Speedo error

[Big twin]

Since I've been riding my bike,one person(member here, Sweb) told me I was going faster than I tough when riding. When I think I'm going 100km/h(55mph for you guys) I really am going 120km/h. Now, a second guy told me I was going faster than I though. That guy had a motorcycle that was written "POLICE" all over it ... Yeah he ticketed me.

Now, I checked the numbers of teeths on my front and rear pulleys. They are both stock for a 2005 XB12s,being 27 up front and 65 rear. What else could be "modified" ? Primary chain gears ?

I was thinking of installing a "Speedo healer", but if I don't know what's been changed, it won't be easy making the adjustements ....

Any help will be appreciated.

 

[ich]

straight vs. helical cut

 

[Big twin]

Ok, that does'nt tell me much .... (I googled it)

 

[Pagano]

Just a thought; It reads off the front tire correct? Might an incorrect diameter tire possibly cause the offset? Dunno.

 

[user_deleted]

most motorcycle speedometers are wildly optimistic. the normal conversion rate i use to correct the error in my head is "deduct 10% and add 3". in other words:
speedo shows 80mph
deduct 10% or 8mph which is 72mph
add 3mph and real speed about 75mph

this simple formula gets you really close. two things here i noticed is that your math is off and you said nothing about the gauge cluster. 100kmh is 61mph..not 55mph as you stated. 120kmh would be 72mph. assuming your speedo sensor is working ok it sounds to me like someone had your gauge cluster off and apart one time to replace bulbs or whatever, removed one or both needles and didn't reinstall them in correct position. it's extremely easy to make this mistake and very tough to reinstall needles in correct display positions. just a hunch.

 

[buelladdicted]

If its a 12, when your tach is at 3000 your speedo should read just short of 62mph.and if so, it can be within 1 to 2 mph depending on actual rear tire diameter,or analog tach.

 

[Big twin]

two things here i noticed is that your math is off and you said nothing about the gauge cluster. 100kmh is 61mph..not 55mph as you stated

Yeah sorry I did'nt google for the conversion, I did an approx. For the gage cluster, when my cluster reads 100km/h I go faster in reality, approximitly 115-120km/h. I know usualy gages on cars and bikes are off, but usualy you go slower than the gage tells you.

If its a 12, when your tach is at 3000 your speedo should read just short of 62mph

I'll check that !

 

[user_deleted]

bigtwin: what addicted also said. i'm betting your gauge needle had been removed and not reinstalled accurately. turn on key, let needles sweep, look at speedo needle and it should be laying at the very top edge of the first mark which is about 1mph. see what you have.

 

[Big twin]

Checked ! The needle is right where it's supposed to be !

 

[ich]

i bet speed indicated is 42/48th of real speed.

 

[Big twin]

i bet speed indicated is 42/48th of real speed.

Ha ha ha ... you don't like to give much info ...[smirk]

I found this but after reading the post I came to the conclusion that it does the opposite of my "issue". His gage must indicate higher speed than he's actually going ...

I'll continue my search ...

 

[buelladdicted]

If it is a 12 and has a 9 primary, it should be aprox 4mph slower then indicated. if it is a 9 with a 12 primary it will be about 4mph faster.

 

[Azxb12s]

use your gps on your smart phone and downlaod a gps speedo app and see what the difference betweent your speedo and the gps is..

 

[Big twin]

If it is a 12 and has a 9 primary, it should be aprox 4mph slower then indicated. if it is a 9 with a 12 primary it will be about 4mph faster

I've got a 12 and am going faster than my gage tells me ...

use your gps on your smart phone and downlaod a gps speedo app

I'm not that well equiped ...

 

[Azxb12s]

does one of your buddy's have one

 

[ich]

I found this

keep on searching. it's all available in the net. and no, primary has nothing to do with your issues, same as tire diameters.

 

[GAXB9R]

Has an XB dash board been installed?

 

[Big twin]

If its a 12, when your tach is at 3000 your speedo should read just short of 62mph.

Just rode my bike and tested it ... At 3000 RPM I'm at 85km/h(53mph)on the gage but I think I'm going faster than that in reality. At 100km/h(62mph) on the gage, I'm at 3400 RPM.

keep on searching. it's all available in the net.

Thanks for your help ...

Has an XB dash board been installed?

No, dash board stock

 

[GAXB9R]

Speedo sensor may be going bad. Mine just did and it worked sometimes and then after the bike was hot the sensor would stop and I would have no speedo at all.

 

[OldGeezer]

Hey Big Twin,

Interesting problem you have there. I don't know what the mechanical or electronic cause of it is, but I can suggest a workaround for you:

1- Find a road with mile markers on it, drive such that you are doing 3,000 RPM in fifth gear as you pass the first marker, maintain a steady 3,000 RPM till you pass the second marker, note the time it takes to go from one marker to the next or use a stop watch.

2- Calculate the speed you were driving at by dividing 1 by the number of decimal minutes it took you to drive the measured mile and multiply that result by 60. The answer is your speed in MPH. Use at least 4 decimal places so as not to get rounding errors.

Example: It took you 1minute and 20 seconds to drive the measured mile, which is equal to 1 + (20 ÷ 60) = 1.3333 minutes.

1 ÷ 1.3333 = 0.7500
0.7500 x 60 = 45MPH

If it only took you 51.4 seconds then:

51.4 ÷ 60 = 0.8571 minutes
1 ÷ 0.8571 = 1.1667
1.1667 x 60 = 70MPH

Knowing the speed you were driving at, divide it by 30 and that tells you how many MPH you do per 100 RPM, or divide by 6 to know how many MPH you do per 500 RPM.

So 70MPH ÷ 30 = 2.33MPH per 100RPM in 5th gear

or

70MPH ÷ 6 = 11.67MPH per 500 RPM in 5th gear

Then work out what RPM you would be at doing whatever speed limits you want to respect in 5th gear.

The same calculations can be performed for any and all your gears, though I wouldn't worry about anything except 3rd, 4th & 5th. A simpler way of figuring out the other gears is to note the RPM increase when driving at a specific known speed and shifting to a lower gear, or you can use the gear ratios from the manual, as long as you know they haven't been changed.

For example, if you are driving at 60MPH at 3,000RPM in 5th and you downshift (without engine braking or accelerating ) and are now driving at 60MPH at 3,500RPM in 4th gear, you know that you can add 500RPM to the RPM for all your calculated speeds in 5th gear and you will get the RPM in 4th gear for those calculated speeds.

When I was a student and couldn't afford 'unnecessary' repairs I drove for a few years without a speedometer (cable or drive gear went), just using my tach and the system above, it doesn't even matter if your tach is accurate as long as it is consistent.