speedometer inaccuracy



I got a speedometer, I'm not going o say who I got it from because that wouldn't be fair and it's irrelevant anyway...but anywho, i was driving along next to my uncle's car, my speedometer said 25 and he said his said 20, and we were going the same speed. So does this mean that whatever speed i'm going, I decreas it by 20% to make it actual speed? I want to see if i can't get an officer to radar me just for the fu of it, but what are the chances of that? just on observation and a thought. So would this also mean that the odometer would also say more milage than actual?

Feel free to mention the where the speedo came from, in the pursuit of knowledge nothing remains hidden.
I think knowing the manufacturer would help us know what exactly you're dealing with.
It may've been designed for a different sized wheel, or tire; which depending on what kind of speedo it is, could easily be reset.
If you know of a cool cop who is interested in the bike, go for it.
If not, seek out a school zone with one of those radar "you are breaking the law this much" displays.
The odo is gonna be off too.
It seems to me that speedometers are almost always off. It could be the car or it could be the speedometer on your bike. This is one reason I prefer digital speedometers, since you can calibrate it to the actual outer diameter of the tire. With cable drive bicycle speedometers its sometimes possible to swap the drive unit, they are each geared for whatever wheel size, but of course can't do anything about tire diameter. In any case, you just boosted the speed of your bike for very little cost :D
I have found the best way to get an accurate speed reading is to use a GPS on the bike itself. I put the mechanical speedos on my bikes but they are by no means accurate, if I break 30mph the needle just starts pinging back and forth and running two bikes side by side I'll get two completely diffrent readings...Kelly
I had one of those digi things. When I put the magnet on the wheel, I put it in the wrong place. I was using the elec bike and it was nice to pick up that 2mph but alas now I know and it's nearly as much fun.
mio digiwalker

I have a Digiwalker by Mio and it says My bike speedo is 2 miles per hour too fast,going around 30. Which means going around 28,if the gps is correct.

Are u sure u calibrated it properly...by properly i mean taking an accurate measurement of the wheelbase of the front tire after it has travelled one full revolution with u SITTING on the bike,then programming that into your computer?
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When I bought my speedom. it came with instructions for the different size wheel and how to so I followed the damrections and it seemded to work ok one of the local cops pulled me over for speeding but did not write me a ticket because I was on a bike and the judge would have thrown the ticket out. I do not go that fast any more the officer was right I was going to fast on a public road for a bike and a bikes breaks.

I never have any luck with mechanical speedom they usualy go flying off the bike in pcs. but the dig works great.