When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I've got a 71 Corvette that i installed an LS3 on. It has a muncie 4-speed but I'm not aware of the rear-end. The problem is that the RPM is way high for the transmission gears, where on the 4th (65mph speed) the RPM is around 3.5 - 4 (x1000), which is too high!!!
So, what would be the best rear-end gear ratio for this setup.
did you use a GPS to confirm you are doing 65 mph?
there is a big difference between 3,500 rpm and 4,000 rpm when you are trying to determine rear end gears, can you be more accurate?
measure the rear end gear by counting the number of drive shaft rotations when turning the rear wheels one completer turn.
the "best" rear end gear is up to you and what your intentions are, low cruising rpm or faster acceleration. You state you installed as LS3 but give no indication as to any modifications, heads, camshaft, etc.
I've got a 71 Corvette that i installed an LS3 on. It has a muncie 4-speed but I'm not aware of the rear-end. The problem is that the RPM is way high for the transmission gears, where on the 4th (65mph speed) the RPM is around 3.5 - 4 (x1000), which is too high!!!
3.08 was the numerically lowest gear set used by GM. Look for the code on the bottom of the diff to determine what you have now. Be aware that changing to a 3.08 would reduce acceleration substantially so most people think using a 5-speed overdrive is the better solution.
did you use a GPS to confirm you are doing 65 mph?
there is a big difference between 3,500 rpm and 4,000 rpm when you are trying to determine rear end gears, can you be more accurate?
measure the rear end gear by counting the number of drive shaft rotations when turning the rear wheels one completer turn.
the "best" rear end gear is up to you and what your intentions are, low cruising rpm or faster acceleration. You state you installed as LS3 but give no indication as to any modifications, heads, camshaft, etc.
No GPS, I was considering the speedometer and relative to the other cars on the highway the speed limit being 75 mph. Based on the RPM, it’s at 3.5 when cruising on the highway at 65 mph on 4th gear, when trying to push on throttle, it reaches 5 rpm.
I’ll the driveshaft revolution a shot to check. The LS3 is stock. What I need is for the car to run normal, speeding when need to. Right now the car takes off like it was possessed. I can actually drive the on 4th gear going on short roads.
3.08 was the numerically lowest gear set used by GM. Look for the code on the bottom of the diff to determine what you have now. Be aware that changing to a 3.08 would reduce acceleration substantially so most people think using a 5-speed overdrive is the better solution.
I’m not sure I want to spend the money and time with installing a new transmission and dealing with the interior adjustment. Thanks though, I appreciate the info. I’m sure a 5-speed would make it work!
But, I actually wanted to know if anyone had done this modification with an LS3, and what type of rearend did they use. My LS3 is stock
Not sure if you're setup is stock or not? If I were you, I would perform the standard test to figure out your current rear gearing ratio:
raise the car so the rear suspension is in the air
make sure the car is not in gear
spin the tire by hand for one rotation
count how many times the driveshaft spins when tire is rotated once
If driveshaft spins approximately 3 1/2 times, you probably have a 3.55 ratio (4 times=4.11, 3 3/4 times =3.73, etc)
Once you know the existing ratio, then you can start to determine what ratio you want. You would want to go to a lower ratio, like a 3.23, to reduce the rpms. That will also reduce the cars quickness. Everything comes with consequences. Only you can decide what your priorities are.
To confirm mph, you can download the speedometer or speedbox app on your smartphone and go for a cruise to check. Works well and you'll know for sure. Then, measure your tire height or calculate using the tire/wheel size and use an online formula to calculate your rear ratio. You may find you have too short a tire, or too much gear, or that your speedo is off a bit, or all of the above. From there, you can determine changes that make sense.
If you're running 3500 rpm at 65 mph on a 27-inch rear tire, you have a 4.33 rear end. If you're turning 4000 rpm at 65, you have a 4.94:1 rear end. Not likely unless somebody has done a bunch of rear end work on your car, or your car has one of the rare special-order 4.56:1 rear ends. One or more of your measurements may be off.
