['68OrangeSunshine]

LESS.
That is, with a "slow" speedometer, your odometer will also count more slowly. Good news is if your tires have a warranty, you're getting better warranty coverage.
Three possibilities:
1] At a speedo shop run on the dynomometer and clock your indicated speed against your revolutions of their rollers and get a graph to post on your dash. So you know that, let's say, 47 indicated is really 55 actual MPH. Callibrations from 10 thru 90 on the top scale, and actual reading numbers on the lower scale.
2] With the above knowledge, some shops can build a compensator box [they're about 1" x 2"x 3"] that goes between your speedo cable and the transmission. Borg-Warner used to make them. With this gizmo, you're cured. Your ODO will read good. [Within 1 - 2.5%, or so]. I happenned to get lucky, once. I had an old comp box - came with a junkyard SM465. It read all wrong, but I kept it anyway. Some years later when I switched from R8x17.5LTs to R235/85-16LTs, I tried it on for laffs. I was amazed, when it matched my GPS speeds within 2-3 MPH. So if you find the little box keep it. A speedo shop can change the internal gears or you.
3] Get a GPS, set it on MPH, and read that when driving. No odometer support however, but it's way accurate.
You could also get a helper to record your speeds, matching indicated marks with actual GPS readings, and make up your own calibration curve chart -- without going to a speedo shop -- like I had to do in the '80s.
Good luck.
Hope this helps.