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-   -   Test your mpg results. (odometer accuracy) (

justme1969 08-02-2013 08:12 AM

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.

doviatt 08-02-2013 10:29 AM

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.

oil pan 4 08-02-2013 11:18 AM

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.

doviatt 08-02-2013 12:40 PM

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

Daox 08-02-2013 12:50 PM

Yes, the SG has speed calibration.

Diesel_Dave 08-02-2013 02:47 PM

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



After the kill switch install it became clear that my odometer was off slightly. Before the kill switch there wasn't much sense in correcting the odometer (nor an accurate way to do it).

In the past, I made a couple attempts to use my GPS to determine the distance accurately, but I had issues with my GPS "assuming" my route, rather than actually measuring it. This was particularly true in one area where I typically cross through a vacant parking lot, and also in my neighborhood, which is only a couple years old and not updated yet in the GPS.

I finally gave in a got a smartphone and found a great free app called Runkeeper. It's intended for running (which I do use it for) and it measures your true distance (not assumed) on-road or off-road. I first used it over 10 trips ove various lengths (totaling 284 mi) before touching the odometer:

Min error: -2.6%
Max error: -2.4%
Avg error: -2.51%

So I got out my Smarty programmer and checked the tire size in the computer. It had been set to 30.55 inches, so I moved it up to 31.30 inches (2.45% increase). This is very reasonable since I looked up the nominal diameter of my actual tires and it's 31.40. Here are the results on the next ten trips (totaling 280 miles).

Min error: -0.4%
Max error: +0.4%
Avg error: -0.04%

In every case the error was within the decimal precision of my odometer, so now I know I'm spot on. There's also a marked 5 mi speedometer course check on my way to and from work that I've checked myself against. Every time I'm within the precision margin.

So, to summarize the kill switch gained ~6% and the odometer another 2.5%. So all told I wasn't accountng for ~8.5% of my miles in the past.

mcrews 08-02-2013 10:11 PM


Originally Posted by doviatt (Post 383338)
The MPGuino has speed calibration in the setup screen.
Can you do this on a ScanGauge also?

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:

niky 08-03-2013 10:22 AM

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.

gone-ot 08-03-2013 02:40 PM

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).

alvaro84 08-06-2013 07:48 PM

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.

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