Because the 21 is probably a little bit larger, and therefore makes fewer rotations per mile, your speedometer will read just a little slower. You could do the math, the diameter of the 19 as a percentage of the diameter of the 21. Say that it was 95% as big. That means your speedo would be 5% off. An indicated 60 would be an actual 63.
Actual being a loose term. Most speedometers aren't very precise, and usually read faster than actual speed. I used to figure actual speed by running as close to 60 as I could hold on a well measured 10 mile stretch of straight highway. (the highway mileage markers are close enough for this purpose) and timing myself. Then I'd do the math. Nowadays I'd just take the Garmin and my OL, and have her tell me how fast the GPS said we were going vs. my indicated speed.
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