I recently mated my Garmin Nuvi 2360 with Garmin's EcoRoute HD -- in my '07 RAV4 Limited. Since buying the car (new in April '07) I have consistently tracked MPG by resetting the RAV's trip odometer and dividing miles driven by gallons purchased at the next fill-up. Since getting the EcoRoute thingamajig, I've also been resetting the EcoRoute/GPS average MPG to zero at each fill-up as well. With both the car's trip odometer and the GPS recording miles traveled between fill-ups, I'm finding that the GPS is reporting more miles traveled than the trip odometer is -- as recorded in the EcoRoute "tools" screen, where you input info from your latest gas fill-up (price per gallon, gallons purchased). This number, of course, gives me a rosier MPG figure than if I go by the trip odometer's number. Is the Garmin's miles traveled calculation more accurate than the trip odometer's? Or should I ignore it?:confused:
Most new car speedometers read slightly high (too fast) however this is affected by many factors such as tire wear. tire pressure, temperature (affects tire pressure) and so on. The same applies to the odometer.
However GPS units measure distance using an elliptical model of the earth's surface which is not always accurate. Most roads are relatively flat (< 3% grade) so the difference is minimal. However when hiking grades can sometimes be steep (e.g. 40%) resulting in GPS distance measurements that are lower than actually traveled.
To add another variable I'm not sure how an ecoRoute HD (I have one) measures distance. It is possible that some vehicles report distance traveled (odometer) while others do not in which case GPS measurements are used. I don't know the answer and am now curious.
The other factor you need to consider is the fuel measurement method. The ecoRoute HD obtains this information from your car's engine management computer which maybe less or more accurate than any given gas pump. Fuel volume measurements are also affected by temperature.
In short there are many factors that can result in differences and it is difficult to know which figures are accurate.
My ecoRoute HD consistently reports MPG figures (2-3MPG) better than my car's computer. However both are higher than the manufacturers so I'm happy ;-)
The ecoRoute HD grossly overestimates the distance travelled, at least in my 2011 diesel Toyota Avensis with my nüvi 3790T. A journey I make regularly is 53km, as measured by the car's odometer and Google Maps, but when I stop the engine, the trip report on the nüvi display says about 67km! I don't know how much I can trust any of the other data it provides.
I have had a support case open with Garmin for about four months now, but there is still no sign of them explaining it, let alone providing a solution.
With the ecoRoute HD disconnected, the trip report gives a more accurate distance measurement, presumably calculated from the track log, but the fuel consumption is only a guess and doesn't look right.
