I generally rent with Hertz, but also use Avis sometimes and some of the others very occasionally. I find the number of UA miles I earn is completely unpredictable. I use whatever code gives me the lowest rate, but very often it is the UA code. Even then, the number of miles earned seems to vary massively for no discernible reason - and, at least as far as Hertz is concerned - they don't employ anyone capable of explaining anything.

I once got 1500 miles from Avis, which seemed like a great deal - so good it encouraged me to rent again from them the following month, when I got 50 (IIRC), which put me off completely. Hertz generally gives around the 500 mark, I find.