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.

[BroncoHauler]

As far as your speedo goes, do you have a GPS unit that displays speed?


Herb

[stanw909]

No gps and no phone with that capability only evert radar that I've ever passed over the course of the last year I've owned the van. Tires are proper size according to door label. 97 Dodge.

[geoffff]

What you found isn't surprising.

Vehicle speedometers are usually set at the factory to read a few MPH high -- because regulations insist that they must never read below actual speed.

This does not necessarily mean your odometer will read high.

I would have no reason to suspect the roadside radar signs are inaccurate (maybe +/- 1 MPH), especially since you found that four of them agree.

In any case, best to just check your speedometer and odometer with a GPS, or use roadside mile markers on the highway.

-- Geoff

[jage]

x(actual) = speedo

x(51mph) = 55mph

x(51m/h) = 55m/h

x(51m/h) * 1 h/m = 55m/h * 1 h/m
(the above is just cancelling the units)
x(51) = 55

x = 55/51

x = 1.07

so
1.07 * actual = speedo

and
actual = 0.927 * speedo

so for all intents and purposes take your speedo reading * 0.93 and you have your actual speed (at least as measured by CHP)

In other words, if you're going 55 mph, that's 55mph*.93 or 51mph

Odometer:

Your Odometer reads, say, 22,222.

actual miles = 0.927 * odometer miles

actual = 20,666

Miles per gallon:
Actual mpg = y * Odometer mpg
Actual miles = y * Odometer miles
actual miles = .93 * odometer miles

actual miles / g = .93 * odometer miles / g

actual mpg = .93 odo mpg

so your 18.3333333 mpg is actually 17 mpg, which if you're posting on the Internet you can claim it's 25 mpg, so it really all works out in the end.

[rionapo]

So, why is my speedo 55/58ths of actual while my odo is 9/10ths?

[geoffff]

Originally Posted by rionapo

So, why is my speedo 55/58ths of actual while my odo is 9/10ths?

Not sure where you got "58". He's saying if your speedo is reading 55/51 of actual then your odometer is also reading 55/51 of actual. So actual is 51/55 of what's showing -- or about 9/10. 51/55 and 0.927 (approximately 9/10) are all the same thing.

However, I disagree with jage's odometer assumption above, as don't I see any reason to believe your van's odometer is calibrated to match your van's speedometer. The factory can paint the numbers on your speedo dial any way they like, purposely making it read a bit high if desired -- while keeping your odometer accurate.

Best to just check the odometer using mile markers on the highway.

-- Geoff

[jage]

Originally Posted by rionapo

So, why is my speedo 55/58ths of actual while my odo is 9/10ths?

They're not- they're both probably 231/250ths EXACTLY. Probably, exactly that.

[rionapo]

Originally Posted by geoffff

Not sure where you got "58". He's saying if your speedo is reading 55/51 of actual then your odometer is also reading 55/51 of actual. So actual is 51/55 of what's showing -- or about 9/10. 51/55 and 0.927 (approximately 9/10) are all the same thing.

However, I disagree with jage's odometer assumption above, as don't I see any reason to believe your van's odometer is calibrated to match your van's speedometer. The factory can paint the numbers on your speedo dial any way they like, purposely making it read a bit high if desired -- while keeping your odometer accurate.

Best to just check the odometer using mile markers on the highway.

-- Geoff

The speedometer usually runs about 3 mph below GPS at highway speeds; hence, 55/58s with tongue firmly planted in cheek. You're correct that the odometer and speedometer are related only in the sense that they're in the same vehicle. Not sure I understand why, though.

[rionapo]

Originally Posted by jage

They're not- they're both probably 231/250ths EXACTLY. Probably, exactly that.

My slide rule agreed, 92.4% of the time. That solves the mystery--I just didn't calculate the actual values but let rounding errors lead me astray.