Console odometer observation

[Crunch]

Been doing a lot of long distance driving lately and I noticed something about the console odometer. On the last trip, which was roughly 1600 miles I was using a Garmin Nuvi navigator. Out of curiosity I reset my odometer as I was rolling off the curb and also reset the trip data on the navigator. When I compared the two, the miles on the console navigator were about 15 miles higher than those on the navigator. That's roughly a 1% difference between the two. The console odometer matched the main odometer. No biggie, unless all states move to a GPS system to replace the built in fuel tax. I've got 142K+ on the clock, which would equal about 14,000 miles of difference for taxation purposes. That's about a year's worth of driving for a most people. Fortunately, any move to taxing us by the miles would result in me being taxed for fewer miles, since a mileage tax would require a GPS device that can be read by the pump. I still oppose on principle the GPS system for taxing, though. Also, for what it's worth I got right at 20MPG on this trip with 3,000lbs in the bed and also for the previous 2500 mile trip, also with 3,000lbs in the bed. Both haul weights were for the outbound side of the trip only. On the return it was only about 250lbs of containers in the bed. Since I also pull trailers a lot I've come to the conclusion that weight has a minimal affect on my fuel economy, but the rolling and wind resistance of a trailer drop it by 20%-25% whether the trailer is weighted down or not. I also keep total mileage and cost records for my truck, so for those who are curious I have run 8,275. 942 gallons of fuel through this truck (including fuel additives), have a total fuel economy of 17. 199746 mpg. The cost per mile for owning this truck (all costs except insurance, safety inspections and registration costs) is $. 4387409805/mile. Yeah, it's a sweet truck. I've come to understand the bumper sticker that reads, "My wife, yes. My dog, maybe. My truck, never. "

 

[HBarlow]

I have never made all the calculations that you have but I agree with your observations.

A speedometer indicated 75 mph has been a Garmin GPS over the ground speed of 74 mph on all three of the Dodges I have owned.

For years I have set my cruise control at 75 mph on the speedometer on interstate highways with a posted speed limit of 70 mph but my Garmin GPS reads an over the ground speed by satellite of 74 mph, acceptable to almost every highway patrol officer in the nation. I've never been stopped but occasionally get a visual inspection as I meet or even occasionally pass a trooper car.

 

[Prairie Dog]

Thats a lot closer than I would have thought it would be!

 

[rscurtis]

Crunch, you might want to check your math on that. 1% of 142000 is 1420, not 14,200. Still not a bad error.

 

[Schrowang]

I'v,e noticed the same kind of difference between my TomTom and my speedometer. TomTom says 63mph and speedo says 65, a 2mph difference.



Could the difference be the amount of time that the GPC locator takes to communicate up and back with the satellite?



Just wondering

 

[mwilson]

Mine is the same way. .



Tom Tom says 74mph, truck speedo will show 75mph.

 

[Crunch]

Crunch, you might want to check your math on that. 1% of 142000 is 1420, not 14,200. Still not a bad error.

D'oh! I guess that's what happens when I post after 20 hours of driving and unloading. I guess the coming tax bite won't be as big I thought while in a haze. And even though I'm not a big fan of navigators for most things, when I drive without it I find myself looking where it usually sits to see what the speed limit is. If nothing else, the Nuvi is very accurate in the speed limit it displays, to the point of being within 100 yards or so of the speed limit signs that change. For GPS fuel taxation, however I still don't trust it. The government induces an error in both positioning and altitude, in part so you can't wire your GPS to a missile, but specifically for national security. I wonder what the acceptable margin of error would be for fuel taxation.

 

[Road Dog]

Why does anyone expect the vehicle speedo/odometer to be accurate? It's just counting wheel revolutions. The difference traveled between a new tire and a bald one is around 3% for typical truck tires. Different vendors have different diameters too.

The GPS odometer adds up a series of line segments so on curves the GPS will be a little less than the true distance. My GPS track log has a point (location) every 7 - 25 seconds at 65mph and calculates the distance from the previous point.

 

[Frenchman]

The mileage will be more on the loaded side of the trip because of tire slipping on every bounce or rough pavement. I noticed that when driveing 18 Wheelers.



Frenchman