To get an exact number, you just calculate the speed difference you want, in percent, and get that same percent difference in number of teeth.
If your speed difference is 5%, then your tooth count difference will also be 5%.
If your speed-O is indicating slow, to speed it up you will need fewer teeth, and vice versa.
5% on 37 teeth is just 1.85 teeth. Now look on the chart at 25" tire, and see that the difference from 3.55s to 3.73 is 2 teeth; see what I mean. The chart assumes that you know the exact rear-end ratio. and it assumes your tires fall on the sizes indicated, BUT a size difference of .5 inches in diameter is actually 1.5 inches in circumference which is what the charts calculations are based on. Therefore, sometimes, as in your case, you would have to guess at a tooth count between the chart numbers, of 25 and 26.
Thus not knowing the exactness of anything, you are looking at a range of Speed-O gears on the chart, between 36 and 39
Whereas, the Percent method will tell you EXACTLY how many teeth to add or subtract to whatever is in there right now.
But I gotta tell ya; from 3.55s to 3.73s is a 5% difference, so maybe you want to revisit your differential ratio calculation cuz 5% at 65 mph is 3.25 mph. So if you got that wrong, the chart won't work for you ............ but the PerCent difference method will.
FWI
The chart is based on increments of 1" tire diameters, which is actually based on the circumferential measurement, in the which, each inch in diameter is actually 3.1416 inches in circumference.
So then
when the chart shows 25 inches, it is hiding 78.54 inches of circumference. and
when the chart shows 26 inches, it is hiding 81.68 inches of circumference.
But, your 25.25 inch measurement is hiding 79.33 inches of circumference
Why is this important?
Well how did you arrive at 25.25 inches. I can almost guarantee that you guessed. What if you're out just .25 inch? That will be .785 inch in circumference, which is what the chart is based on. So then, what you measured as 78.33, could be ~ .785 in either direction or
From 79.12 to 78.55 which is a range of exactly 2%. So right out of the gate, your already about 3/4 of a tooth off the chart, just by the error of .25 inch..
To get a better more accurate number, you have to determine the exact CIRCUMFERENCE of your tire, and convert that number to diameter by dividing it by 3.1416. And you can't be using a metal tape wrapped around the tire! which will NEVER be accurate. You need a cloth tape, or a calibrated section marked out on the floor, with some way to determine a start/finish indicator on the tire. Some guys put a daub of grease on the tire, then roll it out straight, then measure between the smears. Yeah good luck with that, as you are still gonna have to estimate, which is a proper gentlemanly way of saying, guessing..
Suppose you did use the circumferential measurement and arrived at 79.33 inches. Dividing that by 3.1416, gets you 25.25 inches. Ok, so in this case you got lucky.
The point is this;
When determining the diameter of your tires, with a tape measure, accurately enough for this purpose, it is easy to fall 1 tooth off what you need.
One tooth is only ~1.75 mph@ 65 mph.
Now, if you bumble thru a speed-trap at what you think is a safe 68 mph, but the cop's radar says 70, are you gonna get a ticket?
IDK, but in Canada the copsareplicks.
