My 87 Civic's factory odometer reads 3.45% higher than it should, even with the stock sized tires on it. Thus there is always a bit of disappointment when make that correction when calculating MPG
I noticed that when I drive by the stationary highway radar stations that say "your speed is __ mph" that they always said I was going a few MPH less than my spedometer read.
My "test" assumes that the green milemarkers on the side of the interstate are dead on. I reset the trip exactly on a marker and then see how many miles it takes to for my trip to be 1 mile off. In my case, it took 29 mile markers for my trip odometer to read 1 mile too high.
I recall an episode of Car talk on NPR a few months ago where an engineer was commenting on how his brand new car's odometer was reading about 5% too high (doing the same sort of highway mile marker test) and thus his gas mileage appeared about 5% higher than it really was. The car talk guys said they had heard many ppl describing the same thing and jokingly suggested it could have been inaccurate mile marker placement by DOT workers
I don't want to say its a conspiracy, but this does make consumers believe they are getting better MPG (and maybe saves them a few speeding tickets
).
I know most of you are really concerned about accuracy and I wanted to see if any of you noticed simular discrepencies on your cars when running on the stock tire size.
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