Wtf?

[tomt966]

Don't know what kind of rear axle ratio I have cause there is no tag. At 60mph and 2800 rpm I should be about 3:73 but when I turn the rear wheel one complete revolution, the driveshaft only rotates about 1.7 turns. What the hell is a 1:70 rear???? And how come I have "high" rpm @ 60mph? This is on a 8¼ rear on a 68 Dart. Got me confused!!

 

[pettybludart]

Something sounds wierd??Anyway I have 3.23 and 4.10 gears for 8 1/4 if interested??I,m in Ontario,Canada.

 

[BigStrok3]

I dont think your turning it far enough. I think mopars lowest 83/4 gear is I believe a 2.70 ratio.

 

[Swinger71]

If you have an open diff, the drive shaft may only be turning half of what it should. What size of tires do you have on the rear?

 

[tomt966]

If you have an open diff, the drive shaft may only be turning half of what it should. What size of tires do you have on the rear?

Open rear with P235/60R14 tires...... and its a 8 1/4 rear

 

[redfastback]

on an open rear the tires have to be on the ground while you roll on a creeper next to the car with someone else in the drivers seat.

 

[Swinger71]

With your tire size (25.1" diameter) at 60mph and 2800 RPM I figure your rear end to be at 3.55. that should be close and would make sense with the drive shaft rotation as well could have been 1.75 rotaions and then double it.

 

[charger70]

I think 71 Swinger is on it.
3.73 should have rotated closer to 1.9 for an open rear.
There are others out there but I use this Calculator to figure what gear or tire size to use.
http://www.richmondgear.com/01calculators.html

 

[pastortom1]

With your tire size (25.1" diameter) at 60mph and 2800 RPM I figure your rear end to be at 3.55. that should be close and would make sense with the drive shaft rotation as well could have been 1.75 rotaions and then double it.

Yup.........That accounts for the rpm's too.........your speedo would be about 10 MPH off at 50 (actual speed appx. 40 MPH or so).