Unfortunately your method does not take into account the inaccurate reading of the odometer. The first thing to do is use zero out the trip meter and do a run of ten miles as indicated by a GPSr. Now you can calculate to exact number of miles to use as correction factor for the odometer when checking the gas mileage. However there are other variables that will throw the calculation off. First would be that the tires wear as you use them, so the factor used to correct the odometer changes with time. Second is that you can't see how high up in the storage system the gas is so you might not be filling it to the same point each time. Third as related to the second the car has to be level and not on a small slope when refilling. Fourth The temperature of the gas at the time of refill might be dramatically different then the gas that was consumed.

So all in all for those **** compulsive engineer types this is a wonderful way to occupy their time and calm them down.

 

Double Tap, your hypothesis that the odometer is off, also, is correct. However, to keep things simple and since the amount of fuel used that the onboard calculates from is most likely the bigger of the discrepant variables, I suggest using the onboard odometer value in lieu of either calculating the odometer's error or replacing that value with a GPS-obtained value.

I need a nap.

 

But if it is all electronic these days ( and no mechanical connection between speed and odo)... is the odo necessarily off because the speedo is? I guess I will try to do a check this afternoon on the way home (assumng the mm's on the highway are close to accurate)

Cheers!

 

And you guys assume the GPS is correct?

 

You have to have SOMETHING to consider as a relatively accurate value. So what't the least of the evils? If you chose one (either the onboard odometer or a GPS) for your distance from fill-up to fill-up and you stick with that, you'll have good RELATIVE numbers, if not dead-to-nutz accurate.

Also, don't worry about the final number but use it to note any adverse trends. Even if it's inaccurate, it'll trend properly over time.

 

It is accurate to approximately 15 feet of actual location. That is truly amazing to me.

 

yes, trending is really what I am interested in, but did want to start with a relatively accurate number! Thanks

 

My vert`s speedo is about 2 fast at 75, my outback off by 3 fast at 75 and the wife`s impalla is actually correct per my GPS. The vert`s onboard milage is about 1 low, the outback is 1.8 high and the chevy is off by about 1.5 low. Pretty normal as far as I am concerned.

If you want better than that follow nuke`s advice.

 

My 05 base v6 also read 1.5-2mph higher than actual speed. When I switched from the stock 16" wheels to some used 17s that came with a set of p-Zeros, it read accurately. You can adjust your speedo with a tuner if you have one. Now you can justify getting either a tuner or some bigger tires!

 

Checked the odo yesterday - it reads between .4 and .5 low after 100 miles