I just bought a new Garmin HD GPS and found that my speedometer is off by 5 MPH.

I am GPS poor and have one in every vehicle and in my boat and have found them to always be very accurate. It seems that radar is the most accurate, GPS 2nd and your speedometer 3rd.

When my speedometer reads 80 MPH the GPS says I am runing 75 MPH. This would explain why I get passed like I am sitting still running 70 MPH on the speedo and why I thought every friend I ride with runs 80 MPH all the time!

Anyone else experience this? Where does the speedometer pick up the reading from? Anyone ever have a dealer to calibrate one?

Thanks

 

none of them are the same reason i say that is i ride a 07 FXDC my uncle an 04 lowrider and my dad a 09 SE road glide. We can all reset our trip tic pull out of the driveway and by the end of the day we have 3 different mileage readings all close but have been off by 5miles or more and we road the same routes right with each other. Granted tire size would change the total mileage some.

And when all three running the interstate side by side can be different on speed just a little bit.

 

They have some signs on the highway up here that tell you your speed and in my car it's usually right on the mark. On the RK it says I'm going 5 mph slower than what is indicated on my speedometer. A buddy of mine with a police bike says his speedometer reads correctly. Maybe mine was made on a Friday?

 

Mine is about 4-5 MPH higher than I am really going, as far as I can tell.

So if it says 45MPH, I'm really going about 41. Might have saved me a ticket or two here and there.

 

My first thought when I was the title was you were talking about the skimpy bathing suits!

Anyway, my Zumo and my speedo are 2 miles off each other...don't know which is correct.

I do know that I took a 150 mile ride with my brother a couple of weeks ago.(he's got an 09 Ultra), we both started and ended in his garage and reset our odometers at the start. At the end of the ride...we were about 5 miles off of our distance traveled. I also notice riding with my friend Mitch, our mileage will vary too. Assume if the mileage is a bit different, our speeds will be a bit different as well. Guess it's tire size/air pressure.

Come to think of it, my new Zumo is 2 miles off just like the old one...that leads me to believe it's the bike and not the Zumo.

 

Mine reads 2 mph faster than the speed indicated on my Zumo.

 

My zumo reads about 5 mph under what my speedo says

 

My police speedo is certified so I guess that means it's accurate.

 

Early HD's and most motorcycles use to have a pick-up off the front wheel(VW beetles also) , Later models now have a pick-up in the transmission off the output shaft. So if you change tires, or final gear ratio then your speed will be off.(Even tire pressure will change this)

Speed is relative, as far as your GPS goes it depends on which Datum it is set up on. Most are NAD85, but there are numerious choices. If you want to check your GPS against your speedometer make sure you do it heading True North or True South. There is a built in error worst on True East and True West. Distance on these two headings are predicated on your latitude, 60 minus the Cosine of the latitude. Here one minute of longitude is equal to about 58.2 nautical miles.(Should be 60 but this only occurs at the equator. Also any minute of latitude is always equal to 60)

So see distance like time isn't easy/simple. And what differance does it make anyway? Radar and Laser are no better and that is why most Police will give you anywhere from 5 to 10 mph.

So if someone's GPS reads different from yours you might check the Datum, if you don't exactly end up at where is programed in then you might check the Datum. If you are in the US of A chose the newest date preceeded by NADxx (North American Datum; some where in Kansas I think)

Anyway that is the way it was, could have all changed by now.

 

The speed sensor is in the transmission. It is extremely accurate at counting how many times the rear wheel revolves. The exact circumference of the rear tire is a guess though. The speedometer may not display the speed accurately and the angle you're viewing it may not make for an accurate reading.