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I have a 1968 GTO that I am trying to determine the rear end gear ratio. I can not find any code on the axel. I am running a Munci 4-speed and the engine is turning 3,000 rpm at 60 mph. Based on the rpms and speed does anyone have a guess as to what the gear ratio is ? I was told it may be a 3:90 . Any help is appreciated.
 

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I use this online calculator
MOTOR - A Rear Axle Calculator
I guessed your tire height (26.5"), it said 60.6440 mph with 3.90 gears.
You can also remove the rear axle cover and look at the numbers on the ring gear. You'll probably have to jack the rear up and rotate the tires until you see the nimbers. It will have a bigger#/smaller#. Divide and you'll get your ratio.
 

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I think the easiest way to check is count the revolutions of the drive shaft on one complete turn of a rear wheel. Lift the rear end of the car off the ground. Transmission needs to be in neutral. Put a piece of masking tape on the drive shaft and a piece of tape on one of the rear wheels. Turn the rear wheel slowly one complete revolution and count the revolutions of the drive shaft. If you have nearly four revolutions, you have a 3.90:1. I suspect you will have about three and a half, in which case you have a 3.55:1. Regards, Matt
 

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^ what he said ( sorta )
1 lift the rear wheels
2 neutral
3 do both wheels spin when turned ?
4 if both turn same way you have posi and can mark the tire and axle and check ratio
5 if 1 tire spins forward and the other reverse you have an open carrier and need 1 tire
on the ground, mark the tire and axle. turn the tire 1/2 revolution and count axle
 

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What Matt said. No way would you be running 3,000 rpm at 60 with 3.90 gears. More like 3300 rpm, at best. I suspect 3.55 gears, which were standard issue with the 4 speed cars. It was a different world then, with .29 per gallon 100 octane fuel. Today, for street use, 3.55-3.90 gears are not a good choice.
 

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Discussion Starter #6
I use this online calculator
MOTOR - A Rear Axle Calculator
I guessed your tire height (26.5"), it said 60.6440 mph with 3.90 gears.
You can also remove the rear axle cover and look at the numbers on the ring gear. You'll probably have to jack the rear up and rotate the tires until you see the nimbers. It will have a bigger#/smaller#. Divide and you'll get your ratio.
I am running stock 14 inch wheels. BF Goodrich 225/70/14 I measured the tire heigth and is was 24.5 in.
 

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Looks like you are about to confirm you have a 3.55. If you are thinking about changing it, Geeteeohguy is providing good counsel here. Not the best gear ratio for the road today, but certainly better than a 3.90. Regards, Matt
 

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With a 3.55 carrier, you can change to 3.36 gears if you can find them. They will make a big difference. You can also change to a 27" or 28" tall tire and that will help a bunch getting the revs down. I changed the 3.55's in my '65 to 3.36 's, and went to a 235/70/15 rear tire, and the car cruises nicely at 70 mph....a tick under 3,000 rpm and right in the power band. It did ok with the stock 14 inch tires, too, which for these cars are 26.5". Your 225/70/14 should be taller than 24.5"....that's a 60 series height.
 

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I think you have 3.73:1

I had 24.5" tyres in my 67 GTO exact same tyre size as you, ran about 60mph at a fraction over 3000rpm. I have a M20 Muncie 4 speed with a 10 bolt 8" rear end. I fitted a 3.08:1 and it runs really well - far more drivable.


Sent from AutoGuide.com Free App
 

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With a 3.55 carrier, you can change to 3.36 gears if you can find them. They will make a big difference. You can also change to a 27" or 28" tall tire and that will help a bunch getting the revs down. I changed the 3.55's in my '65 to 3.36 's, and went to a 235/70/15 rear tire, and the car cruises nicely at 70 mph....a tick under 3,000 rpm and right in the power band. It did ok with the stock 14 inch tires, too, which for these cars are 26.5". Your 225/70/14 should be taller than 24.5"....that's a 60 series height.
Agreed. I would try to find a taller tire and see what difference that makes.
 
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