Hubby has always said my speedometer must run a little low.  He has followed me on both of his bikes, and his truck too.  Then I had a guy from work follow me to work the other day and he said I was running a little over 60, when I thought I was going about 57-ish.  Is this normal for this bike?  I still have the stock tires, need to replace rear this summer, was planning to go with a 140/90 instead of 140/80.  Is that going to make a difference?

Checking the speedometer against another vehicle is always iffy, because you have no way of knowing how accurate the speedo is in the other vehicle. And it is quite common for makers to have speedometers read a little high. That is reputed to be their method of avoiding lawsuits from people who get speeding tickets.

The easiest way to check your speedometer is to use a GPS. Another good way is a stopwatch and the mileposts along the Interstate (or anywhere else). At 60 mph, it takes 60 seconds to travel a mile. Within the range of 55 to 65, it is "close enough" to assume 65 mph takes 55 seconds, and so on. The difficulty with the stopwatch method is in maintaining your speed exactly for a mile. A third good way is to find one of the "Your Speed is - " units somewhere along the road. Problem there is making sure you are the vehicle it sees. Motorcycles are relatively small targets, and the radar has an easier time seeing the semi behind you.

Changing tire size will affect the speedometer reading. A bigger tire will make the speedometer read lower. If you have ever watched a drag racer do a burnout, you may have noticed the rear tires expand as they speed up. This happens on all tires as speed increases. This, along with innate inaccuracy of the speedometer itself, can mean speedometer error changes with speed. The Savage/S40 drives its speedometer off the transmission, so rear tire size is the one that affects the speedo. Some bikes, and most cars nowadays, drive the speedometer via the engine control computer, so makers can cause the speedometer and odometer to read anything they please.

Stock speedo usually runs a little optimistic... a 140/90 will put it close to accurate...

I'm always comparing it to the bicycle speedo w/ trip meter I have mounted on a turn signal arm. I programed it to the exact (measured) circumference of the front wheel, and it shows the bike speedo to be very close,....maybe 1 mph slow @ 60mph.

Find a good chiro. YOu need a good neck, bike or no bike. Took me many, many years to get to the "I am no longer in pain daily" spot after landing on the shoulder/side of head at amost 60 mph, but w/o chiropractors, I woulda never made it.

Thanks all.  When hubby first commented that I was driving a little fast, he was on his Vulcan, so I didn't think much about it, not knowing its accuracy either.  Which one was correct, right?  Then with his C90, it still seemed  that I was running a little faster than speedo showed.  Then he followed me with 2 different vehicles, same result.  Then when the guy from work followed me and he commented, that's what prompted my question.

Never paid much attention to the odometer, and I don't ride much Interstate with mile markers, but I do know of an easy straight stretch that would work nicely for these tests.  I'll do it.  

But I'm still confused with a couple responses.  My speedo "seems" to be reading lower than I am actually going.  If I go from the stock size to the 140/90, Charon said it will make the speedometer read lower, but Bot said it will make it more accurate.  If the speedometer will read lower, isn't that less accurate since its already lower?  (Sorry, blonde moment, like daylight savings time.... I can never figure out whether its gonna get darker earlier or later...  lol)

Odd as it sounds, both Serobot and I could be right. The larger tire will make the speedometer (and odometer) read lower when travelling at the same road speed. If the speedometer started off by reading high, then the resulting lower reading might be more accurate. Note that the speedometer and the odometer, even though housed in the same case and driven by the same cable, are two separate and independent instruments. It is very common for them to have different errors, and actually fairly unusual for them both to be right. The speedometer measures how fast its drive cable is turning, and the odometer counts how many times it has turned.

GPS does not actually measure speed. It measures and calculates its position on the Earth's surface. It repeats the measurement fairly often. It calculates the difference in position between successive measurements, and knowing the times of the measurements, it calculates a speed. Because there is a small error in position measurements, it "filters" its results to smooth the readings. Because a finite amount of time is required to measure, calculate, and display results, a GPS always lags by a short time. GPS gives excellent results when you are "straight and level," but if you are accelerating or braking, or turning sharply, the GPS unit's filtering might give odd results for a moment.

In general, all speedo's are calibrated under actual speed.
Just think about it, 'sorry officer, I didn't know I was speeding.  This is a brand new car and the speedo should be accurate' and how many suits will arise from that.