Topic: speedo say 45. cop stop says 39

[worlddrum13]

driving my 750 down the road today and i past on of those radar gun signs the cops use. i went past it at 45 and it read 39. is this normal for these bikes? or am i over-reacting. are those things even acuate with bikes? granted my needle does bounce up and down a little but 2-3 mph at most but i dont see it 6mph off.. is this fixable? should i replace the cable? would that help? never done any speedo work before.

[Gordon]

These speedometers are usually a little on the optimistic side, but 13% is farther off than I'm used to them being.  Could be a combination of the radar being off some, the speedometer being off some, and the cable being old and corroded inside the sheath as indicated by the needle bouncing. 

[Laminar]

It's normal.

[333]

A new or well lubricated cable will get rid of the bounce, but this era of bikes is notorious for inaccurate speedos. Usually in the area of 10%.

[mgbgt89]

Better for the speedo to read fast than slow. I got clocked at 89 when my speedo said about 75. I got a ticket.

[betterthanurs916]

I've seen a lot of talk about speedometer inaccuracy lately...I may play around with my speedometer on a bench and try to come up with an appropriately scaled face layout where the interval increases to make up for the inaccuracy that seems to grow exponentially...would be nice to have.

[MCRider]

They were inaccurate when new, 10% not uncommon. Add to that 30+ years of condensation cycles, gravity pulling on the lube, overall corrosion, what can you expect? Even if its low mileage, age takes its toll.

Get a cheapo portable GPS and check it for yourself. The cop is right.

[worlddrum13]

does this effect the odometer to?

[MCRider]

does this effect the odometer to?

Good question. The odo is gear driven and I don't think it would vary from its original accuracy. I've never really heard or read a comment on Odo accuracy.

The speedo needle is drawn over by magic and lots of things can influence it.

[TwoTired]

Stock tires an rims?  Change the circumference of the tire and that changes the odometer reading and the speedometer reading.

The odometer is a geared linkage.  If the tire and wheel is stock, it should be pretty close.
 The speedometer is a magnetic coupling.  Magnets do change their strength over time.  Which usually makes them read lower than actual.  However, the magnets spin around a bell.  If the bearings are worn and the magnets get closer they can increase the needle deflection.  They can also drag a bit on the bell, but that is usually VERY noticeable.  Lastly the needle mechanism has a spring return.  If the spring "relaxes" from metal fatigue of constantly bending, it is easier for the needle to deflect by the magnet's pull.

Used to be that cops had calibrated speedometers.  They had to be checked/ tweaked periodically to keep them accurate.  If you got a ticket for speeding, you could ask the cop to see the calibration card.  If it was out of date, the ticket would get thrown out of court.

Cheers,

[Gordon]

The cop is right.

Yes.  Typically a radar gun used by a police officer is going to be correct, but I've always been a little suspicious about how accurate those radar trailers that show you how fast you're going are.  I'm not saying they're going to be way off, but I don't know if they're calibrated as often or to the same accuracy as a radar that's used to issue speeding tickets is. 

[MCRider]

The cop is right.

Yes.  Typically a radar gun used by a police officer is going to be correct, but I've always been a little suspicious about how accurate those radar trailers that show you how fast you're going are.  I'm not saying they're going to be way off, but I don't know if they're calibrated as often or to the same accuracy as a radar that's used to issue speeding tickets is. 

Sure I agree. I was speaking "theoretically" 

[flybox1]

yep, even with a lubed cable, you'll still see up to 10% higher speedo reading vs actual.
Those signs can be fun 

[Gordon]

Those signs can be fun 

In the same way speed bumps ramps can be fun!