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I'm in the middle of a rear end swap on the 56. I'm installing a 1979 Camaro 8 1/2 inch 10 bolt rear that has a 2.41 gear ratio. I'm removing the stock 3.55 geared rear. I'm wanting to increase my fuel mileage and keep from overworking the engine. I drive on a 75 MPH turnpike pretty regularly and everyone is passing me!! I keep it about 65. I did notice my driveshaft is going to have to be shortened. I have removed the Camaro perches and have brand new perches from Danchuk to put on. I also replaced the pinion seal, axle seals, rear cover gasket, wheel cylinders, gear oil, and brake shoes. This rear end measures 61 1/4 to mounting surface for the wheels. SO, is there a way to figure what my RPM would be at 65MPH with a turbo 350 trans, 2.41 gear, and 205/75/15 tires? I don't have a tach but I know its gonna be significant :party0031:
 

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Hi Hafrod, According to the Engine RPM calculator you will be 1994 RPM at 65mph with a 27.1" 205/75/15 tire and a 2.41 final drive ratio automatic. I attached the link so you can look at other gear combinations. mike

http://www.csgnetwork.com/multirpmcalc.html
 

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Almost 2000 RPM did not sound right to me. I guess I'm not figuring this correctly. It's showing with the 350 trans' final drive ratio of 1.0, 75 tire, 2.41 gear at 65 MPH = 702 RPM :confused0006: When I recalculated to see the RPM with the old 3.55 geared rear it showed 1071 RPM and I know thats not right! :confused0006:
 

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mph = (rpm x tire diameter)/(gear ratio x 336)

and

rpm = (mph x gear ratio x 336)/tire diameter


I'd account for the load on the tire.
You want to measure the loaded radius and multiply times two for the diameter.

At 75, it will be about an 1,100 rpm drop from 3.55 to 2.41. About 3,400 to 2,300.

At 65, it will be a little less than 1,000 rpm drop from 3.55 to 2.41. About 2,900 to 1,975.
 

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1956 chevy 210 del rey sedan
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where it asks for tire size it is asking for the diameter in inches not the aspect ratio size of the tire . where you put in 75 it should have been around 27.1 as mentioned in the other post just measure the dia. of you tier and put it in . good luck :party0031::party0031::party0031:
 

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I'd account for the load on the tire.
You want to measure the loaded radius and multiply times two for the diameter.
The load only occurs on the bottom half, like it's only flat on the bottom. :) If you double that radius, you'll have too much load and no round up. Round up will change with the speed also.
 

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The load only occurs on the bottom half, like it's only flat on the bottom. :) If you double that radius, you'll have too much load and no round up. Round up will change with the speed also.
Think about it.

The loaded radius is the only distance that the axle sees in relation to the ground. So, the diameter used in the calculation should be the loaded radius times 2. (Not accounting for round up).

The loaded radius really can't be calculated. It depends on tire width, load and air pressure (which is also a moving target due to heat).

I don't think there would be a way to calculate round up either.
 

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mph = (rpm x tire diameter)/(gear ratio x 336)

and

rpm = (mph x gear ratio x 336)/tire diameter


I'd account for the load on the tire.
You want to measure the loaded radius and multiply times two for the diameter.

At 75, it will be about an 1,100 rpm drop from 3.55 to 2.41. About 3,400 to 2,300.

At 65, it will be a little less than 1,000 rpm drop from 3.55 to 2.41. About 2,900 to 1,975.
That sounds right. On my 235 with 3 speed manual, stock 3.70, I run almost 3K on my tach at 60 mph.
 
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