Garmin GPS vs KZ Speedometer

Bought a Garmin GPS a couple years ago and love it. It has a feature that shows among other things your mph. My Frontier at 70 shows 67 with the GPS, wifes car at 70 shows 71 with the GPS. So it got me wondering how accurate was my 650's spedo? The old spedo (85mph) showed 6000rpm at 70 but the replacement (140mph) now shows 5500rpm at 70. So being the top flight mechanic and KZ owner that I am, I decided to take a high tech look at this age old question. "How accurate ARE KZ speedometers"? So I did what any good KZ owner would do, I taped my GPS to the handle bars and went for a ride! This is what I found...

In town..

KZ GPS
40 37
45 42
50 47

On the freeway..

KZ GPS
70 64
75 68
80 72

I've tried looking on the net for how accurate IS GPS vs spedo? Have not found anything yet but would think that multi bazillion dollar satillites would be fairly accurate? This test told me that my old spedo was almost right on and this one is way off. Thought you guys might find this interesting. Anybody else have a portable GPS?, do what I did and let us know how it comes out...

Interesting. I have the stock speedo on my 650 and have been meaning to take a ride to do some geocaching. I'll see what I come up with.

I can say that the gps (Garmin Nuvi 780) seems to be pretty much dead on in a cage...but I've honestly never tried it on the bike before. hmmm, maybe this could be an excuse to buy one of those Garmin Zumo's. They claim they're "Vibration-tested, waterproof and built with fuel-resistant plastics to withstand the elements".


...wouldn't want to hurt the Nuvi, after all...

I've tested my cars, a few trucks and both my KZ's against my portable Magellan. The 'vette and my new GMC 1500 pickup were right on, but both KZ's read about about 8-10% higher than the GPS at highway speeds.

As long as you have and maintain good satelite reception, it is very accurate. Within 9 feet on most civilian units. Going under a bridge may provide monetary blups, but for the purpose of testing the speedo just use a clear area. I do it all them time. I used my gps a lot with my 900RR since I changed the sprockets - drive ratio. After the change I was recording more miles on my odometer than actually driven and the speedo was about 10or 15% faster then actual.

Soooooo... is there anything I can do about this? Can I change anything internally? Can a speedometer shop recalibrate it?, or do I need to go through 5-10 ones till I find a good one? :woohoo:

You could print up some really nice speedo faces and slap one in. Or maybe a new one every week. It might become tiresome after a while but you'll always have a fresh look.

John:
Here's your data with a third column for percent error:

In town..

KZ GPS % Error
40 37 8.1%
45 42 7.1%
50 47 6.4%

On the freeway..

KZ GPS
70 64 9.3%
75 68 10.3%
80 72 11.1%

I've always thought that, the faster you go, the more error you get. Seems like your speedo is most accurate at about 50 mph. Or maybe even 55, since that was the spped limit at the time?

I also have run my GPS on the bike and my cars.
My KZ650 is off just about like yours.
I don't think I would try to recalibrate it or go through 5 or 10 more to fix it. I would just ride it like the 28 year old bike that it is and be happy you know the variance by your GPS and not from a LEO!

When I was on my vacation trip to Pa. last month, which was planned for 3 weeks but turned out to be 1 1/2 instead,, , to break the monontony of the 119 mile ride on I91 I checked my calibration a few times for a 20 miles distance using the roadside milage markers as my measurement.
I would reset my trip odometer at one marker and get up to 60 and start the check at the next marker I'd ride the 20 miles keeping a steady 60mph. On all of them I gained less than 1/10 of a mile on the odometer and the ride times were all within 10 seconds of the necessary 20 minutes at 60.
I know, not as scientific as a GPS but, accurate enough for me,, :laugh: :laugh: :laugh:

I have way to much time on my hands.

I agree with Milkman's results for the odometer. Mine seems very accurate. When I ride to work (same route as in my car), the trip odometer reads within 1/10 of a mile on a 110-mile round trip (I need to move closer to work).
As far as my speedo, I think I'm doing about 65 when I'm indicating 70.

I'm getting the same with the gps. My counterpart and I just went to pick up a new car for her and I had her follow me with the gps on the way home. In town, under 40mph the gps and her speedo were pretty much even, and read me about 2 mph less than my gauges.

On the freeway, I was 64 per the gps, "about 65" to her volkswagen speedo and 70 on mine.

Well, gents, I'm not shocked at the error amounts for 3 reasons:
1) the Japanese are famous for not wanting their customers to get tickets and blaming "that crappy Jap speedo", soooo they make 'em read somewhat higher than they should.
2) MOST gauges (speedo, tach, temp, pressure, etc etc) are designed and built so that they are most accurate in the middle third of their span. Thus if your speedo has a max of 120 MPH, it will be most accurate in the 40 - 80 MPH segement.
3) if we could easily take the flippin' things apart and lube them properly from time to time so the bushings and bearings wouldn't wear out so fast, I believe the accuracy would be vastly improved.

My .02......