They are not necessarily tied together the mileage comes off of a hall effect sensor that sends pulses these are counted and a certain number of pulses yields a mile. The speed is a movement dependent on the resultant voltage from how close the pulses are together and this voltage drives the needle around. If this needle isn't calibrated correctly you get the situation where your mileage is right and your speed is indicated incorrectly.
2005 1100 Custom
ORKed 5/16" Lines
PODs K&N RU-600s W/PCV
PCS XXX/2 cams
Jetted & PMS Tweaked 20.0/152.5 3.0 turns
Wimmer Machine Custom Oil Cooler & Stash Tube
Dyna 3000 Step 5/ 6500 redline
6" Fwd Control Extension W/Boards F&R
Memphis Shades Bullet Fairing
Progressive 465 rear
Progressive Springs and GVE front
10,000 Monkeys throwing crap at the glass and this one thinks he's Picasso...