Dave C.
If nothing else, speedometers remind speed-freaks to slow down when they might not be aware how dangerous they're getting. Automatic transmissions definitely require them since it's hard to know what gear you're in. I drove a stick for years with no tachometer but it's easy enough to tell what's going on with RPM by sound and feel. Of the two, I'd take a speedometer since I'd rather not get tickets.
Once you're in top gear, a speedometer is as good as a tachometer for gauging efficiency. Higher RPM in top gear means more fuel burned in all but the rarest circumstances (fuel pump problems, etc..) The whole purpose of having an overdrive (top) gear is to *reduce* engine revolutions to increase efficiency! Once you're in overdrive, the only way to improve MPG is to keep revolutions as low as possible without lugging the engine. Do you deny that?
When you open the throttle, you let more air into the cylinders, which draws in more fuel (via carburetion or EFI computers), which creates more engine cycles, which produces more power. You can't get better MPG when you're drawing more fuel into the engine. If someone made a system that *choked off* fuel at higher RPM, your bbutt-ackwards figures might work, but it would be a pointless design.
C.T.
