If I got a fine for speeding in a rental car from the company I would refuse to pay it. There is no way that a company can fine you based on GPS evidence. (I'm an electrical engineer, I do know what I'm talking about here.
) I wonder if they (rental companies) realize that the Department of Defense adds a random error to the GPS used by the general public. This *may* ( and does ) cause spikes in the speed measurement as they change this. In addition when you round a corner or go up a hill the GPS tracker has no way of calculating your true speed. (Radar has this limitation as well.) This is called the cosine effect. (You are measuring the cosine of the angle with relation to whatever heading the GPS unit "thinks" you are going at the time.) Like radar, GPS units measure speed in one direction only. This means that when you round a corner your speed will decrease on the GPS unit until you straighten out again. (Postition measurements are not affected, only speed.) The same thing happens when you go up or down a hill. GPS accuracy is also effected by the number of satelites the receiver is tracking at the time. (3 needed for postion, 4 needed for altitude and postition.) Accuracy generally improves as the number of satelites being tracked increases.
As for wear on the car, that is is double sided sword. I can tell you ( my father owns his own auto repair shop ) that many of today's cars are geared to run most efficiently around 65 - 70 MPH. (My car does not even shift into overdrive until you get to 50 MPH. ) This means that wear on the engine is less at 65 Mph than at 55. Wear on the tires is greater at higher speeds though. IMO I'd say that wear on the brakes is less if you aren't tapping them all the time to stay at exactly the speed limit.
If I were in this rental car, I know what I would do - I'd put aluminum foil over the GPS receiver's antenna.
If I did receive a fine I would probably tell them to take me to court, because I know that I could beat them. GPS (as of today) is so horribly inaccurate (often > 250 m ) with so much potential for small errors that I could just overwhelm them with evidence. Only differential GPS (which uses a ground based transmitter in addition to the satellites - fixed position translates into a more accurate position triangulation. ) is what I would consider accurate. (<50 m)
There, I'm through venting for now.