notice a lot of engine reving in 4th.
Last edited by bj1k; Aug 28, 2008 at 09:14 PM.
Melting Slicks
Joined: Apr 2001
Posts: 2,030
Likes: 363
From: Lubbock Texas
I don't have a 4 speed. My '73 is an auto with 3.08s. But I don't think you can get where you want with just a gear swap. I've got the same issue and swapping gears to lower highway RPMs will not give me much of a launch. The only answer for me is a an OD tranny, preferably one with a better 1st and 2nd for launch.
I think your best option is to keep the 3.36s and swap the tranny for a 5 speed.
DC
Melting Slicks
Joined: May 2001
Posts: 2,812
Likes: 4
From: Gig Harbor Wa
What RPM are you turning at 60 or 70?
I agree, you won't be happy with the 1st and 2nd gear performance if you go with anything higher than 3.36. Those seem to be a good compromise of performance and mileage.
I also agree that you should consider an overdrive trans. With my T56 I am getting between 25 and 27 MPG with cruising RPM on the highway under 2000.
I agree, you won't be happy with the 1st and 2nd gear performance if you go with anything higher than 3.36. Those seem to be a good compromise of performance and mileage.
I also agree that you should consider an overdrive trans. With my T56 I am getting between 25 and 27 MPG with cruising RPM on the highway under 2000.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
IMHO 3.36:1 is as tall as I'd go behind an M20, and suggest leaving the diff as is and getting a TKO. If such a mod is out of the question, you'll either have to leave things be or decide if better highway economy is worth sacrificing performance, as you just can't improve both by simply changing to a 3.08:1 FDR. I'd re-evaluate your priorities before proceeding...
Drifting
Joined: Oct 2005
Posts: 1,528
Likes: 148
From: East London/SW Essex UK
My 68 has an M20 4 speed with 336 gears in the rear and on the interstates it seems like it needs to shift to a higher gear. I want to take the stock rearend out and put it in storage and replace it with a rear with higher gears. I'm looking more for gas economy on the highway and I realize it will also make 1st 2nd and 3rd higher and I'm looking for the right rear ratio that will still have reasonable power for starting out in 1st gear and less engine reving in 4th. Has anyone experimented with this ? I would be interested in hearing what combinations you are running if you are running a 4 speed and don't notice a lot of engine reving in 4th.
Tech Contributor
Joined: Aug 1999
Posts: 15,193
Likes: 4,016
From: Connecticut, USA
The only other gears to use would be 308 or 273's. I set up a 273 for my buddy who has a ST-10 with 2.64 first gear and he loves it. The Muncie is 2.54 if I recall so it might be a slug off the line. If you have a BB then you might get by, a SB you'll probably be riding the clutch starting out.
As the others said, an OD is probably going to be the way to get you in RPM range you want. The question is, is it worth $3500 to you? The M-20 336 setup is a very good one but then gas was .30 a gallon then too.
I'm running a 336 in my TH400 72 instead of the 308. There is not much RPM difference between them, maybe 250-300 RPM.
As the others said, an OD is probably going to be the way to get you in RPM range you want. The question is, is it worth $3500 to you? The M-20 336 setup is a very good one but then gas was .30 a gallon then too.
I'm running a 336 in my TH400 72 instead of the 308. There is not much RPM difference between them, maybe 250-300 RPM.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
IMO, in a light car like these, any swap of only a 10% gear change is not worth it, go for the overdrive, but to stick with a stick you going to have to spend bux.....years ago I did a swap over to automatic on account of price and left the 336 ratio alone, my car was a Muncie also....I wound up with a 200 4r that I built in there now, and it's fine....cruise at 80 roughly 2300 rpms.....I have a <400 hp 355 roller engine.....so get 22.2222 mpg at 90mph cruise we FLY here in Florida....
I think the 200 is the way to go, and it's not that big a deal to swap over, basically a bolt in costing some 600 bux total IF you can do your own rebuild......
I got a core for 60 bux, then a pump, converter, TRANS GO shift kit (NO OTHER), and soft parts kit and TCC solenoid as an afterthought..
and it's been fine....
did my own rebuild in the garage here which took me two days, but I learned from the ATSG book and so can YOU.....
I think the 200 is the way to go, and it's not that big a deal to swap over, basically a bolt in costing some 600 bux total IF you can do your own rebuild......
I got a core for 60 bux, then a pump, converter, TRANS GO shift kit (NO OTHER), and soft parts kit and TCC solenoid as an afterthought..
and it's been fine....did my own rebuild in the garage here which took me two days, but I learned from the ATSG book and so can YOU.....
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If you are a 'casual' driver [don't care about drag racing the car, etc.] and do lots of highway driving but not much 'city' driving, put a 2.73 rear end in the car. If you need to do a lot of stop-start, city driving, leave the rear end alone and just spend a little more at the pump when you go on the road. You can't financially justify the expense and trouble of installing an overdrive tranny in your car unless you buy a salvaged 200R and do the rebuild and installation yourself.
