Ok so I've rented with Hertz twice. I turned 25 in November and previously rented with Enterprise and Thirfty since they were the most lenient with the age thing. I got #1 Gold from my company's plan and have rented with Hertz twice, rented once with Thrifty and once with Ent since I turned 25 as well, since sometimes Hertz is just way too much money.

I will say that Hertz cars are MUCH nicer and often are more premium. For instance they're more likely to have leather, sunroofs, climate control, etc. Thirfty always rents stripped chryslers.

Anyways, both times at Hertz I've rented midsize with free upgrades to full sizes. First time I got a Taurus, this time an 06 Camry. FWIW, the Taurus is much nicer.

Apparently, midsize is a Corrolla or a G6; is that true? A G6 is much nicer/bigger than a Corrolla. In fact, I'd prefer a G6 to the Camry or Taurus.

Is this true, or just a weird LAX or SNA Hertz thing? What other cars are worth getting in each class?

the corolla is a (c) class - midsize
the g6 is a (f) - fullsize (according to the keyfob of the last g6 i rented)

i believe hertz also considers the fusion as a (f) fullsize for some odd reason

Hmm, strange.

Last G6 I had was considered a YC and the last Fusion I had was a C.

Classes vary by location. In Canada there seem to be some Fs which might be C or YC in many US locations.

Four-cylinder Fusions I've gotten have been C. Six-cylinder SEL models on the other hand were YC. Most of the G6s I've had have been YC as well.

What I have seen appears that the Y designator is a non Ford vehicle. Until recently Ford had controlling interest in Hertz. Now they are owned by The Carisile Group. So a Taurus is class F but an Impala is YF.

Technically the "Y" modifier in the front is supposed to mean "premium" within that class. Thus "F" is full-size, and "YF" is premium full-size.

I have to admit quite frequently I can't tell the difference. The only thing I know is that I'm not allowed to pick up Tauruses unless I'm by myself (while I may not care, someone else does).

Over at my side at AVIS...

Group C - Intermediate
Ford Fusion
Pontiac G6

Group E - Full Size
Chevrolet Impala
Ford Taurus

Anyhow, many of us have a Ford Taurus block in our profiles and well as for the dreadful HHR/PT Cruiser...

There is a pretty clear difference with the Y, and your premium sums up the designation.
Y_ means:
you cannot reserve that specific class. it is only available on day of upsales and upgrades.
premium--usually some of the following: leather seats, sunroof, special trim packages, a car slightly more luxurious than the rest in the class, but would not fit into the next class.

I supposedly have a Taurus block and they give them to me anyway.