My car lies about its gas mileage

One of the few changes between my '03 and my '06 Sentra was the addition of a neat little digital display in the center of the instrument panel. It can do several functions, such as outside temperature, distance to empty, and average fuel economy. Here's what it's reading now:

I filled up the car earlier in the week, and it took 10.6 gallons. Let's see - 267 miles / 10.6 gallons = 25.2 mpg. Pretty good, especially since there was an autocross in that tankful. However, it certainly doesn't come close to the dash display.

So how does the car calculate its mileage, and is it affected in some way by the turbo installation? I know the OBD system can measure air flow and vehicle velocity, so does it perform some calculation based on average air/fuel ratio and those values? Since the turbo blows through the air flow sensor, you wouldn't think the calculation would be that far off if that's the way its done. I don't think it has a fuel flow sensor anywhere, does it? What else could throw off the reading?

Did you change the injectors?

The fuel economy is normally calculated purely on fuel flow, which is calibrated to the original entire fuel system. Nissan may do something different, but I'm not sure why- the calculation is pretty easy.

Hmmm...

To be honest, I don't know if the turbo kit came with higher flow injectors, though that would make sense. I'll check on that.

How does it measure fuel flow?

The kit includes:

Components List:

Turbonetics T3/T4, ball bearing upgrade available $350 extra Turbonetics Evolution Wastegate Treadstone 347 Stainless Steel Cast Turbo Manifold Treadstone TR8 Intercooler with brackets Treadstone Polished Aluminum Pipe Kit Treadstone MAF adapter Treadstone Silicone and T-bolts Clamp kit Treadstone 304 Stainless Steel Downpipe (removes first cat) Treadstone Oil Kit HKS Blow off Valve, Greddy/Tial/Turbonetics also available Treadstone Air Filter Upgraded Fuel injectors with clips Upgraded Spark Plugs Vacuum line, Hardware, Gaskets APEX AFC

Aha - the smoking gun. That explains it.

jstein77 wrote: How does it measure fuel flow?

possibly by counting injector pulses. computers are very good at counting.

Most of them use injector pulse. The upgraded injectors would definitely throw off the computer.

jstein77 wrote: How does it measure fuel flow?

computer opens injectors for a specific amount of time... is all it knows is that it has to open for X amount of time... go from 10cc injectors to 20cc injectors (easy math not real life) you are going to spray 2x the fuel in the same amount of time as before unless you recalibrate the meter...

I don't suppose there is any way I could calibrate it?

Not only what Toyman says, but I remember my friends dads car back in high school had a little display that constantly changed to give you the average if you continually did what you were doing at that instant. i.e. High acceleration, you're getting 5 mpg, two seconds later you are idling and you're getting 25 mpg.

Does that slowly adjust or does it jump around depending on how you are driving at that instant?

(I'm kind of assuming it tries to average it for you.)

If you can tune the actual ECU instead of using a piggback/calibrated MAF it would probably work. Or you could find the data stream and make a black box that modifies the signal before it hits the dash.

My country squire has an instant and avg MPG and gallons used function. Still accurate with the motor and ECU swap, big injectors, etc. Ford EEC-4 is easy to tune though.

ECU calculates MPG from inputs like fuel injector open time, manifold pressure, throttle position, fuel pressure, vehicle speed, etc. Then uses some algorithm to combine those things into an output. Can't adjust for all environment conditions, so at best it is only 'close'.

Changes you made have really confused the computer, maybe can be adjusted but wouldn't be high on my list.....

just doing something as simple as upping the fuel pressure will throw the calculations off- more pressure=more fuel for the same injector pulse.. i'd assume the fuel pressure regulator is boost referenced to raise the pressure as boost goes up..

I'd be interested in seeing some test data on the accuracy of these dash displays. I know the one on my wife's Audi A6 was very optimistic. We owned the car from new and it never had any modifications done to it. It would routinely show average mpg near 26 mpg, while filling the tank and doing the math would yield numbers closer to 22. I think most of them are there to make you feel good, and have little connection to real world fuel economy.

