:)

Another Way
 

The best way I've found to correct an odo reading is a GPS unit reading, followed by the mile marker.

The ScanGauge allows percentage corrections on both the fuel consumption rate and the speed, Plus or Minus. Makes it nice for decent reading.

RH77

You bring up some very good points, especially comparing with other vehicles. I would like to get a GPS and find out how accurate my readings are, I have compensated by 7.1% for my tire size being larger. My readings after compensation are very close to google maps.

Has anyone compared their gps mileage reading to an online maps reading? I would be interested to know how accurate google maps are.

I check my odometer against Google Earth. For instance, the trip from my gas station to home is 6.44 miles according to Google Earth and my SG and odometer read 6.4. I'm calling them dead-on. It would be interesting to see the results if my odo and SG did 1/100th's like Google.

Since I don't have a GPS, I also used Google Earth. It took about 45 minutes to measure a 28.1 mile distance between 2 freeway offramps at a high enough resolution. My CRX HF has 14" rims with 185/60/14 tires and I thought it would read low. To my surprise, it was just about dead on in mileage on the 28.1 distance. This would indicate to me that with the stock 165/70/13 (I think), the odeometer would be reading high.

I think it's a valid point, but I'm just gonna ignore it. Well, unless my new tires show a decrease in FE. ;-) I calculated something like 2.0% for the difference in my old worn out ones to my new fully treaded ones. I would probably be able to see (but not prove) a sudden, consistent 2% drop.

When it comes to measuring distance as an odometer, a GPS is very accurate. The GPS speed reading is also very accurate. Anyway, checking the odometer is a good reason to get a new toy!

Q

QDM -

I agree that the GPS is the best. In every sample run I did, either 20 or 30 mile segments, I always got the same difference when I compared it to the odometer.

I do think that the GPS needs to have a "clear sky" view to the satellite, aka keep it front and center on your dash. Otherwise, I think that when it loses connection, it "auto corrects" with a straight line between points even though you may have been curving along and/or made some turns when the GPS was disconnected.

If someone doesn't have a GPS, find a friend that does!

CarloSW2

My speedo is about 15% optimistic :) But the actual odometer is dead-on, so it is OK for my calculations.

My speedometer is dead on, but my odometer is about 2% high (it reads 10.2 at the 10th mile marker). I generally ignore the difference because my odometer doesn't count when the engine is off, so every time I do an EOC there is about 2 seconds of driving where I have the car off before I go back into the on position where the odometer is not counting. I live in a hilly area and do a lot of EOC so I be that is close to 2%.

If I ever get around to installing an injector off switch so that I can EOC without taking the key out of the on position I will have to start correcting my MPG numbers by 2%.