Speedo/Tire size

[Pooker]

Hello, can anyone help with this debate.
My "Speedometer/odometer" is way off, reads fast and puts on phantom miles.
The bike has a speed sensor that reads off the front pulley. I was told that if I put a tire with a taller profile on the back it would correct this problem.
How can that be? I can see the speedo changing if I put a different size pulley on, but not by putting a bigger tire on.
So, the question is; Will a taller tire change the reading on the speedometer?
Thanks

[old codger]

Changing the rear tire will have no effect on the speedometer, all the cycles I know of read off the front wheel. Changing the rear tire will make a difference in RPM for a given speed. Taller front tire will change the speedometer but may have an adverse effect on handling.

old codger

[Pooker]

Originally Posted by old codger

Changing the rear tire will have no effect on the speedometer, all the cycles I know of read off the front wheel. Changing the rear tire will make a difference in RPM for a given speed. Taller front tire will change the speedometer but may have an adverse effect on handling.

old codger

I do know that all the older bikes have a speedo cable that runs off the front wheel. My understanding is that with electronic speedos, digital and analog do not use a cable from the front wheel or the transmission. They all use some sort of electronic sensor.
Please correct me if I'm wrong.
Thanks

[DrBob]

If your speedo sensor is on a drive pully, or transmission pully, the the size of the rear tire will indeed have an effect on its accuracy. The sensor simply counts the rotations of the pullies, and the speedo convers that number of rotations per second into the distance traveled per second, and thus speed/ miles covered.
If you change the profile of the rear tires, making it shorter or taller, the distance covered per pulley revolution changes, making the speedo inaccurate. If, for example, your stock wheel was 16-inches, and the tire added another 8 inches of height (four top, four bottom) then the diameter of the tire is 24 inches, and the tire is 75.4 inches around. So for every pulley rotation the speedo thinks you covered 75.4 inches. If you swap to a slightly lower profile tire, which adds only 3.5 inches top and bottom, the tire is now only 23 inches tall, and for each pulley rotation the tire only covers 72.2 inches, not the 75.4 inches the speedo thinks you did. So just that slight change in tire height just caused a 5% error in the speedo.

So to correct the error you can adjust the tire size, or find a speedometer shop that will calibrate the speedo to read correctly with the existing tire size.

[texrider]

Another thing that should be cleared up, is the relation of Speedo size vs. Spare Tire size.

The man with an average spare tire should avoid wearing a Speedo at all costs. Especially one that is too small for him.

Additionally, there is the matter of the guy who is too small to be wearing the Speedo at all...

[S83]

[Pooker]

Originally Posted by DrBob

If your speedo sensor is on a drive pully, or transmission pully, the the size of the rear tire will indeed have an effect on its accuracy. The sensor simply counts the rotations of the pullies, and the speedo convers that number of rotations per second into the distance traveled per second, and thus speed/ miles covered.
If you change the profile of the rear tires, making it shorter or taller, the distance covered per pulley revolution changes, making the speedo inaccurate. If, for example, your stock wheel was 16-inches, and the tire added another 8 inches of height (four top, four bottom) then the diameter of the tire is 24 inches, and the tire is 75.4 inches around. So for every pulley rotation the speedo thinks you covered 75.4 inches. If you swap to a slightly lower profile tire, which adds only 3.5 inches top and bottom, the tire is now only 23 inches tall, and for each pulley rotation the tire only covers 72.2 inches, not the 75.4 inches the speedo thinks you did. So just that slight change in tire height just caused a 5% error in the speedo.

So to correct the error you can adjust the tire size, or find a speedometer shop that will calibrate the speedo to read correctly with the existing tire size.

Thank you, you have made it so simple even I could understand.

[old codger]

{I do know that all the older bikes have a speedo cable that runs off the front wheel}

You may be correct that some bikes run the speedometer from the rear wheel. I have owned several bikes over the years and all of them got the information from the front wheel including my current cycle, a 2006 model, the 2007 is the same.
I think it is time to delete myself! A good day to all.

old codger