My CooperS is routinely optimistic by 2 mpg, but my recently acquired Astro seems to be only 0.5 mpg optimistic.

The one in my '94 Passat GLX is spot on. I reset the fuel mileage display and the trip odo before a road trips and it would constantly predict almost exactly how much gas it would take to fill it up. And it just has a needle fuel gauge, not one of those displays that tell you how much gas you have or how many more miles you can drive.

My Tahoe MPG display is pretty accurate. The instantaneous gauge is pretty much worthless, but it does tell me if I'm on 4 or 8 cylinders so that's what I use it for. When I'm cruising on the highway in 4 cyl mode the instant gauge reads well into the 20's but the MPG gauge doesn't agree although I reset it while on the highway.

Did a quasi economy run awhile ago with my Fiesta. 120.3 miles- 2.3 gal.= 52.3 mpg On board mpg computer- 51.2. I have seen it go the other way, but not to a large extent.

peter wrote:

jstein77 wrote: How does it measure fuel flow?

possibly by counting injector pulses. computers are very good at counting.

Exactly- it's a very simple integration routine. The injector flow is part of the core calibration, so just integrating the known flow via pulse widths will give you a pretty darned accurate amount, and then combine that with distance traveled, and you have a very good estimate of fuel economy.

Change any of the non-measured components, and you change that calculation- fuel pressure or injectors will do it. A piggy back computer that adds more pulse but w/o the ECU knowing will do it, too.

jstein77 - I'm just guessing, but larger injectors would flow faster in a linear proportion. So your calculated mileage was 77.3% of the display. It's probably reasonably accurate to simply multiply your display by 3/4 to estimate your actual mileage.

I double-checked the mileage reading on my Subaru and it was within 0.3%. I should do some additional checks to verify but right now I'm taking it as accurate. The distance to empty, however, seems to be 10-20% off.

David

I'm astounded to see that the consensus appears to be that these gadgets are actually somewhat accurate!

Also astounded that there are so many privileged elitists on this forum. Aside from my wife's Audi and her 2011 Hyundai, I've never owned a car with a gas mileage doodad.

the trip computer in my 07 MS3 was pretty accurate, but for miles per imperial gallon. distance to empty usually was at zero right at or shortly after the low fuel light came on, so that may be the discrepancy DWNSHFT is seeing.

The one on our 2011 cruze seems pretty accurate, but errs on the optimistic side. Our honda insight before that (1st gen/5spd) was dead on as far as I could tell, and the instantaneous was shown as a bar that slid back and forth, it was very nice actually. Our cruze eco just gives you the number, so it's hard to follow, one second pegged at 99mpg the next at 20mpg (as opposed to the smooth transition given by the insight).

1988RedT2 wrote: I'm astounded to see that the consensus appears to be that these gadgets are actually somewhat accurate! Also astounded that there are so many privileged elitists on this forum. Aside from my wife's Audi and her 2011 Hyundai, I've never owned a car with a gas mileage doodad.

The fuel side is a super easy calculation. Speed/distance- some have issues. Especially if the tires are changed from spec.

In a case like this, where you get into "trouble" if you rely on the distance to empty closely. But all here are smart enough to trust the fuel guage, and not push it. Thankfully, it's an info gadget, and not likely to be a problem.

are you sure the car was full before resetting the trip counter? did you reset the avg mpg at the time of the last fill up? from my experience they are accurate but maybe yours isn't!? my truck likes to calculate the DTE after a fill up based on the rated highway milage, of course my actual milage is far lower as a matter of fact my average is lower than the city rated milage, btu the DTE figure adjusts itself and gets pretty accurate towards then end of a tank, then I fill up and it forgets ;)

The mileage computer on my 318ti is consistently 6mpg off. No mods, just the way it is.