Just got Ultra Gauge today!

[Fat Charlie]

Euromodder- the gas stop was simply on the way to work, and as an added bonus it's at a light where I have to turn left. When traffic's light I can game the green light by waiting until someone else pulls up to it and trips the sensor. Then I start up and drive off through the now green light.

SentraSE-R- I will check the car against GPS, but I'll also have to see if I can figure out how accurate the GPS is being at any given time. The old PLGRs that I used to use would only give information as accurate as they could get, from a 4 digit grid (accurate to within 1km) up to a 10 digit grid (1 m). Today's smartphones whip everything up into a very pretty display, but the user is impossibly far removed from the actual data. How much of that log is hard data and how much of it is my phone making calculations based on spotty satellite readings and Google's map database?

I do trust the car, though. It was very well manufactured and its instruments have been... independently verified by various police departments over the years (a somewhat expensive and time consuming process, and always when I had somewhere that I needed to be in a hurry). Mile markers? I just can't bring myself to use street signs as precision measuring instruments.

[California98Civic]

Originally Posted by Fat Charlie

the gas stop was simply on the way to work, and as an added bonus it's at a light where I have to turn left. When traffic's light I can game the green light by waiting until someone else pulls up to it and trips the sensor. Then I start up and drive off through the now green light.

Nice tactical thinking, Fat Charlie! Darrell makes excellent points about the GPS and tire size and such, but your questions about GPS reliability make me wonder: which are the good GPS systems and how can a person tell before buying one? (I don't have one.)

[SentraSE-R]

Current GPSes are accurate to within 15 meters (~50 feet). That's not much error over a 10 mile distance. The different map databases and programming algorithms used by the GPS companies probably introduce more error. I know my Tom Tom GPS isn't nearly as user friendly displaying trip distance as my Garmin GPS or the Magellan I gave to my niece.

I've found mile markers to be notoriously inaccurate. The highway workers and contractors will straighten a road, and move milepost 3 to the 2.8 mile location. Highway departments used to have speedometer test sections. I haven't seen one in decades.

[Fat Charlie]

My issue with GPS isn't how much accuracy the system is capable of when locating a particular point, my issues are with how much varying accuracy and refresh rates are combined with map databases, churned around in a computer program and spat out as route information.

I've watched the number of satellites that my system was tracking fluctuate and the quality of grid it was able to give me fluctuate along with it. You can lay a slick UI on top of that to give pretty pictures and reports, but that UI is making a lot of assumptions. I'm going to use Torque and My Tracks to see how close they are, but I lean towards trusting a rev counter and OE tire size. At least I know for a fact that they account for elevation changes.