I just went out and drove mine and took some measurements, then looked up my diff gear ratio. (What do you expect, I'm laid off and bored.) I have no way to check actual speed...

I have the stock 8" 2 pinion 4.10 open diff, (B03A on the door jam).

I have 31" MTRs, w/13K miles that also measure at 30.75".

At 70mph (speedo) in 5th, I was at 2600rpms; in 3rd, (the way I normally drive), 4500rpms.

It doesn't prove anything, but it killed an hour! LOL