When I drive my 72 240z (with the 4 speed) I have an RPM of 2700 for 50 mph. This seems like my car is geared too low. I have been told that I have a 3.36 ratio differential. On a stock 240z what should my RPM be for 50 mph?

That sounds about right but I thought that the diff was more like 3.9:1. It's definitely a higher RPM than you'd expect and I often want to shift into 5th when I suddenly remember there is no 5th. It's actually quite easy to calculate the speed in a given gear at a certain RPM. Here's the first Google hit for a "speed in gear calculator":

http://www.kabamus.com/garage/gears.html

With a stock 4 speed and a stock 3.36 rear end you should be at approx 59 mph @ 2700 rpm. It's possible that your speedo is reading incorrectly, it is after all quite old. Go here for a calculator.

http://www.geocities.com/Z_design_studio/transmission.html

MikeW, the 3.9 was not stock for the 240Z but is a common to the 280ZX and also a common upgrade for those swapping to the later NA 280ZX close ratio 5speed trans. The two are a perfect match.

I agree you must be running 3.90s. I bought a R-200 that was supposed to be 3.70 and experienced about the same thing you are. When I pulled the diff and removed the cover it turned out the diff was a 3.90. Went super off the line but suffered at the top end. I found a diff with a set of 3.54s out of a automatic Z . Now I have the best of both worlds.