I mounted a phone with a speedo app on my handlebars, and found that my speedo reads about 4-5 MPH high (I tried 2 different apps, both were the same). Doing a quick search, it seems that it's common for these old Japanese speedos to read about 10% high, perhaps to hedge against reading under. I guess legally they can read a bit high, but not low. No big deal, but I was curious if they can be calibrated? I did change the front tire from 3.25 x 18 to 3.00 X 18, but the tires seem the same circumference, side-by-side.

They cannot be internally calibrated.
There may be some external gear box that could be placed in-line with the speedo cable, but I have never seen one for an enduro.

I appreciate the reply Deet, and won't worry about it now. I had suspected it was reading fast from riding by radar signs a few times, but now it's confirmed.

Memory is a bit fuzzy... one time I changed to a different size front wheel rim... I think it was to a 21" as that makes most sense but I'm not completely sure... used the original speedo drive gears. I was worried the speedo would be way off but it actually was almost exactly perfect after the wheel swap.

Mark, that's a good idea and could be an option.

Another option would be to swap the speedo drive gears in the hub.... IF there are some that would interchange and slow the speedo down. You might have to experiment with it.

And now that I think about it... here is a little nugget of information that most folks don't know:

The indicator needle is magnetically driven and can therefore vary in accuracy. The odometer is gear driven and will NOT vary in accuracy.

The speedos on all of the Suzuki GS1000s I previously owned were very generous in their readouts!

I do understand that the odometer is gear driven, while magnets and a hairspring are involved in the speedometer needle, but I'll bet the odometer is also reading fast. Also that if it had a 21 inch wheel it would probably read accurately, or very close.

Based on DEET’s input you could verify if any type of replacement gearing or wheel size is needed by observing the odometer as you ride a verified 1 mile stretch of road. If the odometer matches the 1 mile then your mechanicals are working properly. The magnetic drive would be the problem.  

Pedalcrazy, I plan on doing that, just for fun. I'm not gonna worry about correcting it, as long as I know it's a consistent 4-5 MPH high at all speeds, which it is. I thought my max speed was 65, but it's 60-61. If I went back to the stock 16 tooth front sprocket, it would probably go a bit faster at higher revs, where the max HP is. However, I like the gearing as-is (17 tooth on front).

it is absolutely true that there is no "adjustment screw" or similar in the speedo or tach...  that said, a good speedometer shop absolutely can change the magnetic calibration...  probably take thousands of dollars of specialized equipment and might be one of those things nobody does anymore as almost everything has gone electronic. 

There used to be a place called United Speedometer in Riverside, CA  that would rebuild and "adjust" the magnetic field inside the speedo or tach with special equipment. We had them do a lot of work for our truck fleet...  in many gauges there is a statement something like "2250 RPM = 60 MPH" and that's what they would calibrate to most of the time.  (The trucks had little gear boxes at the trans end of the cable that we would swap gears in to compensate for tire size etc.)  

I did have them recalibrate the Veglia speedo in my 69 Fiat 124 so it would read correctly when I installed a Mazda rotary engine and trans. That was about 40 years ago though.