I/S speedo vs GPS Speed

[old grouch]

 ???Went for a ride today.  320 miles.  Highest speed I saw on the speedo was 100.  When I got home my GPS showed a max speed of 139!!!  Garmin E trex hand held.  When comparing the speedo w/ GPS, speedo consistantly  shows 5mph slower than GPS, on many runs.  Any thoughts?  Can the speedo be tired?  Sticking?  GPS bad?  Stan

[BonS]

My IS speedometer is 3 mph slow @ 35 and 4 slow @ 70 mph when compared to my GPS. Since my GPS continuously displays my speed and is fully electronic I believe it to be the more accurate device. Mechanical speedometers are notorious for speed measurement errors of several mph +/- over their span. Your 139 mph reading is likely just a flaky signal received somewhere along your journey. Sounds like you had some fun!!

[old grouch]

  EVERY ride is fun!!!  Any suggestions as to how I might check/calibrate the gps? 

[Ferris Leets]

There is no calibration to be done.  As was stated probably a fluke reading.  Mine will give you an odd position reading every once in awhile which would also affect you speed and distance traveled.

[Grandpot]

My I/S speedo reads 5mph slower than my GPS at 65mph.  My two cars read within 1mph of the GPS.

[Anthony]

your tach is depending on the circumference of your front tire,
which varies by make (Avon vs Metzeler vs Bridgestone makes a big difference ) AND by wear of the rubber.

manufacturers want to avoid you blame them for speeding, so they usually prefer that the tach reads faster then reality, instead of opposite.

my GPS (Garmin Zumo 550) is calculating my speed as an average of +/- 3 measurements :
if i brake hard at a red light, by the time i come to a complete stop, i see the GPS speed still reading 60 / 30 / 10 / 0 km/h, each for about a second.

if you are going through a long tunnel, the GPS will mis some readings. the zumo assumes that in the tunnel you are maintaining the same speed as when loosing the signal, and has to correct the distances/speed/etc when you are coming out/getting back signal. Nobody is perfect.

But, are you sure you had reset your max speed just before the trip ?

Anthony

[SgtBunny]

My Zumo 660 shows 70 when my speedo shows 75.  Traffic also nearly runs me over.

[Kingbee]

When I buy electronics, I like to let them "burn in" a while before installing them, as failures (statistically about 10%) often occur very early on.  I let my Garmin run on the bench for 40 hours, after which it showed I went 4 miles, with a top speed of 139 MPH.  Needless to say, I returned it for another, which subsequently passed the same test.

[Rocketman]

If your GPS antenna loses sight of a satellite or two (or gets a bad reading), it may lose track of where you are.  Then when it recovers, the speed is measured incorrectly, since it's using time to go from point A to point B, and is susceptible to large errors if you have inaccuracies in the locations of points A and B.  Therefore, max speed recorded can record a transient error.  If there's any way of recording a log file, then inspecting IT for your top speed, ignoring any unreasonable spikes ("I don't REMEMBER accelerating from 30 mph to 139 mph in 1/2 second, then braking to 30 again..."), that would be the best way.
Where do you keep your antenna?

[Michvalk]

I have an older lowrance that always shows a 3 MPH difference. I think that the gps is correct and the speedo is fast. I have no real evidence this is true, it just feels right

[Steve K (IA)]

As stated, the circumference of the front tire makes a difference as to how the speedo reads.  Also motorcycles are noted for reading slower than the actual speed.  I have a Zumo 450.  It shows that the speedo is consistently 2 mph slower than the GPS. 

[Rocketman]

The difference between speed measurements should be a percentage, not a set number.  My truck reads ~10% low, so if I'm indicating 50, I'm really going 55, but if I'm indicating 70, I'm really going 77.
I haven't checked the Valkyrie as definitively, but I believe it to be 5-10% high, so that it's the opposite.  Whenever I have the wife driving the car along behind me, she tells me that I'm going slower than I think I am, and it seems to be ~5-10%.

[Baloo]

I have a 99 I/S and a Garmin Zumo 550. My cage is a 2008 Jeep Wrangler. When on the Wrangler with its original summer tires, the Zumo shows exactly the same speed as the Wrangler's speedo. If the speedo shows 100 (km/h around here), the Zumo also shows 100. But when I switch it on the Valk, when the Zumo shows 100, the Valk shows around 105. So the Valk speedo in my case is a bit too fast. I would say your GPS is ok, the Valk speedo is off.

[TearlessTom]

As stated, the circumference of the front tire makes a difference as to how the speedo reads.  Also motorcycles are noted for reading slower than the actual speed.  I have a Zumo 450.  It shows that the speedo is consistently 2 mph slower than the GPS.  

I have to disagree here.    Harley Motorcycles are usually more accurate on MPH but not necessarily in odometer readings.

Ma Honda bikes are usually optimistic in their reading.  Most are reading a bit faster than acutal. Usually about 5-10%.

My ST1300 is 5% my Valkyrie is Spot On Even.  I guess I am one of the lucky ones.

As to the max speed reading in the GPS History. I think it has something to do with interference.  I live between two comercial airports and two military airports.  

I have had readings sitting in my office on the computer of over 1600 MPH Which proves I have the fastest bikes in town.

Tom

[Momz]

Anthony,

The "Tach" has no realation to the speed of the bike or tire circumfirence. The "Speedometer" should give (but is not very accurate) speed indication.

The "Tachometer" gives the engine RPM at any given time or at any given speed (depending on what gear you're in).