Using Interstate Mile Markers to Calibrate an Odometer

Mr. Kite · #1 06-10-2007, 05:04 PM

After my first fillup in my HiHy, I realized that the display fuel economy was a bit higher than what I calculated using gallons put in divided by the change in the odometer. After starting a thread on this, it was mentioned that the odometer is actually off a bit as well. The few that mentioned it said it reads low.

This weekend, we drove from the Denver area to western Colorado. Somewhere along the way I decided I would try and calibrate the odometer using mile markers on I-70. From the measurements I made, I would say that my odometer miles need to be increased by at least 2.6%. However, it could be as high as 3.6%. During the relatively flat stretches of interstate, my trip odometer showed 19.3 or 19.4 miles for every 20 miles of interstate. During the stretches of highway with lots of steep grades, my odometer read higher. The extreme was one 20 mile stretch of interstate that actually gave 20.1 miles on my odometer. Overall, I had two stretches of ~ 100 miles that gave the 2.6% value. In addition, I had one 40 mile stretch that gave a 3.1% value.

Anyway, this leads me to a few questions. How accurate are these mile markers? Do they take into account the actual distance a car travels (including elevation changes), or are they just map based?

diamondlarry · #2 06-10-2007, 05:13 PM

My guess, from travelling on I-70 between St. Louis and Indianapolis, is that they don't seem to take into account elevation. Although, that stretch is fairly flat. It would be interesting to compare a route with GPS and Google Earth.

xcel · #3 06-10-2007, 07:28 PM

Hi Mr. Kite:

___Honda odometers of the recent past are all about 2% over reporting which falls ~ within what you have discovered. Honda was actually sued for it just recently in fact …

___Good Luck

___Wayne

Mr. Kite · #4 06-10-2007, 07:56 PM

Yes, I remember getting something in the mail concerning my Odyssey about odometer class action lawsuit. I wonder if this is an issue at all with my Civic with navigation. It has a hidden diagnostics screen that provides a lot of cool information. One of the options is tire calibration. By default, it is set to automatic. I would assume that the tire calibration and GPS determined speed are used to calibrate the speedometer and odometer.

SkeeterVT · #5 06-10-2007, 08:44 PM

Calibrating your vehicle's fuel economy by utilizing highway mile markers requires two things:

1) that you're driving at 60 miles per hour (literally, a mile a minute) and no faster, and

2) the mile markers are accurately placed.

If neither condition is met, there's no way you can accurately calibrate your fuel economy by that method.

Mr. Kite · #6 06-10-2007, 08:54 PM

Quote:

Originally Posted by SkeeterVT

Calibrating your vehicle's fuel economy by utilizing highway mile markers requires two things:

1) that you're driving at 60 miles per hour (literally, a mile a minute) and no faster, and

2) the mile markers are accurately placed.

If neither condition is met, there's no way you can accurately calibrate your fuel economy by that method.

I cannot think of any reason why #1 is required. If the mile markers are accurate, what is wrong with driving 100 miles according to mile markers and comparing it to the odometer mileage? Besides, how do you know what 60 miles per hour is if the speedometer/odometer is not calibrated? As for #2, that was one of my questions. How accurate are these things?

psyshack · #7 06-10-2007, 09:27 PM

Well if you drive 60 mph than you can also see if you speed-o is any where close to right along with the od. 60mph is a mile per min. Helps you get a better fix.

psy

johnf514 · #8 06-11-2007, 09:01 AM

But then you have to make sure your speedometer is completely accurate as well . . .

ILAveo · #9 06-11-2007, 08:16 PM

A game I used to play to help stay awake when I was driving overnight was to reset the odometer and my stopwatch at a mile marker and try to drive a steady 60MPH for half an hour. The conclusions I drew were that if the mileposts were off in Minnesota, Wisconsin, Iowa, Illinois and Michigan , they were off consistently (or more likely my odometer was off) and that as I grew more tired my speed control grew worse. My speedometer seemed (on average) to be off by about the same % as my odometer.

Mr. Kite · #10 06-20-2007, 07:39 PM

I actually bought a handheld GPS to calibrate my odometer. It is a basic, WAAS enabled, Garmin eTrex. Anyway, I haven't taken it on a long trip yet, but I did take it out for a short test drive. After 10.1 miles according to my Highlander's odometer, the GPS read 10.5 miles. I'll need a longer trip to get a better number, but it seems that my odometer reads at least 3% low. Once I get a better number, I'll use it to adjust my miles driven. I'm currently making an adjustment to my odometer readout based upon my interstate trip.

I think it is silly that my vehicle has a GPS navigation system and I am doing this tedious stuff. I plan on eventually doing this in my Civic as well, but its GPS system seems to be linked to the speedometer/odometer already. We will see.