The problem I've seen with standard DC Motors is most of them will spin to fast at their minimum voltage to even get it to spin. The ones I've been able to find at least are like that. I also haven't been able to find any with an acceptable voltage range to be able to range it that much with any precision. That is why I was thinking a stepper motor, much more precision and much easier control over that precision. Also stepper motors usually can handle torque resistance a little better from what I've seen which there may be some of it in a cable going to the speedo.
If you know of any DC motors that can actually be modulated in this range though I'd take a look at them.