I just put a 21" front wheel on the ultra. It has a left hand mechanical speedo. I just made a dinner run I have made many times BUT!..............when the speedo said I was at the speed limit I felt that I was going much faster! Especially when the police pulled me over! :-( What is the change to my speedo? Did not get a ticket because she ( she, female police officer) got side tracked when she discovered I was carrying a 45 semi on my left hip. :-) I have the license and she was cool, she just forgot why she pulled me over! LOL!!!!

 

Is the 21 inch tire a larger diameter than the stock 16 inch one. If so the speedo will read slower than actual speed. Measure the diameter of both tires and go from there. The larger will cover more ground with less revolutions.

Barney Biker
07 EG
PGR

 

Look here

http://www.miata.net/garage/tirecalc.html

You actual speed is about 19% faster than the speedo reading

 

Logic dictates that if your original speedo works with the larger tire, then you will be going faster than what the speedo says. No way to escape it. Bigger wheel covers more ground.

 

The good news is that your MPG will go up.

EDIT: On second thought, maybe not.... rear wheel the same size, then about same MPG

 

With the bigger wheels your speed will be faster than showing... your odometer will show fewer miles traveled than actual... your mpg will go down in theory only.

 

So when I was in the 65 mph zone I was really doing about 78 mph! Thus the blue lights. :-)

 

get a GPS and temporarily mount, This will show your true speed
Just take note and adjust.
My 07 Ultra shows 3 miles slower on the fairing speedo than actual (GPS), This is true only above 45-50.
At 75-80 on speedo GPS reads 4-5 mph + than fairing speedo