5% discrepancy seems to be the norm. My bike @ 80, GPS says 76. At 100 GPS says 95.

Same topic came up on the Kawi forums I use to frequent, and same for a work associates Honda forums.

 

Jap bikes do the same thing. Off by 4-5 mph at highways speeds.

 

I get the same readings with mine, in fact....in a pack of 8 bikes I rode with recently, only 1 was close to correct.

 

Not that it should always be taken as gospel, but the Wikipedia article on speedometer error states that as of '97 the US regulations are for within 5%. The international rules are that the speedometer must never indicate a speed less than what you're actually traveling. Also, the article later correctly states that the GPS readings are more accurate the faster you're going as compared to slower speeds.

If you want to get your speedometer reading dead nuts on any given day, throw the appropriate figure into your bike's ECU with a TTS tuner. That's just one of the several features that were added to the device once H-D cut their ties with the provider of the SERT.

 

The GPS is taking a mathematical formula between two points to get your speed. Whether or not which is true I myself would base my riding on the highest speed reading to avoid a speeding ticket. My Zumo is always less then my speedo by 3-4 mph. Civilian GPS's are not dead on anyway. I have a Zumo, Roadmate and a handheld Eagle, I took a reading on a spot and all 3 were off in position by 20-30 feet. The most accurate was the Eagle when compared to my brother-in-laws surveying GPS

 

off by 2 vs gps

 

My Garmin has always been spot on in my cages. Also was spot on with my low rider. My street glide reads 2 mph too fast at 50 and 4 mph too fast at 100. Hard to believe something that simple isn't more accurate. Guess it's like the ambiet temperature gague.

 

I just checked this over the weekend. My '09 SG reads 2 mph faster than my Zumo. That's pretty good actually. My Beemer reads 4 mph fast.

Here is an interesting observation to try. Notice that when you go through a turn at a constant speed your GPS will drop down and then when you complete the turn and start going straight again it will pick up speed. When you're in a turn your actually road speed is X but your relative point to point speed which the GPS is calculating is less than X which is why the GPS reads slower.

Marc

 

My Garmin reads about 4 mph less than my SG too, and I trust the Garmin. We were riding down to Milwaukee in August and when we stopped for gas one of my buds mentioned that I was "getting a little ahead of myself". I asked what he was talking about, and he said I was movin' out a little too fast on the highway. I pointed out that the speed limit was 70, I had the cruise set at 72, and the GPS was only reading 68, and if they'd only get their butts in gear we might actually start exceeding the speed limit. He thought his speedo was dead-on and was sure I was exceeding the limit.

We rode home without him and made some real time on the way back...

 

Lucky! My K1200LT read 7 mph fast! Doin' the math to try & stay close to legal drove me nuts till I did the speedo mod (it was an '02). After that it was within 1/2 mph of either of my GPS units. The change to the 2.5 to 3 mph difference with the Road Glide got my attention for the first couple of rides, but I hardly notice it now.

BTW, your balancer keeps me & a few of my friends smooth on the road. Thanks for a quality piece of equipment.