Ok, so I've read that the stock speedometer is about 14% off.  Does it read too fast or too slow?  In other words, if my speedometer is reading 50, am I actually going 57 or 43 mph?

I've also read that a 140/90-15 rear tire will help the speedometer read more accurately.  How much closer to actual speed?

Has anyone ever checked the stock speedometer against a GPS or a calibrated bicycle speedometer?

Most bike speedo's read 5 to 7mph higher than you are actually going... (probably some legal thing,... or just to make you feel fast)...
...
90 profile rear tire does a fair job correcting this...

I live in a small town where 30 mph means 30 mph, not 31. So if I change to a 140/90-15 I should be ok and not worry too much about getting a ticket (provided I ride at the speed limit)?

I always thought my speedo was a little slow but, the gps said it was bang on.  Why does the tire size matter though?  The measuring device is in the front and it doesn't know what's going on in the back?  

pg wrote on 02/22/14 at 22:50:17:


Some motorcyles use the front wheel to drive the speedo.  This was done so that rear wheel spin would not show a false mileage, and sprocket/gearing changes would not affect the speedometer accuracy.

The Savage speedometer drive is from the transmission - so having a larger or smaller back tire does make a difference.  Larger tires travel more distance with each revolution......and the speedometer is not able to compensate for the tire changes.

Well, you could do what I did. I put a bicycle and a gps on the handle bars, that way if you get stopped and the police officer says you were doing 31 in a 30, you just tell him his gear is off and needs recalibrated cause you have three instruments telling you the same speed.
If you get the dell 100 bicycle speedo, the top/avg. speeds are stored, so if the tell you that you were doing % over the speed limit, just call it up and say... um nope... looky here.

I ain't gonna lie, down here on Highway 98, the mean speed is 10 miles an hour over posted (minimum), if you aren't doing that you are causing a problem (I cause a lot of problems). Even through a posted 35 mph construction area (with a double the fine sign) people are doing 50, but not in my lane The area was previously 45mph, but they have cones and concrete to block off the construction crew areas (concrete walls were they are actually working) because they KNOW someone would run off the road and hit some worker or equipment when it rains.
About once a week the state or county will put up a speed trap, but its not often enough, they drive VERY offensively down here, and I don't mean offensive as in the other side of defense... I mean offensive in the term used for Aholes that don't care about others and think they own the road.

I agree completely.  Any time I've ever been caught speeding I had absolutely no reason to complain.  I was speeding...no excuses.  Do the crime, do the time.  My money is better spent elsewhere.