How accurate is vehicle speed on a GPS ?

As others have noted, the gps seems to be the one to pay attention to. I find that mine is always right on the money with radar signs and speedometer checks unlike my odometer.

 

Check the odometer against mile markers. I have seen speedo's dead on and odometers off. I have also seen the opposite. I don't think 3 mph is gonna be a noticeable difference. My GPS seems accurate by the radar signs but seems slow to respond to changes in speed

 

Thanks: Never checked odometer by markers only speedometer. On a hundred mile check that could really add up, eh?

 

Me too!

 

I agree about the mountains.sometimes my GPS reads up to 5mph higher than the speedometer when it's very WINDY going thru the mountains.

 

gps is completely wireless with the satellites. None of us can really tell the accuracy of any type of measuring device. But gps is the most reliable source.

As for the speedo. Which is completely mechanical. It will be slower with brand new rubber then worn out rubber. You're talking roughly a 4 inch bigger circle. 2 inches on top and 2 inches on bottom of brand new tread.

 

I say this. That speedometer is running slow because Your company set it to provide false information.

There was a little game I always played in Wisconsion to ease the boredom and that was the yardstick game. 60 seconds = 60 mph. 55 seconds = a value 50 seconds a lower value and so on. One trip with a dollar truck yeilded a 28 second mile for approx 67 miles.

At the time with FFE, that truck was not working out the yardsticks correctly. I fired up the laptop, attached the GPS to it and examined the data in real time at max speed of 65, it actually displayed 63.

I saved the information as evidence using both screen shot and fraps recording video and took it to safety with a shop order to fix the speedometer or increase the truck speed to properly show advertised 65.

They took me into the back office and said I needed to shut up about the speed. It's set that way on purpose for insurance purposes. I was willing to do that provided Dispatch quit making trip reports because my average fell below the fleet speed average of 45 mph. We were a freaking team stopping for only 20 minutes every 30 hours to fuel the tanks with around 280 gallons of fuel.

They were nice about it. But... something you all consider.

As far as accuracy, It was good to 50 feet until President Clinton wrote a executive Order to the USAF Space Wing to allow data to me much more accurate to around 9 feet give or take a tad. Enough to maintain a interstate lane in a total white out. One episode of driving by satellite with a gigantic fear of hitting a car or truck that has stopped in my lane...

It's promising but not ready for that sort of thing. That year Cadillac introduced a HUD (Heads up Display) with cameras capable of using heat information project the image on the glass in front of the driver so they can see Deer for example before the human eyes actually pick it out.

We can use such a HUD in trucking. I promise you that.

 

Next time you take a flight bring your GPS along. It's fun to watch

 

The GPS units that you can buy are extremely accurate. I was doing the same exact route for 9 months. When I say same exact route I mean I stopped at the same places every single day. Tried to park in the same exact spots every day. Left and got back at the same times. Every day the GPS would tell to the tenth of a mile how far I drove and it would be the exact same every day. I actually fix speedometer calibration errors with the GPS units. Had one truck that was off by 7 mph. I sat in the passenger seat and let the driver drive down the road and when he would get to a steady speed we would compare speeds and I would adjust the ecm readings to accommodate it. It takes about 15 minutes to get it dead on.