This program calculates which speed you will have at a certain engine speed, depending on the type of wheel/tyre and gearbox and differential ratios on your car. As a bonus the program also shows which speed the car will do at 1000 rpm.
If you want to experiment with wheels or various differential ratios it is possible to see the effect it will have, the program even calculates the effect on the speedo meter!.
Fill in the original spec's in the first column and the new spec's in the second column. You will see that fitting larger tyres or a longer reduction (lower diff.ratio) will result in the speedo indicating a lower speed than you are really driving. Watch out for the fines!!
Also notice changing gearbox ratios doesn't effect the speedometer! this because the SD1 measures the prop shaft speed as an indication of the car's speed. Everything you change after the prop shaft affects the speedo. Everything you do before the prop shaft doesn't.
The standard values are showing what happens if you fit a standard SD1 (185/70HR-14") with Vitesse wheels (205/60VR-15").The speedo than will show 2% less than you are actually doing. Well off course this program doesn't work just with SD1's. Adjust the gearbox/differential ratio's and select the right tyre, and off you go....