I know that typically they read high vs what you calculate by hand, but 6mpg?

For the longest time, I would always calculate 2, maybe 3 high. Now all of a sudden it's jumped to 6 for the 2nd fill up in a row.

Onboard showed 32.8mpg, 358.6mi on the ODO, put in 13.3gal, calculates out to 26.9mpg.


I noticed this started to happen after I had my car at WMW, but nothing he did related to fueling or the computer. He just replaced a coolant sensor. Im stumped.

 

Bueller?

 

I've had worse. I've had the computer be up to 7.7 mpg adrift from the hand calculated. Both quantities are subject to uncertainty, so the difference between then will also vary. (In other words, neither is exactly right all the time.) Hand calculated milage is not necessarily totally accurate, it depends exactly how full you fill the tank. If you average the mpg over more tanks they variation get less.

 

Correct me if I am wrong, but isnt the average mpg showing on the computer an over all mpg until you go in and reset the computer the hard way, not just pushing the reset button? It keeps an ongoing total of mpg over the course of time. Tell me if I'm being ignorant.

 

Average MPG is an overall mpg until you specifically reset the average MPG. On the boost radio you do it by bringing up the on board computer display and pushing the button under the mpg. That then gives you a reset button to push. Same with average speed.

I reset them with every tank.

 

Yea, I reset the AVG mpg every time I fill up. Reset the trip, avg mpg, avg speed

 

Man, it happened again. OBC reading 36 on the dot, calculated 30 on the dot. Different gas station, hell, different state. Im stumped.

 

My understanding is that the OBD mileage is fixed. You need to go into the computer and change it manually.

This thread gives instructions: https://www.northamericanmotoring.co...procedure.html

I track my mileage on Fuelly.com and have tracked my mileage for the last 33k miles and I'm averaging 31.9 mpg. I need to change my OBD mileage too.

 

It is optimistic. I have the same issue with mine. Not sure how the computer has things set up, but it seems to make sense given the car also shows me going 3 mph faster than I actually am on the digital readout (vs. my garmin GPS and iphone GPS). My MINI usually shows mpg as being 3~4 mpg optimistic. 6~7 is really high in your case.

 

Wow, thats interesting. Im more concerned about how the difference has seemed to increase. Before, It was reading about 3 high, now its up to 6. Like I previously stated, WMW worked on my car in June, so maybe they did something? I may call this weekend and pick their brains.

 

I'll assume that MPG is calculated by the computer using volume and not mass. Don't forget there are summer vs. winter blends of gas, as well as ethanol and non-ethanol gas. The energy per volume of these is different, so a sudden change in your calculated MPG could be due to blend changes.

I don't know enough about the fuel industry to know any more than that, but I did stay at a Holiday Inn Express last night.

 

Tell me about your wheel and tire set up listed in your sig: Enkei RPF1 w/RE01R.

When running "stickier" tires with wear ratings of 180 traction "A" ( your Bridgestone RE01R included ) somewhat lower mpg's are expected due to higher rolling resistence. For example, I recently switched from a 280 wear rated tire to a 580. My MPG increased by 2.7 mpg.

 

Interesting too that it was noticed after a thermostat sensor swap though. One of the side effects of my having a cooler engine using the M7 180* thermostat is lower mpg....

Do you have a ScangaugeII that you can refer to and see what your operating coolant temps are ?

 

The tires are the OEM size 205/something/17, but I noticed the change before I got the wheels and tires put on, and nothing has change MPG-wise since I changed the wheels.

I do not have a Scangauge, but I do know how to get into the secret OBC temp read-out.

 

If you're wanting exact figures, lots of luck. There are some external things to consider that don't show up in the eqations.
Have you ever noticed that after of few days of running around residential areas at 30 or 40 MPH your computer calculates your range at XXX miles and then when you get on the highway and ramp it up that after a few miles your calculated range actually increases - sometimes dramataically? Three other things that affect milage that the OBC doesn't consider are relative humidity, ambient temperature and barametric pressure which are all three, changing constantly.

Don't forget gear ratios, accelerating up hill and coasting down hill.