WD40 check this out, bold print especially. Hmmm.....
"To measure the VSS counts per mile get on the highway and find a mile marker and reset you VSS counts to zero. Drive ten miles as marked by the mile markers and note your VSS counts. Divide by ten to get your VSS counts per mile. Take an average of this process if you really want to nail it down, but I do not feel that is necessary. You may need to test this count every 6-12 months to readjust this number."
VSS - Vehicle Speed Sensor
The VSS count is fairly straight forward. There is a device on your car that rotates and creates a certain number of pulses per revolution. The sensor is usually attached to your transmission and it has a direct gear ratio to your tires that I will call the rolling ratio. This sensor is used to feed the vehicle computer the speed and distance data. The actual gear ratio has only one main variation, and that is the tire diameter. For the MPGuino to calculate the the MPG, this input is critical for determining the distance traveled. The VSS count is typically measured in counts per mile.
Theory vs. Reality
As with any engineering of a product you will have fudge factors. The important thing to realize is that you always design to get the fundamentals close and then expect to calibrate to meet the accuracy needed for the function. For this product we will evaluate the best way to calibrate and adjust to improve the accuracy of the numbers. This may not be necessary at all if your goal is to get feedback on your driving habits, then the accuracy does not matter, as long as you realize that the MPGuino is changing your driving habits to conserve gas AND best of all save you money.
So first let's discuss the factors that will affect your accuracy.
Factors NOT in our control:
Is it possible to control or measure these factors? yes, is it practical? not in my opinion. So these factors will introduce errors that will need compensation in the fudging process. I feel it is a pretty safe assumption that over time these factors will average within a range that will permit fudging.
Injector Pressure - Your car has a pressure regulator to manage your fuel manifold pressure, and it will regulate to it's setting. You could measure your pressure, but then you would have to know how that affects the fuel flow through the injector. We also do not know how the pressure changes over time and this can affect your fuel flow.
Injector Efficiency - Well, since MPGuino is a device that was targeted to older cars, then you have to expect that injectors tend to clog and become less efficient over time. The only way to calibrate that problem is to measure the actual flow through each injector. Have fun if you want to tackle that project. `,~)
Rolling Ratio - The rolling ratio is a constant changing ratio that will affect your VSS counts per mile. The main reason it changes over time is due to tires. As a tire gets more worn your VSS counts per mile will increase. When you replace that old set of tires with new tires, you will have less counts per mile.
Fudging/Calibration process
There are two areas I want to focus on to get this system as accurate as possible. Let's consider fudging the fuel flow rate and measuring the VSS counts per mile. I am taking this approach, because measuring the VSS counts is a practical option and it can be done with some accuracy. In contrast the fuel flow rate used to calculate the total fuel usage is not very measurable and therefore should be fudged.
To measure the VSS counts per mile get on the highway and find a mile marker and reset you VSS counts to zero. Drive ten miles as marked by the mile markers and note your VSS counts. Divide by ten to get your VSS counts per mile. Take an average of this process if you really want to nail it down, but I do not feel that is necessary. You may need to test this count every 6-12 months to readjust this number. You can compare your numbers to you odometer, but understand that the odometer does not take into account the tire diameter as accurately as your measurement, but it can be used as a verification that you are in the ball park. The best verification would be if you noticed that you have a fairly repeatable offset from the odometer and your calculation.
To measure the fuel is much more difficult. Really the best way to do measure the fuel would be to drain the tank and run the tank dry of fuel. Now weigh the amount of fuel and put it in the tank. Run the tank dry again and compare the actual fuel usage with the MPGuino. Again do this multiple times and take the average. Personally I would not do this at all, but it is the only real way to get acuracy. For a much more practical method, start a fuel usage log. Go to many different gas pumps and take an average over time. I have noticed that the cheapest gas stations tend to calibrate to the high side of the legal limit, if not actually outside the legal limit. Try to keep a standard procedure, like only filling to the first click off. Compare your fuel usage average to your MPGuino fuel usage average and look for a trend. Now fudge the fuel usage to match.
MPGuino - Combustory