kerry3, your taller tires will cause the speedo cable to rotate more slowly than it did before at a given speed, yielding a lower reading on the speedo. The fix is to speed up the speedo cable with either a smaller cable drive gear in the trans tailhousing or an inline overdrive box. Beware of the speedo shop prices on the overdrive, especially in a rural area where there is no price competition. You can actually buy those overdrive boxes directly from the manufacturers.
I just went through this when I converted my '36 Chevy pickup from 4.11 to 3.55 gears. The exact overdrive ratio can be determined by driving a known mile and looking at the odometer distance measurement. In my case, the measured distance needed to be increased by 17% to give a correct reading. I gave that information to the manufacturer of the in line cable overdrive box and it was problem solved.
The gears inside the box are replaceable if you make another tire or gear ratio change in the future.
