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I have 1977 L48. I was just wondering if there is a way to tell if speedometer to tachometer will tell you the rear end ratio. At 70 mph. I'm exactly at 3000 thousand rpms?
Hi jh,
The differential case was stamped with a code for the ratio with which the differential was originally assembled.
It's stamped along with some date and source information on the bottom of the housing rear flange.
Might take a peek?
Regards,
Alan
I believe these are for 77 cars:
OA 3.08:1
OD 3.36:1
LR 3.36:1
OB 3.55:1
OC 3.70:1
Based on the info from Alan 71 and your tire size. My calculator shows your car with a 3.36 ratio. This is based on 4% slip (120 rpm loss) which is very close to what a stock converter should be in a TH400. The speed I show is 69.9 MPH @ 3000 RPM.
Don't assume that the factory tach is correct with its readings. My 78 OEM tach reads 300-400 RPM higher than the actual RPM's...check out the thread about tach accuracy. 78 L-48 autos came only with 3.08 and 3.55 gears. Check out the specifics for 77's at :
GM heritage center.com
My guess is that 77 L-48 autos used the exact same gears.....
Last edited by jb78L-82; Oct 18, 2017 at 05:01 PM.
My 73, TH-400 with 245-60-15 tires, with 3.36 gears runs at 2964 RPM @ 70 MPH.
If you want a more accurate way to check the rear end ratio that takes the tach error and tire size out of the equation, lift "BOTH" rear tires off the ground, mark the inside of the outer rim lip with a marking pin, and mark a single line on the bottom of the drive shaft, rotate the wheel one full revolution of "forward movement" and count the number of turns the driveshaft rotates. 3.36 turns and you have a 3.36 gear ratio, 3.55 turns and the rear ratio is 3.55, 3.08 turns 3.08 ratio, 3.7 turns 3.70 ratio, and so on.
It's the most accurate way without pulling the diff cover and inspecting the ring and pinion gears.
Last edited by OldCarBum; Oct 19, 2017 at 04:31 PM.
My 73, TH-400 with 245-60-15 tires, with 3.36 gears runs at 2974 RPM @ 70 MPH.
If you want a more accurate way to check the rear end ratio that takes the tach error and tire size out of the equation, lift "BOTH" rear tires off the ground, mark the inside of the outer rim lip with a marking pin, and mark a single line on the bottom of the drive shaft, rotate the wheel one full revolution of "forward movement" and count the number of turns the driveshaft rotates. 3.36 turns and you have a 3.36 gear ratio, 3.55 turns and the rear ratio is 3.55, 3.08 turns 3.08 ratio, 3.7 turns 3.70 ratio, and so on.
It's the most accurate way without pulling the diff cover and inspecting the ring and pinion gears.
I think the only way to really accurately determine the rear end gears is this method ^^^^^ unless you have owned the car since it was new....
My 73, TH-400 with 245-60-15 tires, with 3.36 gears runs at 2974 RPM @ 70 MPH.
If you want a more accurate way to check the rear end ratio that takes the tach error and tire size out of the equation, lift "BOTH" rear tires off the ground, mark the inside of the outer rim lip with a marking pin, and mark a single line on the bottom of the drive shaft, rotate the wheel one full revolution of "forward movement" and count the number of turns the driveshaft rotates. 3.36 turns and you have a 3.36 gear ratio, 3.55 turns and the rear ratio is 3.55, 3.08 turns 3.08 ratio, 3.7 turns 3.70 ratio, and so on.
It's the most accurate way without pulling the diff cover and inspecting the ring and pinion gears.
The problem is when you are laying on your back, turning the tire and trying to count the driveshaft rotations, its pretty much impossible to determine the distance of for example, 3.36 and compare to 3.55 on the shaft itself. What circumference does that come out to? A fraction of an inch. Some axles have enough play in them to fool you on the counting incorrectly. Even when marking the driveshaft with a piece of masking tape and a felt pen it can be difficult to measure accurately but will give you a rough idea.
Last edited by HeadsU.P.; Oct 19, 2017 at 08:44 AM.
The problem is when you are laying on your back, turning the tire and trying to count the driveshaft rotations, its pretty much impossible to determine the distance of for example, 3.36 and compare to 3.55 on the shaft itself. What circumference does that come out to? A fraction of an inch. Some axles have enough play in them to fool you on the counting incorrectly. Even when marking the driveshaft with a piece of masking tape and a felt pen it can be difficult to measure accurately but will give you a rough idea.
I calculate that the 3.55 would turn approx 1.5" further than a 3.36, assuming a 2.5" drive shaft diameter (ballpark, I don't know for sure that it's 2.5"). It should be pretty obvious if you've gone 3 + ~.3 turns vs. 3 + ~.5 turns.
I calculate that the 3.55 would turn approx 1.5" further than a 3.36, assuming a 2.5" drive shaft diameter (ballpark, I don't know for sure that it's 2.5"). It should be pretty obvious if you've gone 3 + ~.3 turns vs. 3 + ~.5 turns.
Could be. I slept through algebra class. Inch and a half seems like a lot though. I was thinking more on the lines of 1/2" difference?
Rear end ratio
