I took a long road trip and had my pocket GPS set up on the dash for directions but I noticed that my speedo is off. With the cruise at 80,I was only going 76. I have the adventure pkg with the larger factory tires so if anything I would expect it to be the other way around if anything. It seems like a 35" tire would make it true.
Can anybody else grab a GPS and see how they compare? If its true that its tuned to 35" tires, the base tires would be off by about 5mph at highway speeds!!!!

 

Not sure if this is relevant or not, but I have 2 different GPS units, a Garmin and a Magellan, and when travelling with them in any vehicle, the speed they report is off by a few km always. When I have better sattelite coverage, the accuracy improves, but has never been perfect, so possibly this is within the accuracy of the GPS system, and each unit itself, and not the vehicle?

my .02

 

I thought about that too but having the GPS in my old jeep liberty, I dont recall it being off by that much. I drove 1,000 miles north and south through populated cities and wasteland and it was the same the whole way so I dont think it was the reception or correction acting up.

I guess I could set the cruise at 90 and let the fuzz tell me how fast it was going

 

IMO GPS is junk. It has come a long way, but still has a long way to go before it will be good. Even the military stuff that is supposed to be more accurate than the civilian stuff sucks. It is good to give you a quick look at where yo are at, but there are too many variables that need to line up to provide accuracy. Most of the time someone with a map and compass can be more accurate with thier location than the GPS can,albiet the GPS is faster at it, and when it comes to speed, they are a few seconds behind the times and can vary as indicated by others.

 

I will agree as far as actual position on amicro-navigational scale once you get down to measuring anything under a few hundred feet. But the derived numbers for speed I believe arepretty accurate.

I remember when the government turnedselective availabilityoff, they reported that speed accuracy of civilian GPS should be at .2 m/s accuracy of velocity. Sure enough, looking at the technical specifications of my GPS unit, on full fix (which was had nearlythe entire time) it operates in a published specification of 0.1 knot RMS. Even after conversion and rounding way up, that means it is accurate to +/-0.2 mph.

I would definately say not to trust the max speed reading on a GPSbut to have a current speed reading 4mphlowfor 16 hours sounds more like the speedo is at fault to me

 

If possible, you could borrow a friends GPS and see what they all read at the same time. Maybe help prove you're correct, or open bigger can of worms. Just a thought.

 

Good idea! sounds like its time for some science!

I love pretending to be a mythbuster...if only I had a high speed camera....

 

So.. which one are you? lol

 

I have a lowrance 100 on my dash and the speedo seems off.

[IMG]local://upfiles/954/6ABB60E744274D9397338D8AE27DAD50.jpg[/IMG]

 

I have a Magellan streetpilot in my H3 and it has proved to be very accurate. When I had the stock goodyears on the OEM rims the MPH on the Magellan jived with the speedo, this also proven against the portable radar/speed display that the local cops put up around town to make drivers aware that they are speeding.

When is put the new 35" tires on there was some change in speedo reading. At 60 mph on the speedo, actual speed as proven by the above method was 65 mph.

Hopefully when I have my PCM reprogrammed they can recalibrate for the 35" tires.