Questions about wheels and speedometer...

zoopman · #1 09-28-2007, 09:49 PM

I have an 07 fxd and found out a few things. If ya stay with the stock rearwheel (17x4.5)and want a 180 reartire---ya have only one choice and dunlop makes the tire for HD only. It seems that no other 180 tire will fit this wheel size. I am thinking of going to an after market 18x5.5 wheel so I can use any 180 brand tire in the future. Been thinking about some sharp lookin chrome wheels someday as they are costly! My biggest question is--by changing wheel and tire size will seedometer still be correct? My understanding is that the speedo cable goes directly into transmission connected to vss (vehicle speed sensor) and that wheel size will not be an issue. Anybody ever look into this?

murphdog · #2 09-28-2007, 11:02 PM

If the diameter of the back tire changes, the speedometer will be off. If the diameter increases from stock, you will be going faster thanthe speedometerindicates. If the diameter decreases from stock, you will be going slower than thespeedometerindicates.

soup48 · #3 09-29-2007, 08:15 AM

There is no speedometer cable. There is only the VSS that gives the info to the speedo.

zoopman · #4 09-29-2007, 11:29 AM

Yes soup48 you are right--I should not have said cable!-- murphdog I understand what your saying and agree but does the vss get it's info from wheel/tire diameter? Was talking to someone (can't remember who dang-it) and they were saying wheels size would not matter as speedo reads from transmission. Not a big deal and sure don't want to beat a dead horse with this--but I have learned much here on this site from all you guys!! Thanks

StreetBobJason · #5 09-29-2007, 11:43 AM

If the diameter of tire changes then the RPM inside the tranny changes. Therefore it would vary speedo.

'06FXDBI, My first HD!

jeffbcn · #6 09-29-2007, 09:38 PM

This has got to be a straight math question. Larger diameter at same rpm would result in greater distance traveled, right?

murphdog · #7 09-29-2007, 09:41 PM

zoop, the speed sensor is looking at teeth on a gear in the transmission. If you change something after the transmission, the speedometer will be off. A larger diameter tire travels a greater distance per revolution. So, for a given revolutions per minute of the transmission gear and rear tire, you're traveling a greater distance per minute (going faster) than the speedometer thinks you are.
I put tires on my truck that are 5% taller than the stock tires. Guess what...the speedometer and odometerare 5%off and yes, the sensor is in the transmission.

zoopman · #8 09-29-2007, 10:49 PM

murph--with this info I will measure wheel and tire diameter to see how much of a difference and then drive along with someone to check speedo at a few different speeds. At least I would have an idea and perhaps avoid a ticket.

Just trying to figure all this out if I do decide to buy the chrome wheels. Thanks again guys!!

mdog2222 · #9 09-30-2007, 12:41 AM

Nice thread man. Good topic