There is more to it than just height and length, although they play a major part.
Raising the antenna up lessens the coupling to both the vehicle, and the surface (ground as it were). The lessens the APPARENT ground loss.
Increasing the antenna's length increases its radiation resistance. To a point, doubling the length increases the radiation resistance by a factor of four. In other words, increasing the length by 50%, doubles it's radiation resistance.
The over all efficiency is based on the total ground loss, stray losses, resistive losses (coil and conductor), and the radiation resistance, divided by the radiation resistance. The radiation resistance if effected by the length and frequency, obviously.
The hidden factor is the ground loss. Although the ARRL Handbook and other sources list the loss at 2 to 10 ohms (10 through 80 meters), in reality it can be much more than this. It is rather hard to measure and/or calculate.
In a recent technical article published in QST, Jack Belrose, VE2CV, lists an alternate way to arrive at a mobile antenna's efficiency, and in a round about way to determine the ground loss.
I know this doesn't directly answer the your question, with out all of the other variable factors (coil Q and placement in the antenna as examples), the question cannot be answered with certainty.