my speedo is NOT 10% off - Star Motorcycle Forums: Star Raider, V-Max, V-Star, Road-Star Forum
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post #1 of 15 (permalink) Old 02-01-2016, 08:02 PM Thread Starter
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my speedo is NOT 10% off

so as most of you know by now, if you own a production cruiser, your speedometer is actually reading out about 10% lower than your actual speed. at least at speeds above 50mph. but i've always suspected that mine was more 'accurate', at least when i found this out. and, i don't know if this has any bearing, but i started taking notice after i changed out my exhaust, intake, and added a fms. i haven't changed out my pulleys or gearing or anything like that. my rear tire is a 180 whereas the stock tire was a 170 with the same aspect ratio.

i've previously run through those speed signs that tell you your speed at various speeds and then looked down at my speedometer and either be right on the button or just a couple mph off. usually this has been at slower speeds like 30-50mph. maybe it was human error, maybe that sign was off, maybe our speedos are dead-on at those speeds, or whatever, but it wasn't very scientific conclusions.

then i got my first speeding ticket on a motorcycle in 2014. i was clocked at about 70mph and i KNOW i wasn't going much faster than that, if at all (i was either in a 55mph or 60mph zone). then this past weekend i went through multiple speed signs with the digital outputs. with my speedo reading at or very near 70mph, the signs would display 67 or 69. again, not a scientific reading, but very telling. and if my speedo were truly 10% off as everyone else's seems to be, my needle would have needed to be near 75 to get those readings, right? is anyone else's speedo more accurate than others? does my bigger tire make the difference?

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post #2 of 15 (permalink) Old 02-01-2016, 08:31 PM
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Originally Posted by bevo1981 View Post
so as most of you know by now, if you own a production cruiser, your speedometer is actually reading out about 10% lower than your actual speed. at least at speeds above 50mph. but i've always suspected that mine was more 'accurate', at least when i found this out. and, i don't know if this has any bearing, but i started taking notice after i changed out my exhaust, intake, and added a fms. i haven't changed out my pulleys or gearing or anything like that. my rear tire is a 180 whereas the stock tire was a 170 with the same aspect ratio.

i've previously run through those speed signs that tell you your speed at various speeds and then looked down at my speedometer and either be right on the button or just a couple mph off. usually this has been at slower speeds like 30-50mph. maybe it was human error, maybe that sign was off, maybe our speedos are dead-on at those speeds, or whatever, but it wasn't very scientific conclusions.

then i got my first speeding ticket on a motorcycle in 2014. i was clocked at about 70mph and i KNOW i wasn't going much faster than that, if at all (i was either in a 55mph or 60mph zone). then this past weekend i went through multiple speed signs with the digital outputs. with my speedo reading at or very near 70mph, the signs would display 67 or 69. again, not a scientific reading, but very telling. and if my speedo were truly 10% off as everyone else's seems to be, my needle would have needed to be near 75 to get those readings, right? is anyone else's speedo more accurate than others? does my bigger tire make the difference?
The tire has alot to do with it. Just like on the stryker and raider. It comes with a 210 and when the 240 is added the speedo is pretty close to accurate.
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post #3 of 15 (permalink) Old 02-02-2016, 05:40 AM
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My V-star 1300 Tourer was 5-6mph off from speedo indicated speed to actual (GPS) speed. However the Stratoliner is only 2-3 miles per hour off. Both speedos read higher than GPS measured speed.

On tires of the same aspect ratio the side wall gets taller the wider the tread is. 170mm x 70% = 119mm - 180mm x 70% = 126 mm. Therefore it will change your speedometer reading. See the link below for a tire calculator that will give speedo changes up to 65mph

https://www.tacomaworld.com/tirecalc

If you don't have a GPS on your bike there are smartphone apps that will give you a GPS based speed. On the iPhone side one of the apps is called SPeedHUD.

And there is a tire size calc app for iPhone called Tyre Size Calculator.
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post #4 of 15 (permalink) Old 02-02-2016, 09:57 AM
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my 650 is off 3-5 mph depending on speed it gets more accurate the faster i'm going according to my gps.

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post #5 of 15 (permalink) Old 02-03-2016, 01:53 PM
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Originally Posted by bevo1981 View Post
my rear tire is a 180 whereas the stock tire was a 170 with the same aspect ratio.

does my bigger tire make the difference?
Yes. The Speedometer works on how fast the wheels are spinning, so with a taller tire, the wheel is spinning faster than with a shorter tire, and this changes the readings at higher speeds, anything about 60mph usually. The same is true on cars and trucks, Lifting a truck and putting bigger/taller tires on it will certainly change your Speedometer.

You can always take it to a calibration shop to have it Calibrated to the size tires you plan to run on it.


Side note, any reason why you changed from 170 to 180 for the rear? Did you have to modify the fender to keep it from rubbing?

2007 VStar 1300 Touring (road trip/weekend bike)
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post #6 of 15 (permalink) Old 02-03-2016, 04:02 PM
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Yes. The Speedometer works on how fast the wheels are spinning, so with a taller tire, the wheel is spinning faster than with a shorter tire, and this changes the readings at higher speeds, anything about 60mph usually. The same is true on cars and trucks, Lifting a truck and putting bigger/taller tires on it will certainly change your Speedometer.

You can always take it to a calibration shop to have it Calibrated to the size tires you plan to run on it.


Side note, any reason why you changed from 170 to 180 for the rear? Did you have to modify the fender to keep it from rubbing?
don't most speedos run off the front wheel? on cars they usually run off the transmission so changing the rear tire size will change the ratio in the drive line. on my 650 i can't see how changing the rear tire size would affect the speedo because the ratio of rpm to road speed on the front wouldn't change. am i looking at this the wrong way. or am i correct?

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post #7 of 15 (permalink) Old 02-03-2016, 05:50 PM
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Unless you have a Cable on your Forks, the Speedo is attached to your Drivetrain which powers the rear wheel.

But now that has me thinking, I will have to check where the Speedo is connected on both my bikes to be sure.

2007 VStar 1300 Touring (road trip/weekend bike)
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post #8 of 15 (permalink) Old 02-03-2016, 06:50 PM
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The 650 is driven by cable on the front wheel and speedometer will changed with tire size.
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post #9 of 15 (permalink) Old 02-03-2016, 11:06 PM Thread Starter
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Side note, any reason why you changed from 170 to 180 for the rear? Did you have to modify the fender to keep it from rubbing?
it's not a big difference. mostly because it looks better having a wider tire on the back. an added benefit is a wider contact patch. there was no modification needed for my fender.

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post #10 of 15 (permalink) Old 02-04-2016, 09:38 AM
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Side note, any reason why you changed from 170 to 180 for the rear? Did you have to modify the fender to keep it from rubbing?
If you are surprised on one size up check out the Stryker and Raiders from 210 to 240 or 250
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