I run a 3.73:1 rear end, which provides excellent standing-start acceleration with 3020 RPM at 65 MPH. The common factory 3.08 rear end will give you 2490 rpm at 65 if you're running a correctly-sized 27" tire.
My bet is that your rpm numbers are not correct.
Here are the 1971 rear axle codes and corresponding ratios with the rpm that the engine will turn at 65 mph with a 27" tire:
AA 3.55:1 2870
AB 3.70:1 2995
AC 4.11:1 3325
AD 4.56:1 3690
AW 3.08:1 2490
AX 3.36:1 2720
LR 3.36:1 2720
The code is stamped clearly into the top surface of the rear end housing:
In this case, the "AZ" code is a 65-67 Vette 3.55:1 Posi.
LS3 s rev pretty well. So any of the factory gears from 3:08 to 3.70 would work well for you. Refer to the chart Lars posted above. The 3;08 in particular is going to give you a nice cruise speed, but will have the laziest acceleration. Sound like right now you could have either a 370 or a 411
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I have a 3.55 rear end by the spin and count method. At 3500 rpm I am going 80 mph, both needles straight up. You either have a 4.11 or 4.56 rear end. The best way to figure it out is the spin and count method that everyone else has suggested. My case stamp says I have a 3.70 rear which it obviously does not.
The most reasonable way to a 5 or 6 speed is to find one used locally, A WC T5 is the least expensive, but they generally wont last depending on whats in front of them and since you have an LS, you probably have a huge amount of horsepower. The next is a T56 which is the next step up from the T5 and are used still. The last and best is a TKO500 or 600.
You will need to do some research as to it bolting up to your bell housing (which the last 2 should) and the shifter modification. I believe the WCT5 and T56 will need a spacer for the input shaft and the T5will need an adapter to align the tranny depending on the model. Just avoid the WC-T5.
The T56 and the TKO will need a crossmember adaptor or a new cross member and the driveshaft shortened.Then you will have to find a tranny that doesnt need a rebuild.
For the best bang for the buck, least amount of headaches, knowledge that its new (not someone elses problem) ,ease of just bolting it in and not having to hunt for parts, I would just spend the money on a complete kit and get the TKO. Then you can decide which 5th gear ratio you want and if you need a 6 speed at all. I would obviously have to save for a couple of years to afford it or go with their pay then play program which is about $300 a month then get it when its paid for.
If you like the way your car launches off the line but you dont want to spend all that money, find yourself a 3.55 or 3.70 rear end for $500 or have yours rebuilt with a new gear for $2000. I have a M21 and a 3.55 gear and it sucks off the line. I just rebuilt my tranny into a m20 for the lower first gear and also went to the 3.70 rear. I will rev about 3700 now at 80 mph but again, its not long trips
I love my car and running around locally but no more than a 5 or 6 hour drive with my 4 speed and revving engine. I plan on some cross country trips some day and a 5 speed will be the only way that will happen. Good luck. You will have to do some reasearch and then find a way to get what you need out of your wallet.
I would suggest confirming speed, GPS download on phone. Confirm RPM, ELM Bluetooth, phone app.
Unfortunately changing rear end ratio in vettes is expensive, but you could easily run a 3.08.
If you keep the rear end, you'll still need to know the ratio and rpm to pick the correct trans ratios. The TKOs are available in a couple different ratios. A Richmond 5 speed or 6 speed (no longer available new) are available in multiple ratios that will work with your current rear. They are also a pretty easy swap for a Muncie. Just require a crossmember mod.
I wanted to mention Waze GPS app shows your MPH and also the posted speed limit and shows you if you are speeding, good if driving in a area you are not familiar with, also it is interactive and tells of police, crashes, etc reported by other users.
