We have always used the mile markers on the interstate and the odometer to pick the gear. But be aware that a family member in law enforcement has radared my car several times and the speedo's accuracy is not the same at all speeds.

Which is why Mopar offered certified speedos on the cop cars. Of course, the certified speedos came with a correction chart in the glove box because even they weren't accurate at every speed.

So I think that you can get the odometer zeroed in, but may have to pick a spot on the speedo to get closer than 3mph while accepting other speeds may still be off.