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California98Civic · 05-18-2012, 06:40 PM

[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

05-18-2012, 06:40 PM	#17 (permalink)
California98Civic Cyborg ECU Join Date: Mar 2011 Location: Coastal Southern California Posts: 6,299 Black and Green - '98 Honda Civic DX Coupe Team Honda 90 day: 66.42 mpg (US) Black and Red - '00 Nashbar Custom built eBike 90 day: 3671.43 mpg (US) Thanks: 2,373 Thanked 2,172 Times in 1,469 Posts	[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 __________________ See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.