EcoPrincess! Welcome!
I think that my best suggestion is to say "Okay, you get X MPG. If you slow down and save gas, we will take the money that you save and then"--dinner, vacation, opera, something that she would enjoy, but maybe something that would be a little difficult for you, so if she sacrifices her overwhelming desire to get there first, you will sit through the opera. No, you would not end up saving money, and you would be stuck going to the opera, but maybe she would continue driving that way without extrinsic reinforcement.
RedPoint, utilities are included in my rent and my roommates are always leaving lights on. One is almost never in his room, he spends as much time in the front room as I spend in my bedroom, he even usually sleeps out there, but he always has lights on in his room. It seems like whenever I leave my room, every light in the public area is lit, but there often isn't anyone there, or they are in the living room, but the lights are on in the dining room and kitchen. After several months of me turning off the porch light and trying to remember to turn it on, my roommate complained about walking up in the dark, so I have left it on, so far, but I just looked into it. I do not like my options too much. Phillips makes fluorescent bulbs with built-in light sensors for $8 and G.E. makes screw-in sensors for $12.
Doviatt, of course she drove 80 MPH to buy shoes! They wouldn't come to her!
I hope that doesn't hurt your MPG too much this tank.
Awesome. Multi-quote failed me. I am trying to remember who else wrote comments to which I wanted to respond.
Frank Lee, I liked your ideas about making her pay her expenses and adjusting times so that you can be punctual, and that was an awesome joke!
Old Teleman, my sister has described me as honest to a fault. I only believe in lying to my enemies and I do not have any. Sometimes, I have been in bad situations, where telling the truth only seems to cause unnecessary problems, but I do anyway. That may violate the concept of self-preservation, but never as much as telling a woman that she looks fat!
Poomanchu, the joke was that had he not married the daughter, he would have been ending his twenty-year prison term.
Xntrx, I thought about the speedometer calibration, too, but is she "keeping up with traffic," racing everyone, or what? She might notice if everyone was suddenly driving 85 MPH, and then if you sell the car, do you tell them that you adjusted the speedometer, and therefore the odometer?
If you could somehow mess with the throttle cable so that the accelerator has the same travel, but never reaches WOT, she might just decide that the car has lost power over time, which does seem to happen, but like with the speedometer calibration, if it is sudden, she should notice.
I do not know what you could do besides install a governor. Maybe you could do that and reduce it 1 MPH per week.