Inaccurate speedometer

I know a lot of our bikes have speedometers that aren't as accurate as they used to be. I haven't had my wife follow me yet to figure out how much it's off but when I do 70 on the interstate, I'm passing a LOT more cars than I do when I'm doing 70 in my truck. Do they get more inaccurate over time due to wear and tear on the speedometer itself or is something else that's the cause?

thanks,
mikeg

 

Checking with another car is only good if that speeometer is accurate.

You can check with a GPS

Time your travel between mile markers

Find a cop with a radar gun when you're going too fast. They're always more than happy to tell me how fast i was actually going. On the plus side a ticket can be dismissed if you get a faulty speedometer fixed.

 

I have checked mine against a GPS and found 115 MPH indicated was just 104 on my Seca 900. It reads 5 higher at 45 - 55.
My Seca 750 is close enough, never more than 2 off.
The real question this thread should address is how to correct a speedo that isn't telling the truth. I'm thinking if I got the "hub unit" from a bike with an even smaller front tire, a Yamaha, it would make my speedo read slower. Thoughts and ideas?

If the internal magnets got weaker, the speed would read lower.
If the internal spring got weaker, the speed would read higher.

 

So Time, it sounds like you're saying that it's the speedometer itself that causes the inaccuracy? Which makes sense to me if your cable spins freely and the unit on the tire works properly.

mikeg

 

Mine seemed a bit inconsistant at times and would even just quit,
only to start up again.
I put her on the center stand, checked the cable rotation with a
spin of the front tire. NO spin.
Took the "hub unit" apart..thorough cleaning and some new bearing
grease and prob solved.
Can't say if it's accurate when turning normaly but the the breakdown
of old grease / lack of maintanance can cause a false reading.

 

After I layed my bike down my speedometer wasn't reading correctly. In fact, I was on the freeway going 55 and a deputy sheriff pulled me over for doing 65 in a 55 but only issued me a warning ticket to get the speedo fixed. I knew the speedo was off because at 60 mph I used to turn about 4400 rpm and when I was issued a speeding ticket I was turning more rpm's. There was a guy in Albuquerque that fixed all speedometers so I brought it there and had it recalibrated and it's been fine ever since. Bottom line, they can be tweaked...

 

I'd like to eventually do the GPS conversion like one guy on here did to his Seca. A GPS is usually way more accurate than an old school speedo. The one on that Seca looks really good too.