The best way that I know of to figure this out is to do this....

Say you are in 3rd gear... Try to keep you speed consistant... say 40 mph.. then shift to 4th gear. Look at how much your RPM's dropped... that is your difference between 3rd and 4th gear. For me it was about 1500 RPMs.

Here's a quick step by step.. lol

1. Drive 40 mph in 3rd gear.
2. Look at RPMs
3. Shift into 4th gear keeping your speed as close to 40 mph as possible.
4. Look at how much your RPM dropped. That is the difference.

Of course you'd want to try this in all gears, because not every gear has the same difference. Basic concept is to stay the same speed (whatever it may be) and shift into next gear and look how much RPMs dropped.

hope that makes sense...

you can also do this downshifting.. it's the same concept.. except now you'll have to make your RPMs go up.. not down..