I have a new CX5 Touring, AWD with the 2.5 engine and the 17" wheels and 1550 miles on it right now.


The onboard average MPG shows (as one example) 24.9 and after driving for 199 miles. That means when I fill up the tank it should be close to 8 gallons, but that is not the case. The pump shuts off at 7.25 and the car is on the same level surface I always use with my other cars. The pump is less than six months old. So I am confident that the amount getting pumped is a consistent shutoff.
Why would the physical calculation for 7.25 and 199 miles be so far off of what the computer is showing? This indicates I'm getting 27.4. That is 2.5 MPG off from what the computer states. Could this be an electronic oops where the factory put in the specs for the 19" wheels? That would show more miles driven than actual. I'm going to take a longer trip with known distance to check out the accuracy some more.

 

I don't think speedometers are deliberately set to read high, but you are less likely to get a speeding ticket that way than if they read too low. If they read high, you can guarantee to be below the speed limit, even when your speedometer indicates you're slightly over.

In Australia the indicated speed must not be lower, but can be up to 10% + 4km/hr higher than the actual speed. This applies when the car is tested as prescribed (correctly inflated standard tyres, unloaded, etc). The testing procedure and accuracy specifications are here (Motor Vehicle standards act 1989, Australian design rule 18/03 (.pdf)):


5.3. The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5. above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).

0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h

Because of this, police in Victoria only let you go about 2km/hr over the speed limit before they book you. As far as I know, New South Wales still allows you 10 km/hr.


Basically they got tired of going to court over speeding ticket law suits so they all read high to one degree or another.

 

Do you always reset your economy meter every time you fill up?
Otherwise it will act as a historical average, not an average of your current tank of gas.

 

I like that answer and my response is no, I was not aware that it could be reset. Any quick clues or do I have to dig back through the 1" manual again?

 

Since I live in a different market I suggest you read your owners manual.

 

Could this be an electronic oops where the factory put in the specs for the 19" wheels? That would show more miles driven than actual.


Uhhh, no. The outer diameter of both 17" and 19" wheel/tire combo is the same, so to the computer they are identical.

 

I would think that the odometer is more accurate than the mpg indicator, assuming that you are resetting it each tank. Mine seems to be pretty accurate as I track each tank with Fuelly. On the other hand, my VW Golf TDI is usually a little lower than the computer, usually 2 mpg to up 4 mpg depending on how I have driven on that tank possibly.

 

To reset the Avg MPG, just scroll down to the screen and then push the OK button and hold for about 3 secs. I do this every time I fill up my gas and have found that the computer and my manual calculations are fairly close.