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Burning Brakes
Joined: Apr 2008
Posts: 945
Likes: 1
Great thread. I was contemplating the same issue yesterday. I have a '79 3 spd auto with a 3.55 rear end. HP was down to 195 that year in the L48 and I'm sure GM was trying to get some acceleration out of the meager HP.
The problem is I cruise at 2800 rpm at 60 mph in the '79. Not acceptable to me. My '08 cpe cruises at 1500 rpm at 60 mph, 2.73 axle and has 436 hp with 28 mpg. I was thinking about putting the original engine in the basement and putting in a 350/300 hp crate motor in and changing out the rear end for something else.
The problem is I cruise at 2800 rpm at 60 mph in the '79. Not acceptable to me. My '08 cpe cruises at 1500 rpm at 60 mph, 2.73 axle and has 436 hp with 28 mpg. I was thinking about putting the original engine in the basement and putting in a 350/300 hp crate motor in and changing out the rear end for something else.
Last edited by Bobz08C6; Aug 29, 2008 at 11:52 AM.
Melting Slicks
Joined: May 2001
Posts: 2,812
Likes: 4
From: Gig Harbor Wa
Bobz08c6 - You are comparing apples to oranges. Your 08 has 4.02 first gear and the 79 trans has a 2.52 first gear.
2.73 in the 08 would be the same as 4.35 in the 79. If you drop those same 2.73's in the 79 it would be a total slug, even if you had the 430Hp of your LS3. Not only is first more punchy in the 08, but 6th is .67. That is the same as running a 1.8 rear end with your 79. Thats why you are getting 28 mpg.
Back to the original poster, If you don't want an overdrive, you are going to have to sacrifice somewhere. Either you have good off the line performance or you have low cruising RPM's and better gas mileage. 3.36 was the factory compromise. Changing that will affect something negatively.
2.73 in the 08 would be the same as 4.35 in the 79. If you drop those same 2.73's in the 79 it would be a total slug, even if you had the 430Hp of your LS3. Not only is first more punchy in the 08, but 6th is .67. That is the same as running a 1.8 rear end with your 79. Thats why you are getting 28 mpg.
Back to the original poster, If you don't want an overdrive, you are going to have to sacrifice somewhere. Either you have good off the line performance or you have low cruising RPM's and better gas mileage. 3.36 was the factory compromise. Changing that will affect something negatively.
Intermediate
Joined: Nov 2005
Posts: 47
Likes: 0
From: Clifton VA
IMHO 3.36:1 is as tall as I'd go behind an M20, and suggest leaving the diff as is and getting a TKO. If such a mod is out of the question, you'll either have to leave things be or decide if better highway economy is worth sacrificing performance, as you just can't improve both by simply changing to a 3.08:1 FDR. I'd re-evaluate your priorities before proceeding...
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
He has a 200 hp engine with a 3000 pound car. It may not get off the line like a [normal] Corvette, but it won't stall from lack of torque. If the guy wants a highway cruiser that will get decent mileage...and doesn't care about 'killer' acceleration from a dead start, why do you care?
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
My 68 has an M20 4 speed with 336 gears in the rear and on the interstates it seems like it needs to shift to a higher gear. I want to take the stock rearend out and put it in storage and replace it with a rear with higher gears. I'm looking more for gas economy on the highway and I realize it will also make 1st 2nd and 3rd higher and I'm looking for the right rear ratio that will still have reasonable power for starting out in 1st gear and less engine reving in 4th. Has anyone experimented with this ? I would be interested in hearing what combinations you are running if you are running a 4 speed and don't notice a lot of engine reving in 4th.
Your motor TQ is what causes you to accelerate. I have an M-21 equiped 67 camero that has had a 2.73 for over 20 years. It's kind of a bog or burn off the line. but once it is rolling, big nice gear spreads.
My Vette with 5 speed OD comes out to like having a 2.56 or something rear gear. I put it in 5th @55 and drive i never have to down shift even going over 9000 foot mountain passes or passing somebody
I drove a car a couple of weeks ago that could go 60-70 mph in first gear with a tiny flywheel and light weight multi disk racing clutch. it sucked getting moving, but the closed ratio was really nice.
doesn't any of these companies still sell complete pumpkins for about $1500 or less?
Drifting
Joined: Jun 2007
Posts: 1,404
Likes: 1
From: Ansonia/NYC CT
IMO, in a light car like these, any swap of only a 10% gear change is not worth it, go for the overdrive, but to stick with a stick you going to have to spend bux.....years ago I did a swap over to automatic on account of price and left the 336 ratio alone, my car was a Muncie also....I wound up with a 200 4r that I built in there now, and it's fine....cruise at 80 roughly 2300 rpms.....I have a <400 hp 355 roller engine.....so get 22.2222 mpg at 90mph cruise we FLY here in Florida....
I think the 200 is the way to go, and it's not that big a deal to swap over, basically a bolt in costing some 600 bux total IF you can do your own rebuild......
I got a core for 60 bux, then a pump, converter, TRANS GO shift kit (NO OTHER), and soft parts kit and TCC solenoid as an afterthought.. and it's been fine....
did my own rebuild in the garage here which took me two days, but I learned from the ATSG book and so can YOU.....
