What is the Most Accurate Way

[Paddy]

I don't know about accuracy, but my odometer is consistent to less than 1%.

[spadnos]

Barring mechanical failures, an odometer should be perfectly consistent. (This is a little less true with electronic odometers.) A mechanical odometer is geared from the output shaft of the transmission (or one of the axles), and so turns at a fixed ratio to the wheel speed.

The accuracy is mostly determined by tire size. If you use big tires, you'll be going faster than the speedometer tells you, and the odometer will be accumulating fewer miles than you actually travel.

- Steve

[degrub]

Tyre size and to some extent pressure, make the most difference. An electronic odometer is counting gear teeth rotating by or the equivalent and will be as accurate as any mechanical odometer. As far as the volumne of petrol you meter into your car, the meter is typically calibrated to the average temperature of delivered petrol so the error produced is very small, less than .005 parts or .5% and more likely half that.

The other variable is the fill pipe "tank full" flow cut off. Can easily vary by .5 gallons or more across pumps. And the cuttoff will vary with pumping rate. If you want to be more accurate, use the same pump each time and measure the mileage over several fillups.

[DukeP]

The most effective way is to calculate it.
Your RPM gauge is precise, its actually counting the revolutions.
Know the gearratio and the diameters of your tire - no matter how much air you inflate it with, if you stay within the norm psi, its not going to affect your calculations (much)
Its all down to Pi.

Thats how its done when driving cars that go FAST, at least.

Used it to clock in an old BMW. Made 320kmh - which even on the German Freeway is kind of scary.

~~DukeP~~

[Brian R.]

How about a fifth wheel?

[Marshmallowman]

"Just" find an accurately marked distance of road,

Then calculate the correction factor using the odo reading and the known distance.

Here in oz we have very accurately marked distance markers on some country roads (1km) use for arial speed check's.

[Fat Tone]

Originally posted by GNEP
...But hey, when I work in kbbl/day at best what do I care about the odd gallon here or there

That's precisely when you should care! A small cc error on one litre or barrel can soon add up on that scale!

In the UK it's ultimately administered by NWML (National Weights and Measures Ltd. They maintain the national standards for legal metrology. I once turned down a job with them...I remember a few petrol pumps were in the labs for type-testing when I had my interview.

As to the accuracy of speedos, there was a report out ~1 year ago that said that on average they over-read a little at 30mph and underread a little at 70mph. If you look at the mechanism, it's not surprising that big deviations are reported: One I took apart because the needle was oscillating wildly consisted of a spinning magnet causing a 'stator' to deflect, which was being heavily damped by a coiled spring.

[GNEP]

Originally posted by Fat Tone
That's precisely when you should care! A small cc error on one litre or barrel can soon add up on that scale!

Nah - I work on the principle that globally all the errors up and down pretty much average out.

Anyway, the plan numbers I'm looking at now are all imaginary anyway...

Off Topic:

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and it would appear that the stockmarket is not anticipating hearing what they want to hear at our strategy presentation this afternoon... oh well.

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[Novdid]

Originally posted by Fat Tone
That's precisely when you should care! A small cc error on one litre or barrel can soon add up on that scale!

In the UK it's ultimately administered by NWML (National Weights and Measures Ltd. They maintain the national standards for legal metrology. I once turned down a job with them...I remember a few petrol pumps were in the labs for type-testing when I had my interview.

As to the accuracy of speedos, there was a report out ~1 year ago that said that on average they over-read a little at 30mph and underread a little at 70mph. If you look at the mechanism, it's not surprising that big deviations are reported: One I took apart because the needle was oscillating wildly consisted of a spinning magnet causing a 'stator' to deflect, which was being heavily damped by a coiled spring.

Let me guess, Ford Sierra???

[Fat Tone]

Actually it was a Vauxhall!

[Paddy]

You suprised me with your knowledge there T!