Test your mpg results. (odometer accuracy)
 

Thought I would put this out there as an officer of the law stopped me to discuss his modding efforts. His records were wrong because of tire dimensions, and a ring and pinion change.
Test your mpg by using an online trip calculator against your recorded odometer/ other distance measuring device.
Gps equipped vehicles can record close enough for most applications but if you have not corrected your speed sensor readings or the speedo gear your mpg may be way off.
I use yahoo maps myself and never even consult the odometer as my Oversized front drive tires have seriously altered that reading. You may find your mpg is better or worse than you believe.
It is simple math to follow and if you keep reciepts from your stops the addr. is on them so you can plug them rite into the trip calculator.
mi2centz.

I was discussing MPG improvements with friends at work and realized most don't even think their odometer/speedometer might be off and so haven't checked it. They rely on the multifunction MPG display. I've never seen one to be correct. Tall tires on my Metro make mine off by quite a bit. I still use my speedometer and odometer but multiply by the constant I came up with during my calibration. I mentally remember where my needle needs to be for my standard commute target speeds of 50, 55, and 60 which of course reads lower. The last thing a hypermiler needs is to drive faster than he intends to. I use my multiplier constant when resetting the trip odo at my fill ups.

My calibration included 4 trips of at least 60 miles each (round trip to work). I took 2 GPS (one on my phone and a portable aviation unit) and also verified my route distance with google maps. I averaged out the GPS trip distances and then came up with the multiplier to try against my odo reading. Worked quite well. I check it again every few months and every time I change tires.
Use the inverse of the percentage delta as a multiplier if your tires are big and your speed reads low. I guess it would just be a percentage if the opposite were true.

Oh BTW good thing to note is any scan gauge or eco trip meter in your car will always be off for the same reasons as it gets its data from the same sensor.

I have to add 3% to my suburban due to a diff gearing change and tire size swap.
I used mile markers and map quest and they came well with in 1% of each other.
On top of that the speedometer reads about 3mph fast at 60mph.

The MPGuino has speed calibration in the setup screen.
Can you do this on a ScanGauge also?

Yes, the SG has speed calibration.

My odometer was off by ~2.5%. Here's a copy of my post from a while back:

as mentioned in scangauge II threads, yes. the key is to switch to the metric setting on the sacngauge and the gps you are setting it from. This will give a much more accurate adjustment. Then switch back to miles when set.:thumbup:

In the years I've been testing using a Racelogic V-Box GPS system, I've found most odometers to be witihin acceptable tolerances on standard tires... about 1-2%.

It's speedometers that are way off. Some over-reading by over 5 mph.

Pays to keep a GPS device in your car... or at least calibrate against a GPS device.

I just take my car to the local speedometer shop and have them test it on their chassis dynamometer at exactly 60 mph and compare their value against the simultaneously read ScanGauge II value, which then gives me a "scaling coefficient" for the ratio between "actual" and what the ECM "thinks" the wheel is turning (which includes tire diameter).

Not having a GPS I check odometer accuracy whenever I get to travel continuous highways with reliable km signs (old roads patched with bypasses won't cut it). I also keep an eye on my normal commute trips' measured length. This was that showed me that the new to me car had wrong sized (too small diameter) tires, so I corrected every distance by -3% until I got new (winter) ones. Then the commute length became pretty much the same as our 'unit bike' (Ciliegia, Hyosung GV250 - has the most accurate odometer among our vehicles).

My bike (Teresa, BMW F650CS) is quite a bit different: she seems to be too modest about distance (the speedo is still over, of course), last time I checked (on a Czech motorway) the error was ~2.5%. I usually correct by +2%, to be on the safe side (I don't want to exaggerate my mileage ;)) - and add the distance I coast with the key off...

When I bought new (Michelin Pilot Road 2) tires I hoped they'll be more accurate than the previous Heidenau K73s (I used +1% correction back then), but the difference just became larger. At least I got a tiny bit taller gears which I couldn't otherwise :D

This difference should get smaller as the tires wear, but now, ~1mm down from new I still can't measure the effect.