[Ford Man]

Originally Posted by California98Civic

[EDIT: I am reposting this after taking it down for some corrections.]

I think my Civic’s ECU uses its Vehicle Speed Sensor for both speed and distance measurements. With slightly smaller than stock wheels/tires on, I have been meaning to test for mileage over-count. I have recently had convenient trips that offered good chances for test, so here are my current figures:

Distance: 13" VX wheels with 175/70-13s tested against a google maps route of 64.4 miles, mostly flat terrain, 1.9% over-count by my ODO. To account for the small rolling terrain sections, I might reduce that to 1% or 1.5%, but I’m not sure yet. I want to do more testing.

Speed: using roadside radar digital readouts I compared my speedometer readings. When my dial showed 40mph, the radar showed 36mph. When the dial showed 25mph, the radar showed 23mph. I did five passes at each speed. That seems to mean about an 8% or 9% over-count, I think.

I will do more tests as convenient, but any thoughts on sources of error or why are the percentages for distance and speed seem to vary differently? I expected the percentage would be the same for speed and distance. It’s important to me because ultimately I want a good number to adjust my FE results in my fuel log.

james

Tire size calculator