Diff ratios

Thought we could have a useful (wtf am I doing making useful prarrie canoe) thread on final drive or diff ratios so people looking to change can figure out what they've got and what to look for if they want to change. So for anyone not so clued up, the ratio is dictated by the crown wheel and pinion within the differential, and is referred to generally by a number. The pinion is turned by the input shaft or drive shaft. The splines at the end of the pinion engage in the teeth on the pinion wheel to transfer this motion through 90*. The crown wheel through the mechanism in the diff passes this drive to the output shafts, the prop or half shafts. For example a diff ratio of 4.1:1 means that the input shaft spins 4.1 revolutions for 1 revolution of the output shaft. What this translates to in practical terms is that on a standard gearbox in a given gear the car will move at a set speed. 

For sakes of argument lets say that at 3000rpm in 5th gear the car is travelling at 50mph, with a diff ratio of 4.1:1. We then change the crown and pinion to a ratio of 3.9:1. This means that the input shaft has to do less turns to produce an output, and that for example to maintain 50mph in 5th gear the engine now turns at 2800rpm. In practical usage this means that a greater economy can be had to a certain point by having a shorter ratio diff. The trade off for this however is that for this economy is a reduced acceleration up to this speed.

Now for informations sake, can people with knowledge provide the 50mph/top gear rpm, I will update this post and chart here