Rear End Ratio ?
Thread Starter
Race Director
Joined: Mar 2016
Posts: 14,325
Likes: 8,130
From: Napa Valley California
Rear End Ratio ?
I have a 1973 Coupe, with the 454, TH400 automatic trans and 3:36 gears in the rear end. I'm thinking about swapping the rear end gears to the optional 3:08's to drop a couple hundred RPM's at cruising speed on the highway. Does anyone have the optional 3:08 rear end ratio? What RPM are you seeing at 70 mph? Does the trans want to kick down on an incline? Does the motor bog down because the RPM's are to low? How does the lower RPM's effect cruising through a winding road? What are the positives and the negatives?
Race Director
Joined: May 2006
Posts: 18,692
Likes: 2,572
A 27" diameter tire with a 3.08 and 70 mph equals 2,683 RPM versus 2,927 rpm currently with the 3.36. The 3.08 would have been an optional rear on a 1973. If your engine is stock there should be no shifting or bogging issues.
Last edited by MelWff; Apr 5, 2016 at 12:49 PM.
Melting Slicks
Joined: Feb 2006
Posts: 3,105
Likes: 96
From: Sunrise FL
Couple things I will add. The more torque, the more a 3:08 would be fine (read BB here). A light car (read vette) will also have less issues with the 3:08.
I have had torquey V8 cars with 3:08/TH400 and liked the gear for a cruiser.
As far as bogging, no problem.
Another thing to consider: a properly matched torque converter can really make a huge performance change with a tall 3:08
I have had torquey V8 cars with 3:08/TH400 and liked the gear for a cruiser.
As far as bogging, no problem.
Another thing to consider: a properly matched torque converter can really make a huge performance change with a tall 3:08
Last edited by mikem350; Apr 5, 2016 at 04:26 PM.
Team Owner
Joined: Jun 2001
Posts: 43,136
Likes: 155
From: Thomson Georgia
I had a 3:08 gear in my car and it was okay with my stock 74. When I decided to put some horse-power in the car I immediately switched the rear end to a 3:73 and installed a 700-R4 and and it really made a World of difference. Wish I had done it earlier.
CF Community Team
Joined: Oct 2004
Posts: 19,357
Likes: 5,242
From: Cape Cod, Mass.
2025 C6 of the Year Finalist - Unmodified
2025 C8 Z06/7/E-Ray of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
2021 C8 of the Year Finalist Unmodified
2020 Corvette of the Year Finalist (performance mods)
2019 C1 of Year Winner (performance mods)
2017 Corvette of the Year Finalist
2016 C2 of Year
2015 C3 of Year Finalist
My 68 427/390 with Turbo 400 and 3:08 rear turns around 2600 RPM's at 70 mph. It's a stump puller off the line and there is no unnecessary downshifting going up inclines or driving on back roads. Big blocks make so much low end torque and the Turbo 400 1st gear is a perfect compliment to pulling well out of the hole. Not a huge difference though, between the 3:36 and 3:08. A bit better on the highway if you like to travel in it.
Thread Starter
Race Director
Joined: Mar 2016
Posts: 14,325
Likes: 8,130
From: Napa Valley California
My plan is to do a frame off restoration next year. Beside everything else, I plan to go through the motor and increase the compression ratio to 9.5:1, do a 30 over bore, install a good bottom end to mid range camshaft, balance, replace the heads, install a good intake, headers, new ignition and a new carb. I plan to cruise down the highway, through the foothills, the Sierras and Lake Tahoe.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
I have a set of 3.08 gears; they will drop the rpm 10% from what you're seeing now.
As far as performance, the 3.08 can work well if one uses a higher stall torque converter such as a tight 10" converter. With my former '78 Vette running a 406 SBC with a Quadrajet carb on an Edelbrock Performer RPM (pn 7104) manifold and a TH350 trans, I could turn easy 11 sec quarters with short times of ~1.75 sec. The advantage of the 3.08 is that it would hook better without blowing away the tires than a lower rear end such as a 3.70.
Mileage wise, I could get about 17 mpg on road trips due to the good hook up of the tight Continental converter.
As far as performance, the 3.08 can work well if one uses a higher stall torque converter such as a tight 10" converter. With my former '78 Vette running a 406 SBC with a Quadrajet carb on an Edelbrock Performer RPM (pn 7104) manifold and a TH350 trans, I could turn easy 11 sec quarters with short times of ~1.75 sec. The advantage of the 3.08 is that it would hook better without blowing away the tires than a lower rear end such as a 3.70.
Mileage wise, I could get about 17 mpg on road trips due to the good hook up of the tight Continental converter.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
Thread Starter
Race Director
Joined: Mar 2016
Posts: 14,325
Likes: 8,130
From: Napa Valley California
Actually, buying a new set of gears and having them swapped professionally can be accomplished for a reasonable price with no modifications. Since I'll be replacing everything in the rear differential anyway the gear swap will only be the cost of the new gears. The stock torque converter should work fine for my application. I'm still putting some miles on the car so I can see what it needs to perform and handle the way I want it to before I start the restoration. After all it is a Corvette and should perform and handle like the sports car it was designed to be.
Drifting
Joined: Jun 2012
Posts: 1,990
Likes: 183
From: Port St. Lucie Fl
I have a 1973 Coupe, with the 454, TH400 automatic trans and 3:36 gears in the rear end. I'm thinking about swapping the rear end gears to the optional 3:08's to drop a couple hundred RPM's at cruising speed on the highway. Does anyone have the optional 3:08 rear end ratio? What RPM are you seeing at 70 mph? Does the trans want to kick down on an incline? Does the motor bog down because the RPM's are to low? How does the lower RPM's effect cruising through a winding road? What are the positives and the negatives?
