Originally Posted by Francis M.
the car with a lower IDR should be more efficient and might actually have better top end from lower rolling resistance due to having lower belt speed... I don't know if you will feel something a difference between a car with idr of 2.0 vs 1.9....
Now when you think of it 3 for seconds longer, the belt speed does not depend on the IDR, it's only determined by diff pulley size. Smaller diff pulley = lower belt speed.
I think the main difference actually comes from layshaft speed and spoolup. With higher IDR, the layshaft has to rotate faster for the same car speed, meaning the spoolup will also take more effort.
Efficiency wise, the best of both worlds would be to have lower IDR by making the diffs smaller, not the layshaft pulley bigger. Then again, the belts run more efficiently on bigger pulleys, so in the end it's all a big mess