Calculate City/Highway from Avg Speed

[spikejnz]

It would be nice if aCar would calculate City/Highway split automatically from average speed, the formula for which is as follows:

(AVG-21)/27 = Highway %
Anything >= 48 == 100% Highway
Anything <= 21 == 100% City

Seems like it'd be simple to implement, and would save me from having to go back and edit my entries en mass at a later date using a Google Docs spreadsheet I have just for this very purpose.