Bocatrip

After many years of LS430 ownership (99% city driving) I recently started using the car's computer for calculating my gas mileage. I compared it to my manual calculation of miles driven divided my gallons used. I find a discrepancy of approximately 1/10 -4/10 mpg more (with car's computer) than when I do it manually. Has anyone else compared the car's calculations for MPG to their own? Thanks all.
PS...With 99% city driving I average between 14-15mpg.

kos

iTrader: (1)

It's close like you said, sometimes I'll go sit in mine at lunch and let it idle, which will drop the "dummy meter" number down, but then when I hand-calc, it will be higher.

Thecar

I just did 1000 miles of highway at the beginning of this month. I found the gauge was only slightly high (a couple percent at most) when calculating my highway fuel economy. Contrast with my former BMW 3series with was wildly optimistic.

Johnhav430

I want to say mine is within 0.4 mpg. Keep in mind it helps to use the same pump, and stop when the pump stops, and take the gal pumped in and compare to the odometer. No lie my BMW is within 0.0-0.1. Every time. My wife's GM isn't even close. We don't panic when it says 9.8 mpg. I have no clue how it, or any car measures it to tell you the truth. There isn't a meter measuring the flow of gasoline. It's extrapolated. And for whatever reason the Germans nailed it. Consistently 0.0-0.1 off from comparing to the pump.

Bocatrip

Pretty much what I'm getting...up to .4 mpg higher on the car's computer. The variables are when the pump does the automatic shutoff. Each pump might be different. Also, if I'm idling for a few minutes waiting for the customer in front of me to pull out, it's calculating 0 mpg for that period of time. I guess it's close enough.

Johnhav430

At one point all I had was the BMW. So my buddy would say I don't go by the dashboard it's way off, on his Lincoln Navigator, and I didn't understand why he was saying that, mine is exact. Once my wife got a new GM SUV in 2011 rated 16/22, I was horrified, the mpg computer would say 11-12. Then I'd do a hard calculation based on the pump and it'd be 14.5. At first, when getting full service in NJ, I'd want them to add some more fuel since the pump would often stop 2 gal shy of what the car actually said. again, not sure how it determines fuel used, it's not metered....

2KHarrier

Over on the Chevy Volt forums, we discuss the MPG/Range/KWH Used meters that are on board the vehicle and they are affectionately known as the "Guess-O-Meter"

You could almost say it's a similar situation with the LS, but I found mine to be within 1/10 MPG or so, much like your findings.

911LE

The fuel used is metered/measured by the ecm. It calculates the correct air/fuel ratio for any given situation (rpm, load, throttle position, intake air temp, cold start modifier, manifold pressure, etc.) and then commands the fuel injectors to stay open a certain amount of time, in milliseconds. Injector flow rate multiplied by open time gives the exact amount of fuel used. Since processing power is finite on any given system, manufacturers decide how much of the processor they want to devote to "features" such as this. Systems with faster processors/more memory can obviously use that power to make very accurate measurements and calculations.

jayclapp

Your mileage will vary depending on your routes, speeds, time at varying speeds, and weather conditions and how inconsistent you are in driving. This makes determination of gas mileage on a single tank of gas an inconsistent result. I find the best method of knowing what you car (and you the driver) is doing is to keep a record of every fill-up you make and calculate the mpg for the milage driven since the last fill-up. You can then easily compare the previous fill-ups versus the current fill-up. You will notice when the mileage is significantly higher or lower and likely know that the driving conditions were different. If you don't have an answer and it persists, then you can question as to whether something is wrong with the car. You can get a better answer by averaging the mpg using the milage and cars consumed over a few months for instance.

I never use the cars computed mileage. However, the instant calculation in the car can be a good teaching tool for learning how to drive to maximize mpg, if one is interested. I could care less, as I will drive the how I want to in order to enjoy it. Maximizing mpg is not one of my priorities.

Bocatrip

Thanks everyone! Very Helpful. Bottom line is that the car's computer is not absolutely on the money but fairly close. As mentioned by Johnhav430, mine also varies by up to .4 mpg (compared to manual calculation) which is close enough. I drive very conservatively and get 14-15 mpg with 99% city driving. However, if I decide to get on it every now and then, I'll notice 12's as my city average. Regardless, It's still a comfortable ride and worth the extra thirst!