Speedo brain teaser

[stanw909]

I passed four radar signs showing my speed yesterday on highway 126 from Valencia Ca. to Santa Paula.Each showed 51 when I was driving 55 according to my speedo.When I got to Ojai from San Bernardino my trip meter showed 137 miles.Is my odometer correct or did I travel less than that?Does my van really have 109,000 miles or less? I went to San Diego a few weeks ago and got 18.3333333 mpg,filling before leaving and filling upon arrival to calculate,but if my odometer is off what did I really get? I'm not sure I want to know but please chime in.

[markopolo]

I'd first check it with a GPS before looking into it more. Maybe borrow one if you don't have one.

Your relatively low roof 2500 series van could very well be getting 18 mpg if you mostly cruise around 55 mph.
I don't know if you track mpg regularly but you get a more accurate mpg number by averaging out consecutive trips and fill-ups.
For example, my last 3 results were 15.86, 18.02 & 15.52 mpg. The 18.02 was an anomaly which also affected the 15.52 calculation. My current average is 16.29 mpg.

[mlts22]

I wonder if the odo and the speedo are on separate circuits. Some are in some vehicles because it protects against tampering. I do know some speedos are caliberated a little bit fast so people go slower than speed limits (har, har).

[stanw909]

I only used tge San Diego trip mpg as an example but I have gotten several high 18s but now due to the odometer they are really low 18s. Iwas in error with the figure. I looked it up and it was 18.666666. I go with traffic at 70 mph.

[markopolo]

You could use the highway mile marker signs to check your odometer accuracy. Find one where you can pull over and stop next to it then reset your trip meter.

[stanw909]

Will try that but evry radar sign since I bought the van can't be wrong in exactly the same way can they?

[booster]

Originally Posted by stanw909

Will try that but evry radar sign since I bought the van can't be wrong in exactly the same way can they?

The radar sign will give you the speed accuracy, but it is not necessarily related the odometer. So if you want to confirm your gas mileage, you have to check the odometer. You can use gps, or the mile markers others have suggested. If you use the mile markers, be aware that many of them, especially older ones before gps positioning, may be a bit off location. We always will us a minimum of a 30 mile test, and then check every mile for 10 miles to see if they match the same amount off. If they do you are good, if not, try the next 10 miles again.