Had it on my girlfriend's phone and we went to California for a trip in the Corvette, Waze kept telling of wrecks on Los Angeles freeways, and police cars, we encountered about 5 crashes in a half hour and all were accurate except one that must have been minor and the cars left.
At the time my old iPhone didn't have enough memory to download it so that's why she put it on her phone, plus made her the navigator. Lol
Just installed on my new phone last night and tried it last night, the mph was spot on.
Nobody has mentioned the T56 Magnum, a brand new transmission that is actually better than factory T56, it's expensive, but one of those things you get what you pay for.
It's extra heavy duty and rated for 700 ft pounds of torque, better shifting.
There are 2 sets of ratios, the one I'm interested in is the wide ratio, which has really deep overdrive gears .74 5th gear, and .50 6th gear, to put into my 1965 C10 pickup, the 3.73 gears and TH400 are not suited for freeway driving.
My 1999 Corvette has the T56 with .74 5th gear, and .50 6th gear, with 3.42 rear gears, at cruise the RPM is low and can pull over 30MPG on road trips on freeway, I love that!
I know a lot of guys make fun of someone who has a Corvette that considers mpg "get a Prius if you want mpg!" but if the Corvette is your daily driver it matters more, especially road trips and being able to drive somewhere without having to look for gas in the middle of nowhere because it sucks the tank dry so fast.
The TKO transmission is good also, made by same company, however it only has one overdrive and can't get the really deep overdrive, can't be revved as high as Magnum T56, can't handle as much power, and doesn't shift as smooth, I have read this all by the company that builds these.
It depends on what rear gear ratio you have, if I recall TKO is about 2600.00 and Magnum T56 is about 3200.00, not cheap!
On the shifter, they have different locations, and adapter parts so you can use your factory shifter location, which in a classic Corvette is important.
I called and asked questions about my 1965 C10 so these may not apply to C3 Corvettes, asked if I have to buy a new transmission cross member, he said no, I verified it doesn't need a different angle or anything, he said some people want a new cross member.
My pickup was originally a 283 SBC and powerglide, then a 327 and TH350, now a 396 big block and TH400, just moved the cross member back in the frame.
Not sure what a C3 cross member is like, maybe the stock cross member could be reused?
The price of this T56 Magnum and related install parts, you could buy a lot of gas and keep your 4 speed, but how much do you drive the car?
I want the T56 Magnum for my 1965 C10, see how it does with 396, it's expensive probably $4000.00 for transmission clutch quick time bell housing hydraulic clutch setup, but I want the C10 to be a daily driver.
Last edited by 1999corvettels1; 02-18-2019 at 12:48 PM.
I've had a TKO 600 in my LS3 swapped 73 Camaro. It was a good choice. I was able to fab up mechanical clutch linkage so I didn't need the hydraulic clutch stuff. I did have to cut the tunnel to keep the drive line angle down to 3 degrees. It shifted OK. Ran 3:73 gears. At 60 mph on the Power Tour it did 25 mpg.
I currently have a T56 magnum in my 68 C10 LS swap. It shifts much better than the TKO. Used a standard T56 bell housing, LS7 clutch and flywheel and 2002 Camaro slave cylinder. Tick speed bleeder. Wilwood 3/4" master cylinder. The T56 is a bigger longer trans than the TKO. I suspect you will need to mod the tunnel to make it fit. Also a new trans cross member. Nice part about the T56 magnum is that it has a VSS signal port and a mechanical speedo port. I looked at rebuilt T56's and buy the time you had it rebuilt it was very close to the new magnum price wise.
To 1999Corvettels1 you will need a high hump trans tunnel cover to clear the T56. Also if you have swb truck and one piece drive shaft check your clearance where the drive shaft passes thru the trailing arm cross member. Mine was really close. A guy who goes buy "CaptianFab" makes some great C10 clutch bracket parts.
02-17-2019, 06:44 AM
71 Corvette LS3/Muncie 4-speed/and ??
