55 profile- speedo correction

wizbit · Mar 24, 2012

Hey a quick question to see if any boffins out there know of a fix for your speedo accuracy when fitting a 55 profile rear tyre. I normally run a 50 but thought I'd try a change but I've found that without realising it, I must use my speedo as a guage when blasting around so a couple of times today when I was 'ahem'....going along briskly, I looked down and i was about 15+ mph out which makes you think you should give it some more beans....which is dodgy.
So just wondering if theres a simple fix anyones heard of like a resistor you fit to a circuit somewhere.

Cheers Dave

nikfubar1 · Mar 24, 2012

Not a cheap fix but a speed healer will sort it out :thumbup

wizbit · Mar 24, 2012

Cheers I'll google it mate....nice nipples

philandpa · Mar 24, 2012

http://www.gearingcommander.com/ Have a look here as the difference isnt that great between 190/50 190/55 i changed from a 50 to a 60 and didnt really notice apart from turn in and ride height. Back to a 55 though now

Tash1 · Mar 24, 2012

The tyre wont affect the speedo as it uses the sprocket nuts as the pickup, same with changing the sprockets the speedo isn't affected.

apostrophe · Mar 24, 2012

Tash said:
The tyre wont affect the speedo as it uses the sprocket nuts as the pickup, same with changing the sprockets the speedo isn't affected.

Is that right Mark? I agree that sprockets do not make any difference but I reckon tyre size/profile does. I assume the sensor counts a revolution or part revolution of the wheel and calculates speed based on some stored wheel circumference.

Wheel circumference differs with size/profile.

The differences are not big. Less than 2kmh in this example

Assume wheel rotating 8 times a second:

180/55 has a circumference of 1978.6 so the speed in kmh is 56.98368
190/50 has a circumference of 1953.4 so the speed in kmh is 56.25792
190/55 has a circumference of 2013.1 so the speed in kmh is 57.97728

The speed shown on the speedo would be the same in each case.

Deviation increases (in proportion) with speed. At 24 rear wheel rotations per second speed is about 170kmh and difference between 190/50 and 190/55 is about 5kmh

Rob

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55 profile- speedo correction

wizbit

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