[Christ]

So now she's got something to shoot for.

For you - instead of telling her what she's doing wrong, try encouraging her to do some more "right" things. Noone likes to be told they're wrong about something, but many people are willing to accept advice on how to do things "right".

Offer advice while you're riding with her, but don't be overbearing about it. Like when she guns it off a red light, say something like: "Ya know, you're probably better off just slightly pushing the throttle until you're through the intersection, if you want to get better gas mileage."

Or like, when she passes someone on the highway, suggest that instead of passing, she just fall in behind them. Let her know that it will only take a few more minutes, and she'll be saving money on the deal.

If she's into math, you can do a little bit to actually show her how she'd be saving money by doing things only slightly differently. Who knows, you might actually get her into modding as well!