In EVP, wheelsToEngineRatio is the ratio that is used to calculate the fictitious transmission rpm value. The average rpm of the driven wheels is used, then it's multiplied by wheelsToEngineRatio, which gives an initial number that is then converted to engine rpm based on the number of gears and the min-max gear rpm settings (gearDownRpm, gearUpRpm).
Basically, first you configure the number of gears and the min and max rpms for each gear. Then you configure wheelsToEngineRatio so the final gear is reached before the car reaches its top speed.