Edit,

I checked the axle rotations with the Billman method, I have 4:57's, they are really close to the 4:77 at crusing speed.

 

According to the gear calculator, 4000 rpm with 4.77 in 6th will give you 64 mph. No need to have someone go out and try it.
4.57s would be about 67 - 68.
It appears you have 4.77s in there.
It may be too late for you, but for anyone who gets gears, count the teeth in the ring and pinion BEFORE you put them in.

 

Sigh...the gears came in a competed diff, I never had a chance to count them.

I thought it felt a little wound out on the highway.


Hey Hardtop guy we need to talk.

 

wow

 

Here's what ya do bruva....

Put the car on stands, in neutral.

matchmark the wheels to the backing plates.

Matchmark the drive shaft to the dif.

Now..using the wheel matchmarks, turn both wheels one turn.

Watch the amount of turns the driveshaft makes. If it turns barely more than 4.5 times, you've got 4.57.

If it turns 4 and 3/4 turns, you've got 4.77.

 

I hope you get it resolved, if a mistake has been made.

 

I am sure Jeff will make it right.

 

Awww... I'd take that diff off your hands if I had the dough and you could just start over

 

Tires can alter this as well. If you have tires that are smaller than stock this will increase rpms as well.

But, it would be rather strange to have exactly the same rpm you should with 4.77s.

 

WOuldn't that be like a free upgrade, or is there a reason that 4.56's would be preferred over 4.77's ??