THE race series for your racing Midget or Sprite

RPM / Speed / Tire Diameter Calculator

Ever wondered how fast you were going when..... ? or how much a faster a 3.7 diff is than a 4.2 diff ?.
Or how fast you drove through Eau Rouge at ?
Fill out the various boxes below & press a button. The answers will magically appear.
I have defaulted the various boxes with meaningful FISC values.

Tyre Details :
Default values in this section are for a 155/70 TR 13 - FISC legal tyre

Width (mm)         
Sidewall Ratio    
Rim Size (inches)

Back Axle & Gear Ratio Details :
Common Diff Ratios for a Spridget are 4.222 , 3.9 or 3.727
There are other ratios e.g. 4.555 and 3.55 Diffs, but these are as rare as hen's teeth (and not used much in racing).

Diff Ratio :

Gear ratios for a standard 1275 road gearbox are as follows :-
1st gear 3.2:1 , 2nd gear 1.916:1, 3rd gear 1.357:1, 4th gear 1:1

There are various ratios available for SCCR boxes.
Ratios for a "Special tuning close ratio" gearbox are as follows :-
1st gear 2.573:1 , 2nd gear 1.722:1, 3rd gear 1.255:1, 4th gear 1:1

- Default value is for 4th gear.

Gear ratio: :1

Calculations :
Since the max. allowable revs for a FISC car is 7,200 I have pre-filled the field. You may obviously change this as wished.
For this section to work you MUST have a value in ALL the boxes above.


at  Rpm

at Mph

at Kmh