After you make tire and/or final drive changes you need to change the internal reduction ratio of the spedometer drive within the transmision, either by changing gears or in the case of modern transmissions with an electronic speed sensor by changing the output of that sensor.

If you start playing with final drive gearing and tire sizes I think you'll notice something that's counterintuitive but true. The RPM you're turning at a given speed indicated by the speedometer won't change but your true speed vs. what's indicated by the speedometer will.

Think of it this way.

The tachometer measures engine RPM.
The speedometer measures driveline RPM after some gear reduction within the transmission but before the final drive, and displays it as a MPH display. They are both measuring RPM but the speedo is calibrated to display MPH for a given RPM reading.

The above is not applicable if the vehicle uses a wheel speed sensor to determine speed though I don't know of any that do...

So even though the ratio of the tire height vs gear ratio that I have now is very close to the stock ratio, my speedo can still be off. This is what you are saying, correct?

Yes. Think of it in percentages. For instance, if you change from the stock tire diameter by 10%, you'll need a 10% change of your final drive gearing to get back to where you started.

Example - we go from a 30" tire to a 33" tire, a 10% change.

Now if we had a 4.00 rear to start, we'd need to change to a rear that was 110% of the old rear ratio to get back to where we started.

4.00 X 1.10 = 4.40 and we're effectively back to stock. But we have bigger tires now...
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Just divide the new tire size by the old one, then use that number to determine the change needed in final drive ratio to get back to the same overall ratio...

Ok, went from 225's (28.4"-Firestone website) with 3.90's to

32's (31.8") with 4.30's

That's an increase of 11.97% in tire size and a 10.26% increase in gearing, so overall a 1.71% decrease, which means that 75 mph should be 73.7 mph. I'm overall 5 mph off, what gives?

Maybe I should measure the diameter of the tire on the ground and use that figure?

I agree. You should be a lot closer than you are. In fact, I'd think you should be about perfect. It means one or more of these things.

The tires are different than the spec sheet says.
The speedo was off in the beginning.

Keep in mind if you replaced worn 225's with new 32's the 225's had lost quite a bit of diameter from wear. A lot of truck type tires will run 16/32 tread depth. Which means if you wear them smooth you've lost 1/2" from each side or a full 1" in diameter...

--
Jim - who feels he's probably not helping you a bit even though he'd like to...

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Just to make this even more complicated, I just measured the 32's on level ground on the truck. They meausred out 30.75". Now, I'm really confused. Maybe the speedo was way off from the factory?