Low instant mpg reading from SGII

[winkosmosis]

When I first put my Scangauge II in my Cherokee I accidentally left it on the engine size setting for my Miata, 1.8 instead of 4.0. I calibrated it, but only with about a 1/4 tank fillup. The correction factor was 106%

Not ideal, but it should have been fairly accurate after the calibration.... And it was. It showed about 21mpg at 65mph, which is what I'd expect with my 17mpg average.

The other day I filled up, reset the gallon counter, and corrected the engine size. As you would expect, the mpg readings halved.

Then I filled up, and adjusted the gallon counter, ending up with a correction factor of 75%. Now it's reading 11-14 mpg on the highway, less than my manually calculated tank averages.

Any ideas?

[winkosmosis]

BTW I used the same gas pump, same direction

[woodsmith]

Instant readings can be misleading.
I use instant readings to decide if I accelerate or slow down at that point in the drive. I use my tank range average to determine true consumption and trip average to determine average for each trip.

I expect 10 UK gallons to give me 600 miles or more and I am annoyed with myself if I can't reach that.

I don't know about calibrating your Scangauge as I use the factory fit computer in my car

[winkosmosis]

Once calibrated though the SGII should be pretty accurate. Is there some other setting that could be messed up? The speed compensation is set to 10, because I have 31" tires vs the stock 28" (confirmed by GPS)

[woodsmith]

If your tyres are oversize then your speedo will be reading slow and short of a few miles. That may explain your differences.

If you have the gear ratios in the owners handbook or workshop manual then you can work out what your speedo should be reading. The tyre diameter should be measured as twice the raidius from the hub to the road to allow for the flat spot due to vehicle weight.

Having siad that you should be able to use your gps to work out your distance travelled and your tank refill to work out a corrected average.

[Wonderboy]

I've actually had the same or a similar issue. It was definitely the fillup thing that threw it off most dramatically. I ran the car with the SGII for a long time not even remembering to calibrate the fillup gallon thing. I messed with it once and my instant MPG was off by about 20mpg. My tank MPG has never matched what the pump says, but it's getting closer, as I've also discovered that my speedo displays at least 5mph higher than radar signs at the side of the road tell me I'm going. It would come in handy to have the SGII give/collect very accurate fuel consumption and distance data, but I've found that it doesn't really matter if the instantaneous MPG number on the screen is accurate enough - just always try to get it the highest you can. If anything it may help to intentionally throw off your fillup calibration so that the SG displays a lower than normal MPG reading so you can strive to make that reading in the range that you'd normally expect - effectively tricking yourself into driving more efficiently.

[winkosmosis]

I need accurate numbers. It's depressing to see 13mpg at 65mph.

I just don't get why it would be inaccurate, unless there is some bug in the software. Calibrating should give you the right numbers, even if you have some settings wrong

[bluetwo]

My Scanguage 2 has given me the most depressing numbers when it was at it's most accurate but that just told me that I needed to try harder to drive conservatively.

My finding is if you're off by say 12% it means you're getting a conservative number, less than actual MPG. If you're off by -12% it's a liberal measurement meaning that you're getting an inflated number instead of the actual MPG.

Please, somebody, if I'm wrong straighten me out because above all I want the true info to be out there and to be right second.