Make it simple; calculate the speed error in percent. That is the same percent you need to change on your trans gear.
If your speed-O is reading hi, then you need to slow the cable down.
If reading low, the cable has to spin faster.
Do this by adding or subtracting teeth to the Speed-O gear; whatever percent you calculated.
In your case;
35/30 =plus 16.7%, and you need to speed it up, so you need 16.7% fewer teeth. and from 21 that would be;
(1 less 16.7%) times 21=17.5, which you can round up or down.
BTW
As you can tell by the varied answers; this is NOT a stupid question.
Doing it by percent, gets you to within a half a tooth every time. By the chart, yur kindof guessing, cuz modern tires are metric and there is no accurate way of translating between the two systems.
Whereas math is just math, just get your percent error as close as you can, and the faster you are moving the closer will be the result.
For instance, you say at 30 actual mph, your speed-o is reading 35, a 17.6666% error. If you increased your speed to 60, you would expect the same 17.66% error. but what if your speed) now reads 69, then the new error is 69/60= 15% and your new gear is (1 less 15%)x 21=17.85 teeth. So now you round to 18 teeth. Whereas earlier, you had 17.5 which could round either way.
