[nayr14]

The last 10 times I've filled my tank, the MPG calculated by the computer is consistently high by ~2-3 MPG. After I bought my car the first few fills were within 1 MPG high/low.

When I installed my CAI and reset the computer I thought it might correct the MPG discrepancy, but it made no change. Any idea why it would be rating high? If I knew more about how it calculated the MPG I would try troubleshooting more but I'm at a dead end. Any ideas?

 

[Bullets]

Just to be clear, did it start doing this before or after you changed the intake? Also, did you change anything else at all on the car?

And does your gas gauge seem accurate?

 

[stiller fan]

have you checked it manually, by figuring it out yourself?

 

[Pilm1112]

The last 10 times I've filled my tank, the MPG calculated by the computer is consistently high by ~2-3 MPG. After I bought my car the first few fills were within 1 MPG high/low.

When I installed my CAI and reset the computer I thought it might correct the MPG discrepancy, but it made no change. Any idea why it would be rating high? If I knew more about how it calculated the MPG I would try troubleshooting more but I'm at a dead end. Any ideas?

I wonder if it has to do with driving style? I drive my car very easy and calculated MPG is always within one or two tenths of actual! My last fill up I calculated 38.2 mpg while computer said 38.3 mpg. Anyway, wonder if those driving their cars hard might get more discrepancy.

 

[nayr14]

The discrepancy started just a few fills after I purchased the car in May. The intake doesn't appear to affect the MPG reading. I have used different pumps and I fill at least half a tank then calculate it manually via the trip meter.

I have started driving it harder, but yesterday I did a 230 mile trip and I started on a full tank. It was mostly highway speeds round trip. For 1/2 the way back I was driving aggressively without cruise control in order to try to beat some rush hour traffic. The car's calculated MPG was more than 2 higher than what the pump said.

Another issue I noticed was on the ride back it was giving me very high instant and "current" MPG readings (going 70-80 w/o cruise control) around 39 MPG which is unusual. I wonder if there is some sensor that isn't returning a true reading to the system. But I don't know how it actually calculates the MPG!

 

[drdiaboloco]

My trip computer averages high, but it's 0.4 high. 2-3 seems to be well outside of what you could consider "acceptable variation".

 

[Gfn8r]

FWIW, mine is about 1mpg high generally. (This is something where I figure that a manufacturer can fudge things just a little and get away with it! :dunno: )

 

[namegoeshere]

This is something where I figure that a manufacturer can fudge things just a little and get away with it! :dunno:

Or the gas pump(s) the OP's using isn't accurate.

Gas pumps not always accurate

When it comes to buying gas in Wisconsin, you don't always get what you pay for.

A Journal Sentinel analysis of nearly 60,000 gas pump inspections shows that more than 2,000 pumps delivered a different amount of fuel than the meter registered in the past two years.

In some cases, that is a good thing for motorists, especially at a time when an average gallon of gas costs more than $3. Most faulty pumps dispense extra gas - a little more than half a teaspoon per gallon on average.

Motorists unwittingly hit the jackpot at Weber's of Brillion gas station in Calumet County. In October, the station dispensed about 4 gallons of extra gas for every 15 gallons pumped.

Then there are places such as Kettle Citgo in Waukesha County.

In May the station shorted customers more than any other in southeastern Wisconsin, according to state data. Customers lost out on a little more than a half-gallon for every 15 gallons bought - close to $2 worth of gas.

Bruce Hanke, the station's owner, said the shortage was because of a broken part in the pump.

"I don't do anything to rip off any customers," he said." (The pumps are) tested yearly, and any problems are taken care of immediately."

The majority of pumps in the state are on target, inspection records show, but about 3.5% of stations inspected didn't put in your tank what the meter registered during the two-year period.

Racine County had the largest percentage of problem pumps in the seven-county metro area with 7% of stations' pumps failing state inspections. Pumps in the city of Milwaukee failed about 5% of the time.

The problem instead is faulty equipment, said Mike Pappas, who inspects fuel pumps for the state's weights and measures department.

"After years of wear and tear and usage, things break," Pappas said. "Usually when we find an error, it is due to the fact that (the pump) is a mechanical device."

.....

Most of the time, consumers question a pump's accuracy after finding that their plastic, portable gas can holds more fuel than expected. Or, for instance, a car that has a 14-gallon tank will fill up with 14.5 gallons.

The plastic gas can, and even a car's fuel tank, isn't necessarily accurate, inspectors say. Plastic cans bend and bulge, and a car's gas tank doesn't always match what is outlined in the manual, Pappas said. Plus, fuel expands and contracts because of weather. When it is below zero, you can get more gas in your car.

http://www.jsonline.com/news/29276359.html

This is why when calculating your MPG by hand you want to refill using the same pump as your last fill-up's.... this helps to rule out any gas pump inaccuracies.

 

[drozzin]

My calculated by car MPGs are higher than real MPG on 1.4 in average. I do a log and can estimate real MPG numbers quite well. I do a lot of highway driving. I thing that difference is less in a city driving.

 

[tkoft]

How do you calculate your own gas usage? Unless you drive until your tank runs dry, there's always a good amount of gas left when you have to fill up again. This might account for the 1-2mpg difference.

 

[03LXV6]

Fill up. Drive x number of miles. Fill up again. Divide miles driven by gallons put in.

 

[drozzin]

How do you calculate your own gas usage? Unless you drive until your tank runs dry, there's always a good amount of gas left when you have to fill up again. This might account for the 1-2mpg difference.

I always fill up and track amount of gas and number of miles since last fill up.
My previous car (hybrid Accord 2007) inflated MPG only by 0.3 (about 1%).