Last edited by Kacyc3; Apr 6, 2016 at 08:13 PM.
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
Go with a 700R4. Can get a tranny and torque converter for $1100-$1200.
A more expensive route, but way more fun is to go with a TKO swap.
I've never heard of anyone going "up" in gear ratio, everyone always wants to go down.
A more expensive route, but way more fun is to go with a TKO swap.
I've never heard of anyone going "up" in gear ratio, everyone always wants to go down.
Last edited by htown81vette; Apr 6, 2016 at 08:20 PM.
Safety Car
Joined: Sep 2011
Posts: 4,109
Likes: 1,229
From: Madeira Beach, FL
2024 C8 of the Year Finalist - Unmodified
2024 C2 of the Year Finalist - Unmodified
2023 C2 of the Year Finalist - Unmodified
2020 C3 of the Year Finalist - Unmodified
My friend, I think you got everyone else's opinion what they would do.
But more important as asked what do you want from the car?
I have a 427/390 M-20 3:08 stock set up.
I can lug in 4th gear through a 15 mph school zone, then accelerate
out of it and be at 50 mph in a blink.
At 50 I can give the edelbrock 1401 a lil juice and be well up into 3 figures.
I too am under 3 grand at 70mph.
I do cruise nights, weekend drives with the 1st Sgt, and some spirited
twisty's along the river near my home.
I can drive all day at any speed in 4th with the 3:08 set up.
I also have AC, P/S, P/B, Tele wheel, so who ever ordered it originally meant to drive it.
If I was looking for a muscle car to do the old high school burn outs
I'd get a chevelle with line locks, turbo hydromatic 350.
This is America's one and only original sports car.
To me it's Americas Austin Healey on steroids.
Marshal
But more important as asked what do you want from the car?
I have a 427/390 M-20 3:08 stock set up.
I can lug in 4th gear through a 15 mph school zone, then accelerate
out of it and be at 50 mph in a blink.
At 50 I can give the edelbrock 1401 a lil juice and be well up into 3 figures.
I too am under 3 grand at 70mph.
I do cruise nights, weekend drives with the 1st Sgt, and some spirited
twisty's along the river near my home.
I can drive all day at any speed in 4th with the 3:08 set up.
I also have AC, P/S, P/B, Tele wheel, so who ever ordered it originally meant to drive it.
If I was looking for a muscle car to do the old high school burn outs
I'd get a chevelle with line locks, turbo hydromatic 350.
This is America's one and only original sports car.
To me it's Americas Austin Healey on steroids.
Marshal
Race Director
Joined: Oct 2008
Posts: 19,171
Likes: 1,668
From: Long Island
I put the 5 speed in my 350 cu in 71, 3:36 rear. I turn 2000 at 80 mph. I have 400hp and 400 tq. It's nice to cruse with the new Vetts and get 24 mph.
Back in my younger days we drove all over with 3:36 and 3:55 rears, no one thought anything of it.
Back in my younger days we drove all over with 3:36 and 3:55 rears, no one thought anything of it.
Thread Starter
Race Director
Joined: Mar 2016
Posts: 14,325
Likes: 8,130
From: Napa Valley California
I also have AC, P/S, P/B, Tele wheel and power windows. The questions I have about swapping the TH400 for an overdrive automatic is:
1. Will it bolt right up to my 1973 454?
2. Will it handle the additional torque and horsepower of a mildly modified 454?
3. What about the transmission mounts, aren't they different?
4. Will I need to modify the frame mounting?
5. What about the tail shaft length? will I need to replace the drive shaft?
1. Will it bolt right up to my 1973 454?
2. Will it handle the additional torque and horsepower of a mildly modified 454?
3. What about the transmission mounts, aren't they different?
4. Will I need to modify the frame mounting?
5. What about the tail shaft length? will I need to replace the drive shaft?
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
I also have AC, P/S, P/B, Tele wheel and power windows. The questions I have about swapping the TH400 for an overdrive automatic is:
1. Will it bolt right up to my 1973 454?
2. Will it handle the additional torque and horsepower of a mildly modified 454?
3. What about the transmission mounts, aren't they different?
4. Will I need to modify the frame mounting?
5. What about the tail shaft length? will I need to replace the drive shaft?
1. Will it bolt right up to my 1973 454?
2. Will it handle the additional torque and horsepower of a mildly modified 454?
3. What about the transmission mounts, aren't they different?
4. Will I need to modify the frame mounting?
5. What about the tail shaft length? will I need to replace the drive shaft?
Burning Brakes
Joined: Jul 2015
Posts: 823
Likes: 57
From: Austin TX
I think you already have a nearly ideal gearset for today's highway speeds. My '72 454 had 3.73s. I made plenty of long trips, but the limit back then was 55 mph. Purely from a gearing standpoint, I agree with the concept of swapping to a 700R4. However, the TH400 is really a brute of a transmission. I abused mine for years, more than 80K miles, and it only finally started slipping because the filter clogged up. For that reason alone, I'd think twice about getting rid of one.
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
My 68 427/390 with Turbo 400 and 3:08 rear turns around 2600 RPM's at 70 mph. It's a stump puller off the line and there is no unnecessary downshifting going up inclines or driving on back roads. Big blocks make so much low end torque and the Turbo 400 1st gear is a perfect compliment to pulling well out of the hole. Not a huge difference though, between the 3:36 and 3:08. A bit better on the highway if you like to travel in it.
I had the same experience with my 74 big block 427 with a 4 speed. If your 454 is running correctly, you shouldn't have any bogging or hesitation. You either need carb work or possibly the hormonic balancer shifted and your timing is incorrect due to the timing mark being off.