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You're invited to talk odometers and accuracy for measuring distance, if you please

From: The Department of Waaay Too Much Spare Time

Optional background anecdotal drivel:
I’ll blame this discussion on our local High School Girls’ Golf Team. :slight_smile: I look forward to Monday Morning Seniors’ League, a fun time, at our local country club. :smile:

This past Monday the weather was beautiful for golf, perfect in fact, but we didn’t play because the girls hosted an invitational tournament and about 11 schools were involved, taking up the entire course. We couldn’t play. :frowning_face: (Actually, I’m not complaining about these fine young students. My daughter and son were on teams for 4 years in high school. They can be on the lawn!)

I religiously keep a log book of daily cardio/strength training exercise and add entries each day. I woke up Monday morning thinking I was going to go for my 7 mile bicycle ride, but at 7:17 a.m. as it began getting light, I decided I’d go around the lake, instead. Of course I had to push myself around the curvy, hilly course, to beat my old record time. :blush:

I got back home and recorded a stop watch time of 1:51:33, shattering my former “old guy’s personal best” record. My wife was just having her first morning :coffee: coffee! :laughing:

I did the math using 27.3 miles (from years ago as measured in our 96 Dodge Caravan) as the distance, but wondered If that was the real actual distance. So over the next couple of days I drove the route in the Grand Prix for an odometer reading. Then I drove around with my cell phone using Map My Ride, a GPS app. my kids put on it.

The actual important details:
Dodge Caravan: 27.3 miles
Grand Prix: 27.0 miles
Map My Ride GPS app: 27.5 miles

These distances vary my average speed from 14.52 mph to 14.80 mph. I’d like to know which one is closest. I’d like the faster time. :wink: How can I tell? :thinking:

Another thing… cars once used cable driven odometer/speedometers, then used transmission speed sensors. Do any cars use GPS for speed or distance?

Run it again pushing one of those little, calibrated wheels they use to measure distance in construction. Those are very accurate.

All of the cited distances are well within the accuracy of the Odo in those vehicles. Nobody relies on GPS for vehicle speed or distance. Always been some sort of output shaft rotation sensor as far as I know.

In the end, the actual distance has little meaning. It is the elapsed time change that matters most. Congrats on the PR!

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The distance will vary even with a calibrated wheel, as it depends on what radius you take on the curves. I would just take the average of 27.3 miles.

You can get a bike odometer for about $10. Since it counts revolutions of your front wheel, it should be fairly accurate. I’d look for one that allows you to input the actual size of your wheel… the few I looked at don’t seem to do that…

edit: this one says you can input the circumference of the tire.

The total span of your 3 measurements is 0.5 miles or 1.9 percent. Pretty darn good! Close enough for government work, as we used to say. Have a beer and enjoy the glory of your new record time!

If you really want to be OCD about it :grin:. Drive the Grand Prix and the GPS 10 miles on the highway and calibrate your odometer with the mile-marker signs. Read the odo and the GPS at the end of 10 miles. Do this a few times, say 5 times. Or drive farther. The farther you go, the finer the data you will collect and the better your accuracy. The road signs are the “correct” distance or the best standard you’ll find.

Or just set the cruise at exactly 60mph and get the time between 2 distance markers. If it’s not exactly 60 seconds, you know your speedo (and therefore your odo) is off, and in which direction.

You’re right about the Department of Too Much Spare Time, but congratulations on getting into the department! And on your physical exercise. I always listen to podcasts while I’m exercising, because I get bored easily and because if I don’t listen to something I get obsessed with some issue and end up generally annoyed.

About odometers - I often wondered if manufacturers were engaging in fraud on buyers when they built vehicles with odometers that were 3 to 5% off, because warranties are expressed in miles (and dates, of course) and if it’s a 36,000 mile warranty it runs out at a true 34800 miles, more or less.

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Sometimes they don’t have a choice. Porsche, for instance, has to set its speedometers very optimistically because Germany follows the EU, which has a law that says the speedo can never read low no matter what, even if the owner puts a different kind of tire on as long as it’s a “normally applied tire.” So if the owner puts on a tire that has a larger radius, and the speedometer errors to the low side, Porsche gets in trouble. So they set their speedos high to make sure that won’t happen.

Not sure that’s a correct statement. They are separate devices each with their own error but driven by the same input. CSA was concerned about that less than 2 % accuracy so I can’t be sure that they wouldn’t offer slightly different answers. Maybe the odo is correct for warranty purposes and the speedo reads high for speeding ticket purposes?

So THAT is another test to be run…

I’d still use multiple mile markers to get a better accuracy, but I am a bit OCD :smile:

At this point I’m feeling like we’re not going to be satisfied until we get laser measurements down to the submicron level. :wink:


I don’t know how to respond except that the answer is “about 27 miles”. Who knows after that? I remember the Ramblers used to have printed on their speedometer “police certified”. So I always felt Rambler speedometers were accurate but all others probably were not-or at least that’s what you could tell the officer. I sometimes check mine against the “your speed is …” flashing signs to see how close I am, but figure the GPS based navi systems are probably the closest. But then the purpose of this comment is to show that I really don’t know much about it.

I prefer CDO.
Same thing but the letters are in the correct order

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Every one of the digital computers i’ve had over the years allows you to set the diameter of the wheel, usually they’re preset to the common size which at least used to be 26in.

Some years ago a participant in one of those time-distance rallies used a cheap bike computer in place of a non functioning odometer in the car. Worked well enough.

GPS based ones now exist starting at around $100

I don’t have GPS. I have also used the flashing speed signs and 60mph cruise control, mile markers, and a stop watch. My Kia is 59mph actual at 60mph indicated. Close enough for me.

You’re pushing the wheel. How could it ever not match the distance you ran? The goal is to measure the actual distance run, so it’s the most accurate measurement because it follows your path exactly.

I have the kind of speedo/odo gadget for my mountain bike uses a hall effect sensor to count the revolutions of the front wheel. I calibrate for the exact wheel circumference by inputting a 3 digit wheel calibration number. So it is sort of like those wheels you see surveyors pushing around. It seems like it is pretty accurate. There’s a flat paved road which can only be used by bicyclists on Sunday nearby, with markers every 1 mile. The bike’s front tire was within a 2 feet of the post at the 2 mile mark.

Thanks! I forgot to mention that I have had a wireless speedo/odo bike computer on the bike for years. I constantly check speed to see how I’m doing and to encourage me to “get the lead out”. I didn’t mention it for a distance source because it sometime “glitches” and momentarily shoots to a higher than actual speed.

I replaced batteries in both transmitter (at spokes) and receiver (on handlebars) and recalibrated it to EXACT tire circumference (as measured in millimeters (2037), linearly, seated on bike), but it still had very occasional glitches.

Wow! I’ve recently be looking at that exact GPS! (Garmin Edge 20 Cycling GPS). I want to be able to see the speed and a cell phone won’t cut it. I carry a cell phone in a plastic bag for emergencies.

I’m seriously considering buying that little bike GPS.

I’m pretty sure I’m not OCD…
It’s normal to line-up the Quaker guy on top of the oatmeal container to the Quaker guy on the front of the oatmeal container every morning when I use it, right? :laughing:


If it makes you happy, does not interfere with your family life, social life, or day to day living… As I get older, things I do get to be called eccentric, instead of a mental disorder.

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Mine only has one battery, at the main display unit. Sometimes mine will go a little crazy, inaccurate, but I’ve always been able to fix it by either replacing the battery or adjusting the sensor position so it is very close to the magnet.