While deciding which differential to fit to my 840ci I wrote a program to visualize what difference it would make to the road speed. I hate just looking at numbers so I came up with this:
You can select the 4, 5 or 6-speed gearbox and choose from six different final drive ratio's. Just click on the rev needle and drag it around the dial and the speedo will reflect the road speed. It can be downloaded from here:
http://www.meeknet.co.uk/e31/Timms_E31_Gear_Monitor.zip
Download anywhere and double-click setup.exe - and it will install the program (hopefully). It works on most versions of Windows. As it is a zipped download your PC will complain, but you can trust me!
You can change the wheel diameter (so can be used for the E38) and the amount the speedo over-reads (4% by default)