Last year I bought a Zumo 550 GPS. One of the reasons I went with this unit is it is made for motorcycles and another is that I can use it in my truck as well. I like the unit a lot.

When I'm running down the highway in my truck (2004 F150) I end up having three speedometers - the original in dash truck speedo, another in my Edge ECM programer, and the one in the GPS. There is a pretty good descrepincy between the three. If I set the cruise control at 80mph by the GPS reading my Edge reads 84 and my dash speedo reads 86 or 87 mph. I always figured the GPS was the most accurate and went by that but always wondered how it might or might not concur with DPS radar. I run an Escort Passport in the truck and the HD Escort unit on the bike to give me a head's up on where those rascals are at.

I had not run the GPS on the bike in a while until yesterday so I got a chance to compare the GPS reading with the bike's speedo. To my surprise they were dead on with no variation in reading between the two. 75 on one was 75 on the other, 80 on one was 80 on the other, 40 on one was 40 on the other.

I don't ususally run the GPS around town because it just isn't needed. But I now know just where the speedos in my truck and on my bike stand. My bike is right on. My truck is way high.

It may seem like a small thing to others but it's something I like to be on top of.

Oh, and anyone considering the Zumo 550... I recomend it highly. It is definately worth the money.

 

I check the speedo on the bike and the Zumo. i set the cruise at 100 kmh and on the Zumo i read 96 kmh when i set the cruise at 120 kmh i have 118 kmh on the Zumo.

 

I find the bike speedometer to read slower than the GPS. It's more evident at higher speeds.

 

According to my Garmin E-Trex Legend, the speedo on my Ultra is about 5% optimistic, at least from 30 mph to around 90 mph.

 

That's odd, in all the past threads on this subject both here and on the Zumo forum, I've never heard anyone say the speedo registered slower than the GPS. Most are about 5%-10% higher than actual speed.

 

My Garmin Quest GPS always shows about 3 mph lower that the bike at 70mph, and 2mph lower on my van. That fits the 5% optimistic values stated by fireglider and Wecroft. Never had to defend either with LEO.

 

I didn't mention - I have an Automax with AutoTune. It completely replaces the stock ECM. There is a value in there that, when changed, will make a difference in the speedo reading. I guess I have mine where it needs to be. If you have a stock ECM you can't change didly squat. That's one reason I went with the after market replacement. I didn't want to be forced to suck on the dealership's tit.