I'd always approached making a car rental reservation the same way as I approached airline tickets: the earlier you reserve, the best rate you get.

Maybe this was totally naive, but this is certainly not the case.

I had an upcoming weekly reservation for a midsize car for $288. I noticed in a recent email blast a $25 off weekly rate coupon, so I went to go use that. But instead of modifying the original res, I started a new one and noticed that the weekly rate for a standard SUV was cheaper than the midsize, both of which were substantially lower than my existing reservation.

Bottom line, I now have a new reservation for a standard SUV, with the coupon applied for $201 for the week all in.

Which leads me to my question: is there a better day to reserve cars than others? Or is it just a question of continually checking what's available and re-reserving if the rates go lower?