ScanGauge: How much calibration required on each tank?

[ShadeTreeMech]

I wonder how much of the problem is related to another problem I'm having, which is the SGII randomly shutting down and restarting. It happened to me on a recent 250 mile trip. On the way down, it showed I got 28 mpg (I was going to a lower elevation on average). When I refueled, the manually calculated MPG was about 26, which I kind of expected since I was going back up in elevation. But my fillup gallons used was off by about 2 gallons or more. I have noticed the miles used since last fillup sometimes don't match the miles on the tripometer by quite a lot, which would seem to point to the same problem. And BTW, I don't stop the engine and coast as other more diligent folks do; my old cars tend to have cranky starters.

The scanguage can be an accurate tool, but it has its faults. If it generally seems to be pretty close, I generally ignore the odd 2/10 of a gallon discrepancy. I'm curious though how accurate the nozzle is to pop at the same place everytime. I suspect factors such as weather, temperature of the fuel in the pump and in the car, and how empty the tank was can affect when the nozzle pops. Once upon a time I would refill the tank to the reading on my scanguage (once I had it zeroed in) and it was surprising how close it usually was.

[euromodder]

I had stopped using mine on Hägar the Diesel stove as it was too susceptible to changes in driving style.

The way I feel about it, it was more calibrated to how you drive the car, than to how much fuel the car uses.

Since getting the up! , I haven't bothered to install the SG again.

[MileMonger]

Sounds like my experience. It seemed to respond really well out of the box, but after using it and calibrating it a few times it seemed inconsistent. Now that its been calibrated every fill up for almost 1 year it's a lot better. I wonder if that has anything to do with it or if it only counts back 1-3 calibrations.

[t vago]

Originally Posted by iveyjh

MPGuino, accurate once dialed in.

My MPGuino is also rock-solid accurate. I have an UltraGauge (cheaper and flashier version of the ScanGauge), and it will vary wildly in its tank fuel economy calculation. It'll tell me that I get anywhere from 19 MPG to 22 MPG, when the MPGuino (and my tank calculations from each fill-up) will reliably report 24-25 MPG.

[PaleMelanesian]

Too soon to say in my Fit, but in my old Civic, my total error between Scangauge and actual pump gallons was less than 1/2 gallon per year. Once it's calibrated it can be amazingly accurate. Individual tanks would vary by a lot more, but that's the fault of the pump rather than the 'gauge. It all averaged out.

[bikenfool]

Does anyone know how the SG or UG actually calculate fuel consumption? I asked the scanguage folks & they wouldn't tell me, its proprietary. Since they're not reading injector pulses they must be doing some estimation using throttle position, MAP, and RPM? It seems like you could get it calibrated to give you a fairly close average MPG for a given driving style and type of route, but if you change your style or drive more or less hills, or highway then the calibration could be way off.

[gone-ot]

Simply:

a) assume A/F-ratio of 14.7:1 (closed-loop ECM);
b) measure Air flow from MAF or MAP sensor;
c) backsolve for GALS/time;
d) and divide: MPG = (MILES/time)/(GALS/time)

[bikenfool]

Originally Posted by Old Tele man

Simply:

a) assume A/F-ratio of 14.7:1 (closed-loop ECM);
b) measure Air flow from MAF or MAP sensor;
c) backsolve for GALS/time;
d) and divide: MPG = (MILES/time)/(GALS/time)

That should work as long at the A/F ratio is controlled tightly and the air flow is measured precisely. So why does everyone have so much trouble calibratiing? Why did the SG folks tell me it was a proprietary algorithm?

[PaleMelanesian]

I'm guessing that most cars run a little rich from 14.7:1. Almost always the calibration is "more gallons" and not less.

[tbear853]

I've been using my SGII on a 1995 Thunderbird (Ford had OBDII on 94/95 T-Birds and Cats), a 2001 Mercury Grand Marquis, a '2007 F150 FX4, and starting this past June, a 2008 Mustang GT (all with dedicated cords and velcro). Occasionally I'll slip it on my wife's '03 Forester.

I tried that calibrating when I first got it, way back in 2010 .... but seems to me I was just calibrating it to fuel tank size. Maybe it was calculating based on AFR / O2 sensor readings? Anyway, I haven't calibrated it since 2010 in any case.

I religiously figure my MPG fill to fill using math and I keep the SGII left side X-gages set to MPG instant above MPG average, and I find that when I drive without shutting off (so no resetting) from fill to fill like one may see on road trips (love road trips) so the SGII has it all .... the SGII average reading is nearly always within a tenth or two of what math tells me.

I know it knows "time", it get's a distance from speed devided by time, I didn't know for sure where it got gallons per hour, but appearantly it get's a pretty good sense from whatever my Fords tell it? I do know by using radar (30+ years LEO) in the past and by timing over mile posts multiple times that with OEM tire sizes, my veh's have pretty accurate odo/speedometers.

I use the instant MPG like a vacuum gage of old, which I still use on my '77 F150. It's good to train one to anticipate stops / slowdowns / hills, etc to maximize the average. It's almost like a video game .... I have favorite stations and I find myself trying to beat my normal average from fill up (reset) to home.