How accurate is the mileage guage?

The display tend to be slightly optimistic but is surprisingly accurate.

 

All I have read indicates that the MFD gauge is very accurate. It uses the pulse time of the injectors and distance traveled to compute the mileage. I also read that it uses the data from the ICE ECU and HV ECU to compare to make sure the numbers "compute".

Using the amount that you add to the tank vs miles driven, is on the other hand, very problematic. Problems include:

1. Capacity of the tank bladder changes based on temperature, so it is not always 11.9 gal as listed in the owners manual. It loses capacity as the temp goes down.

2. Not all pumps shut off at the same threshold, so the amount of fuel added depends a great deal on the particular pump you use.

3. The venting of the tank is quite different from other vehicles (for lowering Evap Emissions) . It is a good idea to pump as slowly as possible (lowest setting on the nozzle) to allow the tank to vent through the system. Different pumps may pump at different flow rates.

4. Overfilling can damage the Evap Emissions system, so its a really bad idea to top up after the pump shuts off. Most experienced owners seem to recommend letting the pump shut off and not trying to add much more.


I have personally found that amt added vs. miles driven can vary wildly from tank to tank, but averaging 3 or 4 tanks calculated will generally agree with the MFD.

 

<div class='quotetop'>QUOTE(G-Man @ Jun 6 2007, 07:57 AM) [snapback]456529[/snapback]</div>

Mine is pretty darn accurate. Over the 1st 20k miles the guage was 1.02% optomistic. I'm up to 26k miles now but haven't yet input all the numbers to see if it's changed, It doesn't seem like it has.

 

<div class='quotetop'>QUOTE(dogfriend @ Jun 6 2007, 08:14 AM) [snapback]456543[/snapback]</div>

Although the MFD itself may accurately report the mpg, etc. don't expect the fuel gauge on the dash or even the output of the OBDII connector to be accurate or consistent. I've seen CAN-View report drops by as much as 5% in remaining fuel followed by a rise back to the original reading on occasions.

I don't believe this is the fault of either the CAN-View or the output of the OBDII. It's probably just because of variations of the mechanism within the gas tank.

Dave M.

 

<div class='quotetop'>QUOTE(dmckinstry @ Jun 6 2007, 01:00 PM) [snapback]456769[/snapback]</div>

Yes, I don't have a CAN-View or ScanGauge, but the Fuel Gauge is very inconsistent, sometimes I can go over 100 miles before the first "pip" disappears, other times less than 50. I have also noticed that it seems very non-linear at the bottom end as well.

From the Tech Training Manual (Fuel and Evap Systems):

Also, in the same document:

 

<div class='quotetop'>QUOTE(G-Man @ Jun 6 2007, 09:57 AM) [snapback]456529[/snapback]</div>

As they say, close enough for government work. I prefer my own calculations.

 

Similar Problem

I have had a similar problem the last two times I filled up. I'm presuming that it is a result of the auto-cutoff not being the same. It really is a frustrating system--my favorite gripe about this otherwise intelligent car. My two most recent discrepancies:

Calculated 52.09 mpg, display said 51.3
Calculated 43.85 mpg, display said 50.1

I think the top one is a result of 'topping off' more than normal where as the bottom is a result of just the opposite(?) Is it a result of too much concern about completely filling the bladder--and the weather turning cooler--do other folks see this happening?

 

Manual calculations on a single tank is a fool's game. At best, you are just going to frustrate yourself trying to understand the differences. There is enough variation in the bladder and individual filling pumps to defy any formula, so give up the attempt and accept the fact that your fills will vary from tank to tank. You will feel better for it.

Tom

 

The one thing you can be certain of is, when the last pip start to flash you had better buy gas immediately.

In the great majority of cases reported here the MFD fuel economy is a little greater than the manually calculated fuel economy. Maybe the MFD is being deliberately optimistic, but note that this discrepancy is also in favor of the gas station: they make more profit by charging you for gas you didn't actually receive from the pump, and this makes the manually calculated fuel economy figure smaller than the true fuel economy.