[Diesel_Dave]

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

Update:

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.