Option 1 Signage

There aren't any guidelines for this which allows us the freedom and curse of authorship. We should consider who the audience is and how to best achieve the intent of the credit and rating system.
If your audience is General Public, perhaps a more explanatory sign could be placed near a grouping of these spaces to supplement the LE/FEV or leaf, etc at the spot itself. The allowable vehicles for the credit is more broad than "hybrid and electric" but if you think that's the wording that will work for people, go with it. Since its more restrictive than the LEED credit I think you stand a great chance.
On a small university campus I called for a sign reading "Reserved ACEEE Green Score 40+". Maybe students and staff are more of a mind to look it up and see how their car, or prospective purchase ranks. I realize they won't do that before parking there, but these spots aren't policed anyway and for this client it at least raises awareness of a metric.
On a corporate project with a huge parking lot, we had over 40 spaces to mark. We used a broad green stripe where the spaces meet the parking lane and repeated the words "Low Emitting Fuel Efficient" about every three spaces within the green stripe. The lot was color coded this way for visitor parking (blue), compact parking (white), and LE/FEV (lime green). It works pretty well! It's big enough to read and recognize at a glance and for the quantity of spaces we had it was very economical.

You need to be very careful with sign language on this credit. We were denied this credit during design review from the GBCI recently for proposing Hybrids and Electric language which fits with the older population the project is serving. Their response stated 'The signage must indicate that the preferred parking is for low-emitting and fuel-efficient vehicles'.' They went on to state that not all hybrids are low-emitting or fuel-efficient and vice versa. That said, I like the green parking stripe idea. I think it visually communicates the sign intent.

We too were denied this credit during the design review for having a sign that reads, "Hybrid and Low-Emitting Vehicles." We received a similar comment saying that not all hybrid/alternative fuel vehicles are low-emitting or fuel efficient. The technical advice said we should provide signage saying "low-emitting and fuel-efficient vehicles only". They put that in quotes. This phrase is extremely vague and the general public probably doesn't know its meaning, but apparently GBCI doesn't want us to try to clarify or make it more understandable.

I have had the same comment on a few projects so I would definately use the exact verbaige of "low-emitting and fuel-efficient vehicles only". We had some latitude with the wording in v2.2 projects, but they have since tightened the requirements.

Parking signage for this credit must typically include the terms "Low-Emitting" and/or 'Fuel-Efficient," with the only exceptions being "Zero Emissions Vehicles" or "ACEEE 40+." Signage using solely terms like "Alternative Fuel Vehicles," "Hybrid Vehicles," or "Electric Vehicles" is not sufficient, because some hybrid vehicles, etc., do not meet the LE/FE definition.

Can the ACEEE 40+ rating be converted to a minimum MPG value? Many users do not know what the Green Score Rating of their vehicle is and these low-e and fuel-efficient designaged spaces are not being parked in because occupants do not understand the requirements. We would like to include the MPG value on our signage, if possible.