I think the 200 is the way to go, and it's not that big a deal to swap over, basically a bolt in costing some 600 bux total IF you can do your own rebuild......
I got a core for 60 bux, then a pump, converter, TRANS GO shift kit (NO OTHER), and soft parts kit and TCC solenoid as an afterthought..
and it's been fine....did my own rebuild in the garage here which took me two days, but I learned from the ATSG book and so can YOU.....
Thread Starter
Le Mans Master
Joined: May 2006
Posts: 5,820
Likes: 414
From: Pittsburgh suburbs Pa.
I don't have a 4 speed. My '73 is an auto with 3.08s. But I don't think you can get where you want with just a gear swap. I've got the same issue and swapping gears to lower highway RPMs will not give me much of a launch. The only answer for me is a an OD tranny, preferably one with a better 1st and 2nd for launch.
I think your best option is to keep the 3.36s and swap the tranny for a 5 speed.
DC
I think your best option is to keep the 3.36s and swap the tranny for a 5 speed.
DC
Thread Starter
Le Mans Master
Joined: May 2006
Posts: 5,820
Likes: 414
From: Pittsburgh suburbs Pa.
What RPM are you turning at 60 or 70?
I agree, you won't be happy with the 1st and 2nd gear performance if you go with anything higher than 3.36. Those seem to be a good compromise of performance and mileage.
I also agree that you should consider an overdrive trans. With my T56 I am getting between 25 and 27 MPG with cruising RPM on the highway under 2000.
I agree, you won't be happy with the 1st and 2nd gear performance if you go with anything higher than 3.36. Those seem to be a good compromise of performance and mileage.
I also agree that you should consider an overdrive trans. With my T56 I am getting between 25 and 27 MPG with cruising RPM on the highway under 2000.
Instructor
Joined: Aug 2008
Posts: 235
Likes: 0
From: Orlando Florida
My 68 has an M20 4 speed with 336 gears in the rear and on the interstates it seems like it needs to shift to a higher gear. I want to take the stock rearend out and put it in storage and replace it with a rear with higher gears. I'm looking more for gas economy on the highway and I realize it will also make 1st 2nd and 3rd higher and I'm looking for the right rear ratio that will still have reasonable power for starting out in 1st gear and less engine reving in 4th. Has anyone experimented with this ? I would be interested in hearing what combinations you are running if you are running a 4 speed and don't notice a lot of engine reving in 4th.
P.S. here's the link: www.classicchevy5speed.com
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
FYI, these examples should give you a better idea of what various manual OD transmissions could do for you. The top lines are gearset ratios, and the bottom lines are those multiplied by your current 3.36:1 FDR; the larger numbers representing more torque multiplication and the smaller ones less...
Muncie M20 & 21 (wide) ratios:
2.52 1.88 1.47 1.00
8.47 6.32 4.94 3.36
Muncie OD (tho I don't suggest this gearset) ratios:
2.20 1.64 1.00 0.87
7.39 5.51 3.36 2.92
TKO 600 ratios:
2.87 1.89 1.28 1.00 0.64
9.64 6.35 4.30 3.36 2.15
TKO 600-RR (my personal preference) ratios:
2.87 1.89 1.28 1.00 0.82
9.64 6.35 4.30 3.36 2.76
Richmond OD ratios:
2.89 1.85 1.31 1.00 0.77
9.71 6.22 4.40 3.36 2.59
T56 ratios:
2.66 1.78 1.30 1.00 0.74 0.50
8.94 5.98 4.37 3.36 2.49 1.68
Using the formula below you can plug in the lower numbers to calculate RPMs for given speeds in whatever gear with your current 3.36:1 FDR.
RPM = (MPH x Gear Ratio x 336) / Tire Diameter
(Note that the 336 in the formula has noting to do with your FDR.)
Hope that helps.
Muncie M20 & 21 (wide) ratios:
2.52 1.88 1.47 1.00
8.47 6.32 4.94 3.36
Muncie OD (tho I don't suggest this gearset) ratios:
2.20 1.64 1.00 0.87
7.39 5.51 3.36 2.92
TKO 600 ratios:
2.87 1.89 1.28 1.00 0.64
9.64 6.35 4.30 3.36 2.15
TKO 600-RR (my personal preference) ratios:
2.87 1.89 1.28 1.00 0.82
9.64 6.35 4.30 3.36 2.76
Richmond OD ratios:
2.89 1.85 1.31 1.00 0.77
9.71 6.22 4.40 3.36 2.59
T56 ratios:
2.66 1.78 1.30 1.00 0.74 0.50
8.94 5.98 4.37 3.36 2.49 1.68
Using the formula below you can plug in the lower numbers to calculate RPMs for given speeds in whatever gear with your current 3.36:1 FDR.
RPM = (MPH x Gear Ratio x 336) / Tire Diameter
(Note that the 336 in the formula has noting to do with your FDR.)
Hope that helps.
Last edited by TheSkunkWorks; Aug 30, 2008 at 02:24 AM.
