I was obviously assuming that the amount of slippage would be similar at a fixed speed and power between 2400RPM without overdrive and 1850RPM with overdrive. It looks as though I must have got this very wrong.
Gear ratio can't vary so slippage % must, mph speed is the same but power is usually less at these lower rpms especially as they get lower and the speed the fluid coupling in the converter is probably less etc.. these must have an effect.
