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Date:         Tue, 4 Sep 2007 01:42:14 -0400
Reply-To:     Kim Brennan <kimbrennan@MAC.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Kim Brennan <kimbrennan@MAC.COM>
Subject:      Re: MPG
Comments: To: David Kao <dtkao0205@YAHOO.COM>
In-Reply-To:  <376307.86811.qm@web82702.mail.mud.yahoo.com>
Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed

Note that I gave errors based on percentage. This is important. Before I calibrated one of my vans, I found out that the speedometer consistantly read 7 mph high at ALL speeds. This was fixed by repositioning the needle. Later I switched wheel/tires. Then the speedo was off by a percentage (in this case 10%).

This means at 20 mph (indicated) I'm actually traveling at 22 mph. At 60 mph indicated, I'm actually traveling at 66 mph.

I once at my speedometer (different van) reading 85 mph. When I double checked the accuracy, I found that meant I was actually traveling at 91 mph. (7% difference).

Various GPS units can often give highly accurate speed indications. Distance is trickier. I usually calibrate on an interstate highway, which the maps will have fairly high accuracy on. I choose a location more than 10 miles ahead (on the interstate), wait until my odometer is at some convienent point, check the "mileage to go". Then travel 10 miles based on the GPS, rechecking what my odometer reads.

Example may make this easier to understand.

I located a spot on my GPS about 13 miles ahead. I waited until my trip odometer read exactly 136.0 miles. At that point the GPS said it was 11.7 miles to my destination. When my GPS said it was 1.7 miles to my destination, I checked the trip odometer, which read 145.2. Using math at this point reveals that my odometer said I had traveled 9.2 miles, but the GPS said I had traveled 10.0 miles. 0.8 difference over 10 miles. Or 8 %.

On Sep 3, 2007, at 9:37 PM, David Kao wrote:

> --- Kim Brennan <kimbrennan@MAC.COM> wrote: >> >> My calculations of margin of error are based on GPS values for speed >> and distance. >> > > I have zero knowledge about GPS values for speed and distance. Does > GPS > give more accurate measure of speed and distance? I understand GPS > is extremely > accurate and reliable to pin point your location and maybe the > distance > between locations. But how does it measure the actual traveling > distances > between two points? Does it track every foot of your track and add > up to > get the total distance you have traveled? Or does it use its software > to add up straight distances of some not too short segments along > the route > you have traveled? If the latter is the case then I have to doubt > its accuracy. > > My house is located near by a school. There is a radar speed > detector with > large digital display on the street. I often used it to check my > speedometer. > I have found my speedometers of all my cars just about dead on > accurate.


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