I have a 2009 Street Glide with a 21 inch rim on the front and an 18 inch rim on the rear. When the bike had stock heavier rims and tires, it ran about 120 mph, with the new rims and tires (lighter and larger) I only run about 105 mph. Why am i running slower?

Larger means getting false speedo reading. Speedo will show slower than actual if you go bigger. Find some highway and hold at 60mph for 5 miles. Should be 5 minutes on the button. If you come in under 5 min its running faster than speedo shows. You can do the math and calculate how much faster by a percentage. That will apply to any speed. If you are 15% faster then 100 on your speedo is actually 115. Or find a friendly cop who will radar you...

BulldogJim is correct. If you don't own a GPS, borrow one and compare it to your speedo at reasonable speeds. You can make a temp. mount and let it run on battery power for a while.

that works too, best if on straight ground unless the GPS calulates rapidly... GPS doesn't do cosign math when you go around a corner. Will show much slower than actual

Jim, I don't own a GPS as of yet so I don't have a clue. Are you saying if I drove in a huge circle over and over, the GPS would say my average travel was 0 miles?

On a lot of freeways, there are Mileage Markers and ALSO Test Zones if you look... DOT can tell you where they are located.. I have done the 5 mile Checks and they are broken down to EXACT 1 mile increments. Just Don't do them at 20 over the speed limit Like I DID and got a warning,,, The officer said "The TEST NEED NOT BE DONE AT 80 IN A 60 ZONE,,,,,, by the way,,, Nice Bike" :hii This will give you a safe way to check.... signed....BUBBIE

Maybe... lets say that the GPS marked your location, then 10 seconds later marked it again, then did the math of how long it took for you to get brom point a to point b... however point a and point b are the same place. Then yes, zero MPH. The GPS assumes you took the traightest route between the two points. If your GPS tracks you every tenth of a second then the speed will be pretty accurate. If every ten seconds, not so much... unless you are traveling in a straight line. Also, it's only averaging your speed. How can it know that you sped up and slowed down between readings? The only way to acurately measure speed is by Doppler theory witha radar operator who knows how to do cosign, or you are traveling directly into or away from the radar beam.

I know this is a stupid comment for this thread but I couldn't care less if my bike would only run 105 instead of 120. As long as I can cruise somewhere between 65 & 80 I'm good to go.

I agree that as long as the bike can maintain crusing speed and the RPM showing on the tach. is correct all is well. However when your speedo shows 65mph and the policemans radar says 80mph you have a problem.

Yes, BUT if the speedometer is OFF and shows 60 when you are doing 70 a Ticket can result... With the BIGGER Diameter tire on the rear, the MPH are Effected greatly... in favor of a ticket.. The Idea Here as I Understand it,,, is to get the speed-o to match the Correct MPH/MPM A electric BOX can be purchased to accurately re-adjust the speed-o to the right MPH 89$ 125$ signed....BUBBIE