2007 E Wagon

Hello All,

Does anyone else have a difference in the overall Accuracy of the MPG on the cluster vs actual calculated MPG? I am noticing a difference of approximately 2-3MPG.


Thanks,

Greg

Mikey53

That's pretty much what I have seen with any car I have owned.

mmaldin

+1
That 2-3 more MPG on the cluster than the car actually gets.

StanNH

My car's computer is typically within 1 or 2% of the actual, calculated numbers. It's the most accurate computer I've had on any car I've owned.

sean1.8t

I'll never trust the cluster. It's too easy to just do the calculation and know it's 100% accurate.

tivoboy

I think one is ALWAYS going to see a difference in the calculations from the onboard computer and doing a tripmeter / gallons loaded at fill up.
The later is always going to vary quite a bit, time of day, elevation, different pumps/stations/nozzles, etc. Just a half gallon difference would be a difference of at least near 1 mpg.

I find that resetting the onboard computer, and then doing a TM/GL calc, it's between 1-2 off for MPG.

That said, the MPG not as good as I thought it would be. I'm getting a combined 24-25, and on long distance at 70-72 AVERAGE I'm getting about 27/28. Speed kills, true, but I figured I'd get something better in 2017.

sean1.8t

I don't think any of those variables are going to effect the end calculation. Pumps are regulated pretty thoroughly and must be accurate in their dispensing readings. You put 15.5 gallons in at gas station A and it's going to be the same 15.5 gallons as gas station B. I would say the only real variable the could be a factor would be orientation of the car [tank]. But I've yet to go to a gas station that wasn't perfectly level so...

But I'm seeing the same mileage on my 450. ~23 combined and upper twenties highway (which I really haven't done too much). I don't mind though. I romp on it quite regularly and as long as it's not in the teens, I'm happy. And this thing does make me happy every time I drive it even after 3 months

MASSC450

Are you complaining that your AWD vehicle with a dual turbo charged 362 HP 3.0L V6 engine ONLY gets 27/28 MPG on the highway?

You're kidding, right? For the record, I'm getting a combined 23 MPG...WAY better than my less powerful single turbo 4 banger 2008 EVO X or 2012 STI which both had a combined 19 MPG.

S500Sport

Combined I get 15.5 mpg, 90% city driving. This is the same combined MPG I got on my former 2014 C300. Given the power differences, I'd say that's a good deal.

Mplayers2006

This may sound dumb, But could someone explain to me how they calculated MPG by hand. I don't see how this can be done without a good size % error being made.

sean1.8t

Math. Division to be specific

dieseldoc

Easy to do. Fill your car up with fuel. Reset trip odometer. Drive car. Refill car. Take mileage from trip odometer and divide by gallons required to refill tank.

Mikey53

The only thing I'd add would be...stop fueling when the pump clicks off, don't top off the tank.

Mplayers2006

LOL, Thats how I thought yall were doing it

But I'm assuming some of y'all are forgetting two things:

1. our gas is priced at 9/10 a cent, so when you do your calculation you must input the price in 4 sig-fig (X.XX9) digits, not doing so adds on an error.

2. How do you know how many gallons are present in the tank before filling ? y'all Have to assume there is XX gallons cause there are x notches on the dash.
(I'm assuming there is no way to find the current volume of the tank from inside the trip computer.)

Doing it by hand will give you a worst estimation than the trip computer. Unless someone can explain another method to me that accounts for 2nd error.

dieseldoc

I am old! I have been driving cars before there were trip computers. It does not take a doctorate degree. It is simple math. Fill the tank till it is full. Drive the car and measure the distance drove until the next fill up. Fill the car back up again the same way you did the first time. The gas pump tells you how many gallons you put in to refill the tank. Take the miles you drive since last refill and divide by gallons put in.

www.mistupid.com/automotive/mpg.htm

Mikey53

Price is irrelevant when calculating mpg.

Don't care how many gallons are remaining. We only care how many you burned. The pump shows you how many gallons it took to refill the tank.

This isn't nuclear physics

dieseldoc

+1 It is advanced rocket science for some people.

sean1.8t

Hahaha!

Mplayers2006

I meant to clarify the first point cause you can take the dollar amount and convert it to gallon amount (volume) and solve from there. But im assuming no one is doing it that way. I was only pointing out a common error people over look.

For the 2nd point - Yeah, duh.. but your not understanding what your doing if you think its just that simple. You are rounding MPG when your doing it that way. Your not taking into account the full potential of the input (gas). You said "We only care how many you burned" How do you know you burned XX amount of gallons of gas (I will assume below)?

If you put 10 gallons of gas in your car (ignoring price), You will not know when you drove 10 gallons out, or do you ?
If your assuming that 10 gallons = 7 notches on your dash then your wrong 10 gallons ~ 7 notches.

Correct me below where I assume or am wrong.
So your using 7 notches as a referencing point, right ? Once 7 notches has dissipated, you are taking the Delta X = total miles (starting miles = X_1 and finish miles = X_2; X_2 - X_1= Delta X). Now your taking Delta X over total gallons = MPG (AVG), right?

Conclusion:
If That is what your doing, and you think that will give you an error free result your WRONG. Here is My argument, doing it by hand creates an error. How ? You dont know when you burned (consumed) 10 gallons of gas. There is no way you can tell unless your calculation the volume of gas in the tank.
I'm not saying your primitive method is wrong to do. All I am saying is you are NOT getting an exact value, your only getting an estimated value. Same thing for the trip computer calculation, but it will result in less error cause the system can better determine; total gallons and total miles. Being informed on techniques rocket science implement to test and formulate formulas would help you understand the loss that your ignoring in your method you "deem" so accurate, FYI.

ALSO, MPG is an Average, Right ? So the longer the test, the better results. That could be another error introduced that some people overlook.

Mikey53

Primitive? Get your mom to figure it out for you.

sean1.8t

You are over complicating this like crazy.

It's very simple: When you fill up, the nozzle stops at the same fill level "X"(the constant) give or take a few ounces(acceptable variation). So when you fill up, you know that you are always at X and that what you need to pay attention to for this equation is the difference "Y". So simply put, you take the miles traveled from last fill up and divide it by Y.

What is so hard about this?

Mplayers2006

LOL, Really.

I am not trying to cast aspersions but Miles/ Gallons = MPG is primitive method since there are more effective methods and formulas to calculate MPG. You do understand that the EPA does NOT determine your car MPG by simply putting just Miles/ Gallons?

sean1.8t

Dude, you are hilarious. Not only are you wrong, but you're an a$$hole as well.

Mikey53

Here's your first mistake. It's miles/gallon. Either use the recommendations multiple people have given you or create your own method.

I've killed enough brain cells on this discussion.

Mplayers2006

Let me put this statement in bold cause I feel people think Im saying their method is dumb or is stupid/foolish.

The method is not wrong(in a sense) , but results in an estimation. It is NOT, Again NOT foolish nor stupid to determine MPG by finding total miles and dividing that into your total gallons. It is foolish though, to think that you are getting a 100% accurate MPG by using gallons to reference on your gauge cluster. You are approximating by doing so which creates a rounding error.