Question regarding average mpg calculator

Ok so I know this topic has been discussed to death but I have some thoughts that I couldn’t find being discussed in previous threads. I recently upgraded my wheels and saw the obligatory drop in efficiency (Boy I sure did pick a great time to do this considering gas prices and world events). I noticed a week or two before changing the wheels when comparing my speed via a Speedometer GPS app on my phone and driving through a radar speed trap that my speedo with OEM wheels was off by a couple miles per hour. After installing the larger rims and tires the speed is essentially perfect when comparing my speedometer and GPS app. Now here’s my question..is the average mile per gallon calculator on our dashboard calculated via the Speedometer? Are the traveled miles on the odometer more accurate now? I would imagine the computers on board use the Speedometer/wheel speed sensors to make these calculations. Just wondering if anyone has any insight if that’s true. I am doing some manual calculations over the next few tanks to try to get more insight but that begs the question if you’re calculating gallons used to miles traveled how can you be sure that the miles traveled are accurate. I imagine you could use an app that would track how many miles you have actually traveled and do the math but at that point I’m out, too much work. Just curious about the onboard computers and how they work.

 

1. Two miles and hour is well within the margin of error at 70 mph.
2. Yes, the instant mpg calculator uses the wheel speed sensor.
3. Yes the instant mpg calculator is only there for convenience.. doing the mpg calcs by hand is the most accurate way.
4. By far the biggest variable is the environment and driving style. A few mpg headwind at 70 mph can cause you to lose a few mpg for awhile and you don't even know it.
5. If you really want to check your mpg, and you want to be accurate, you need to do this for a whole year. This takes variables like winter gas, winds, stop and go, temperature and everything else out of the equation. Doing it for 20 minutes and a few gallons is a waste of time. It means nothing.