One Class Upgrades - how do you know to what?
 

I'm looking to book an SUV and as a First member I know I get a free one-class upgrade. Therefore rather than paying to book an SUV, in theory I can book the class down and get a free upgrade. But what is the class below that?

Looking at the "Car Guide" on avis.com doesn't help as I don't believe they are listed in order (otherwise I could book an Impala and get a BMW 528 :D), is it done on group letter?
If so, it would sound like I could book a Group E - Full Size Car and then hope for an upgrade to Group F - Intermediate SUV.

The only reason I didn't think it was as clear-cut as being the group letter as above, is that Group W is a Standard SUV, and if you do a test booking it is the cheapest SUV category. An Intermediate SUV Group F is much more expensive than the Standard SUV Group W.

So how do I figure out what my one-class upgrade would be if I booked in to a certain category?

Considering upgrades are "subject to availability", which boils down to the whim of the renting location, I would reserve what you really need and enjoy an upgrade from that if you get it.

You get a one class car upgrade if it's available.

It's not applicable to SUV's.