Glenn and a million other guys have said how to do this
My understanding:
Put car/truck in Neutral
If its "posi" put it on stands and turn the rear wheel 1 rev and see how many times the drive shaft turns.
If its limited slip, lift one wheel and leave the other on the ground. turn that wheel and count the driveshaft revs.
On my 62 Uni:
Well I jacked up 1 side in N and it wouldn't turn.
I jacked both sides and it turns but the opposite wheel turns the opposite direction.
I left both wheels up in the air and rotated it 1 rev.
The drive shaft turned 1.5 turns.
What do I have cause 1:50 doesn't make sense?
Also if its X2 then 3:00 is what the tag says but the thing is at like 1billion RPMs (no tach) at 60mph.
It's C6 is in 3rd gear too. If its a 3:00 gear I should be able to do 100mph all day long.
As it is it sounds wound out (speedo is off by like 20mph at approx 60+/-) and everyone is passing me.
Axle ratio- How to determine?
Axle ratio- How to determine?
Someday I'll get another slick 
