I'm thinking that if your 30mph/40mph ratio is taken out to highway speeds, you get 55mph/73mph. Your wife would be zipping by everyone thinking she was only going 55mph, but in fact going 73mph. Unless she chronically gets speeding tickets, or knocks people out of her way wondering why people are "so dang slow", my guess is the problem is with your speedometer. Is your's in kilometers, has anyone changed rim dimensions on the bikes or car? Knowing how wildly my speedometer spins around, i'd throw in a few percentage points for error, and maybe a slightly underinflated tire could count for something. Good luck.