What heppens to the speedometer reading when installing sprockets with different gear teeth count?

Question

Hi

I've got to do a chain and sprocket job, and have been trying to find out what happens to the speedometer reading when installing sprockets with a gear teeth count that's different then stock.

The bike has 15/45 teeth stock. I plan on using the same size front sprocket, but am considering installing a 47 tooth rear sprocket. Will this cause the speedometer to read higher then the actual speed, or lower?

For example, when the speedometer says 100 mph, what speed will the bike actually be traveling at, with a 47t as opposed to a 45t?

Thanks in advance for all responses.

This bike has an electrical speedo/odo with the pickup sensor on the front sprocket.
This bike does NOT have a mechanical speedo driven off the front wheel.

Answer 1

more than likely 100mph as most speedo drives are on the front wheel,

Answer 2

With the sensor taking readings from the F sprocket, the speedometer will give false readings.
The difference will be so minimal, I wouldn't worry about it.
If you're that concerned, get a friend to drive next to you in a car.
Have him drive at 30 for a mile, accelerate to 40, 55, etc (tell him what speeds to stay at before going faster) then record your speeds.
For those that don't think sprocket size will make a difference, consider this -
With a 15 tooth sprocket on the "rear" - the chain will move 15 links to rotate the wheel once.
With a 75 tooth sprocket on the "rear" - the chain will move 75 links to rotate the wheel once.
If the sensor reads a different amount of links per second, it will think the wheel is moving at a different speed.

Answer 3

Assuming that you have a 600 sportbike, I am guessing that because of the sprocket sizes you listed, your speedo will be very close to perfect. These bikes come from the factory will very optimistic speedos (about 10% is the norm) and gearing the bike down will not only improve acceleration but as an added benifit will make your speedometer read closer to correct. Adding two teeth in the rear and going down a tooth in the front made mine read correct.

Answer 4

You will be going slower than the speedo reads because when the front sprocket is turning a set # of revolutions the rear wheel will be turning slightly fewer with a 47.
I'd guess 100mph, with a 45 will = approx. 94-95 with the 47.

Answer 5

leg bone's connected to the hip bone... I don't have an answer for your gearbox output shaft sensor, mine is located on the front wheel hub, BUT I highly doubt that a minimal increase in sprocket size would create a significant inaccuracy of your speedometer that would cause any need for concern. Cruising with traffic a bit is always a good cheap test for accuracy. Do we ever pay that close attention to the speedo while riding anyway? Personal "feel" is a great gauge.

Answer 6

Multiply the speedo reading by .96 will give correct reading. To find correct correction factor, divide old sprocket teeth by new sprocket teeth. This gives the correction factor which you multiply the new speedo reading by.

45/47 = 0.9600, 100mph indicated X .96 = 96 mph, corrected.
If you change front sprocket, reverse the above, divide new teeth by old.

If you change both: Divide old rear teeth by old front teeth, this is R1. Divide new rear by new new front, this is R2. Divide R1/R2=C1, this correction ration, multiply speedo reading by this (C1) for corrected speed.

Tomcotexas

Answer 7

Reading should be the same as the speedometer reading is taken from the front tire, not the back

This is as long as you do nothing to change the circumference of the front tire. The only thing that would change your speedometer reading would be going to a larger or smaller diameter front tire.

Your additional details state that your speedometer is driven from the front sprocket. This changes everything. You will be going slightly slower than the speedometer reads. A little more than 4%