[Sequoia]

Each time I get a new vehicle, I check the odometer against the mile markers on the highways. I always take the mileage over a longer trip to weed out poor placement of individual markers. And, I try this on various highways to compare the results. So far, my 08s have all matched up pretty well. Currently, I am 1/20th of a mile worse off than the highway markers. Meaning that the truck calculates 1/20th more than the markers. I know there is a variance that manufactures are allowed to be off. 3-5 % I think. Which, would make this off by 5% if the markers are accurate.
Anyway, the point of this post is that I used to own quite a few other brands and many times, the speed and odo were off quite a bit. In the 10% range. I was wondering if everyone has checked this out and figured it into their MPG calcs? Also, some of the people with other brand trucks might have inaccurate ODOs too giving them the illusion of better or worse fuel economy. So it might be difficult to compare a Dodge getting 20 mpg to a Chevy getting 17 MPG

 

[SmokeyWren]

It takes at least 10 miles on an interstate highway to get in the ballpark of tripmeter accuracy. 100 miles is a really good test. 50 miles is a decent compromise. The 10-mile test gets you within one percent of actual. The 100-mile test gets you within one tenth of one percent of actual.

Regardless of miles, you cannot cross any state lines or change highway numbers or take any exits - not even rest stops. So plan ahead before you begin a 100-mile test. /ubbthreads/images/graemlins/crazy.gif

For the first mile marker, pick one that is not near an entry, exit, overpass or underpass - so the highway crew could have placed the marker where it belongs. At the end of the 10 or 50 or 100 mile segment, pick the last mile marker that's also not near any of those obstructions. If the 100th mile marker is near an entry, exit, etc., then go to 101 and do the math. Then you'll have accurate numbers.

On a SuperDuty pickup, the tripmeter, odometer, and speedometer are all driven by the same computer, so tripmeter mileage and odometer mileage should be the same. The analog speedo will usually show about one MPH faster than you're actually going, but I always get my tripmeter accurate and let the analog speedo fall where it may. Normally, with my tripmeter accurate to 99.7%, 70 MPH indicated is 69 MPH actual.

 

[KINGTUT]

Why not just use GPS with WASS enabled. The amount of error is around 3-5 feet. Much more accurate than mile marker signs. BTW Honda just settled a huge class action for inaccurate odometer readings.

 

[Sequoia]

Wow. There is someone who knows his mile markers! Thought I was doing well with a 30-40 mile test.

Was thinking my difference might be from the 20 inch wheels. Anyway, the readings were way closer than my Dodges but then again that was the mid nineties.

Would imagine GPS would be the way to go.