63 impala rear end - Impala Tech
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post #1 of 6 (permalink) Old 08-27-2017, 10:53 PM Thread Starter
Join Date: Sep 2012
Location: California
Posts: 45
63 impala rear end

I have a couple of questions I'm hoping some one can answer for me. I have th350 transmission in my 63 impala now. I've never been able to get the car over 50 mph with it. I was planning on going with a 700r4 transmission but I started thinking, could the rear end gears be the ones holding the car back? If so I'd rather invest in new gears first than maybe later change the tranny. I know the car was factory 283 with a power glide. The rear end appears to be a 10 bolt. I've had the th350 looked at by a shop & they said it looked fine.

Any other ideas?
Frankthetank is offline  
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post #2 of 6 (permalink) Old 08-28-2017, 01:43 AM
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Your 1955-'64 drop out ten bolt rear end was made by Chevrolet, back when GM made everything used in their cars (that started to change in 1963 when AC-Delco stopped making their own spark plugs buying rebranded Autolite plugs from Ford).

The highest gear ratio that will bolt into your differential case is a 4.88 gear. You would probably notice a 4.88 or even a 4.56 gear as you would indeed be RPM limited with a gear that steep and hydraulic lifters. These gear ratios required solid lifters and a ten grand tach for a reason. You are going to be spinning the motor up to hit any kind of speed.

A four series carrier also supported a 4.10 gear set which was a popular choice back in the day when people regularly ran solid lifters and pinned their rocker shaft studs in the head to keep them from being pulled out by the valve spring pressure required to spin a motor to ten grand.

Today you can not get a hydraulic cam past 6,400 RPM due to lifter pump-up, and weaker valve springs (though all modern heads now use screw in rocker arm studs). Because of this limitation the hot rodders choice for a "performance" gear set is today a 3.73 ratio. This is mostly because there isn't a new 3.55 gear set being made to fit a twelve bolt rear in a car (a gear choice used if you had the towing option on your full size to tow that Air Stream from the frigid North to visit the sunny South upon retirement). The 3.55 used in a truck won't fit in a passenger car 12 bolt housing because of the size of the pinion bearings, so the gear is common in trucks but rare in a car. The 3.23 and 3.07 are very common ratios, and probably running neck and neck with the 3.07 is the 2.73 ratio in terms of gear sets encountered.

Your rear end doesn't have an inspection cover that can be popped off to count gear teeth (how you determine your ratio) so you have to either guestimate by counting revolutions (highly inaccurate), or drop your center section out for inspection and refreshening with new gear oil and seals.

I would also say if you have a tachometer you could look up your rear gear ratio if you know your tire size, speed in high gear, and your RPM. There are a lot of these gear ratio calculators on line.

Big Dave
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post #3 of 6 (permalink) Old 08-29-2017, 10:04 AM
Join Date: Jun 2017
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Jack the rear of the car up in the air. Put a mark on one tire and a mark on the driveshaft. Turn the tire one complete turn, and note how many times the driveshaft rotates. For example, if you turn the tire one complete turn, and the driveshaft rotates exactly four times, you have a 4.00 gear.

Even with a 4.88 gear you should be able to go faster than 50mph. Something is wrong. I think a 700R4 has an over drive ratio of 0.70, so even that is only going to get you to 70mph if you do the math.
MikeN is offline  
post #4 of 6 (permalink) Old 08-29-2017, 10:49 AM
Join Date: Jul 2009
Location: Canada
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Yes, something is wrong. My 65 Impala 327/powerglide/3.08 car easily tops 70mph at cruise, with lots of power left. My 65 C10 230/manual three speed/3.73 cruises easily at 60mph.

These non-overdrive cars were built in the Interstate era, they will all do sustained 65mph with no problems.

Two doors, four doors, wagons, and ragtops.
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post #5 of 6 (permalink) Old 08-29-2017, 12:22 PM
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Since it takes ten years to make any progress on an interstate roadway due to (planning, land acquisition, construction, and construction of the feeder roads to upgraded to handle traffic. All of this takes time but mostly it is waiting upon the money. It wasn't until 1956 that Ike started the implementation of the interstate system based upon copying the Third Reich's military road system (autobahn) that he admired while in Europe.

The first contract signed was for upgrading a section of US Route 66 to what is now designated Interstate 44. On August 13, 1956, Missouri awarded the first contract based on new Interstate Highway funding; this work began on US 40 (now I-70) in St. Charles county Missouri. In October 17, 1974 Nebraska becomes the first state to complete all of its mainline interstate highway. It wasn't until October 14, 1992 that the original Interstate Highway System authorized in June of 1956 was proclaimed to be finally complete with the opening of the last section of I-70 through Glenwood Canyon in Colorado.

In 1965 GM engineers were still designing their cars to handle an average road speed of only 45 mph. Yes the interstate had been on the books for nearly a decade, but there wasn't enough of it built to justify raising the speed limit for their cars as yet (that happened in 1972). This delay is because GM designs a car to last only seven years and designs it to be able to run in conditions anticipated five years ahead of it's construction build year.

Big Dave
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post #6 of 6 (permalink) Old 08-31-2017, 06:56 PM
Join Date: Jul 2016
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Frank: I don't care what rear gear ratio you have (and I HIGHLY doubt it's all that deep) you should be able to go over 50 MPH. Let's start with a couple of basics. First, if you truly had that deep a rear end your engine would be screaming! Let's assume it's not. Is there a possibility that your throttle linkage is not allowing the carb to open past a part throttle? This could be caused by a whole bunch of different things...the linkage hitting the bottom of the air cleaner, the linkage hitting the firewall, the linkage being so far out of adjustment that when the peddle hits the floor the carb is only partially open. Can you rev the motor up fairly high in neutral? Next, is it possible the transmission is not shifting into high gear? This could be caused by any number of things. A turbo 350 shifts twice....first to second around 10 mph, and second to third in the low 20 mph range (under normal throttle pressure, not being floored). As a last resort, borrow or buy a cheap tach and you don't even have to mount it. Just wire it and set it on the seat. Now you can see the engine RPM's as opposed to speed. You shouldn't be turning over about 2,000 at 50 MPH if the transmission has shifted into 3rd gear. If you really are in the upper 4K to almost 5K range and only going 50 the transmission is the problem.

Back to the rear gears for a second.... the rear end that came with your 283 Powerglide should have been a 3.06. It's possible that someone else changed it, but even so a 4.10 would still take the car to 100 mph at 5,000 RPM. As someone else mentioned, you can get a rough idea by jacking up the car, mark the drive shaft or U-joint with tape, mark the tire and make TWO full revolutions of the tire. I know this sounds strange, but I've been told by two different sources and watched a video and you must turn the tire two times, not once because when only one wheel is turning is actually "halves" the ratio. I proved it on my own car. Did it several times to be certain and it works. You should get about 3 and a fraction revolutions of the drive shaft to two rotations of the wheel. This would give you somewhere in the low 3 to 1 ratio.

Good luck. Hope you find the problem.
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