Perhaps this could be of use to somebody hence my post...

You folks have too much time on your hands, or perhaps it's raining where you are even more than it has here in Central NY, so that you can't be out riding and actually racking up the (significant to .5%) miles, instead of resurrecting zombie threads.

Perhaps you should study "significance arithmetic."

+7
Besides, digital displays have their own accuracy percentage +/- 1 digit.
Worrying about 1/4% is absurd. Your tire diameter probably varies more than that due to pressure loss on a long ride.

In my experience, chart values are always larger than rollout ones.

Why don't people use a tailor's tape to measure their wheel's circumference?

Also, do people reset their computer's after a while to take into account the tyre losing tread?

Cateye directions that come with the Strada computer has a single reassuringly simple choice: 2105

Great idea! Thanks!

That's hard-core. OTOH, tires tend to stretch over time too so what was once a 25mm tire might become 27mm after a couple years of being inflated at 100 psi.

Of course, tire pressure, rim width, and rider weight make small differences as well.

Some of my computers (I'm looking at you, Cateye) can only be calibrated in cm, so they have up to 0.5% error built in.

Whatever settings you decide on, you could always check them on the road, if you can find a stretch with accurate mile markers.

I never thought about a difference between front wheel sensors and rear. How 'bout my trike? I have the sensor mounted on the right side. That wheel will go farther than the left when making left turns. This means that if I ride a loop going counter clockwise I will go further than the other way round. Hmm, I will have to measure the turning radius and calculate the diff. On the other hand, I think I'll just save myself the worry and go riding instead. Mileage is not a measure of effort, it's just a number that needs to be qualified by hard, easy, uphill, downhill, into the wind, etc.

OK AnkleWork,

Correction: . . . Circumference ≈ 2051.46000279 mm . . .

∴, I rounded off to the nearest whole number to account for the variation due to temperature, my weight, and tire/tube condition, and air pressure.

Feel better now?

I recently began using a Garmin Edge 500 in addition to my Sigma 1606 computer.
For my 700 X 23 tires, the Sigma indicated an calibration figure of 2133.
this was always too high (fast,) and I had been using 2115.
However, this did not agree with the Garmin. Still too fast.
I had to reduce to 2189 in order to get the 2 machines to agree.
I had checked the Garmin against MapMyRide on routes I mapped, and it was dead-on.
Now I don't have do decide which one is correct....

Maybe people do, but it would seem very awkward and not necessarily accurate, given the difficulty of keeping the tape on the center of the tire and the inability to put any weight on the wheel, as one can on a rollout.

...and folks, again remember we're generally talking about 1/2% difference. Instead of obsessing about how far you're riding, how's about just get out and ride.

I was envisioning using the tailor's tape on a wheel that was completely off the bike, but I hadn't taken into consideration the weight on the wheel issue.

However in now thinking about it, one could easily press down with body weight on a wheel and do a pretty good job of simulating the weight you would put on when actually riding.

In so many ways, biking is the perfect past time for obsessives.

I now use this website for circumference: BikeCalc.com - How to calculate Bicycle Wheel Size

This is just bizarre on several levels. After three years I still feel great and you still don't understand significance. Oh, well.

most basic.. ride thru a paint blob, [Dog poo is useful too) measure how far apart the next mark
made by the rolling tire of the deposit you rode thru is from the original one.

most basic.. ride thru a paint blob, [Dog poo is useful too) measure how far apart the next mark
made by the rolling tire of the deposit you rode thru is from the original one.

A snippet from my calibration experience of 5th wheel speed and distance measuring system . Here is a critical application to get the circumference right since it's a measuring device, but same can be used for a bike if you want that kind of precision (yet the instrument may be lacking)

So the best way to measure rolling radius and circumference is by rolling it while loaded the same as any ride of yours. Find a straight section of road and mark the biggest distance you can with a tape measure. (I've pinpointed by GPS 2km with +- 50mm accuracy and permanently planted two nails as a reference and install optical barriers at those locations). So go 50m or 100 or 200 or whatever you can measure accurate with a tape measure or laser rangefinder or ....

Setup for start with the wheel at start line. (for the 5th wheel device, i start a few meter behind since the optical barrier is the trigger that starts and stops the measuring). Ride in a straight line until past finish line. The speed sensor from the fifth wheel gives exactly 1000 impulses/ revolution. So between the two points exactly 2km apart I have a set number of impulses and calibrating consists in associating a set number of impulses for each meter. So the math is simple here.
Now since a bike owner will not have optical barriers nor a specialized encoder to measure the angle of the wheel, you need a reference value first. Wrap a tailors tape around the center and find an approximate initial circumference (let's say you measure 2110mm). Then carefully setup the wheel with valve stem down (even better is to add some marks with a sharpie or white corrector paint on the tyre itself). Start rolling from the start those 100m and carefully stop the tyre in the same position with the mark down near the 100m mark. So make another mark. Now you know a number of complete revolutions will get you that distance (100m plus something small where the tyre stopped.. measure with a ruler or tape measure that distance. so let's say you stopped 1m longer at 101m final distance)
The question is how many revolutions were done? (one could count by riding, or setting a video camera or use some math: divide those 101m by the initial circumference known of 2.110m =47.87 revolutions.. so the nearest is 48 revolutions in this case)
Divide the distance to the exact number of revolutions 101/48=2.1042m
Be very careful in identifying the correct number of revolutions, since a bad initial measurement may throw the calculus by as much of one revolution. The longer the distance the more chance you have to miscount the number of revolutions if using the initial measurement math determination.

Another approach you can use is just calculating the % calibration needed between a trusty indicator and the one you want to calibrate.
A good indication is the normal GPS in any phone. At constant speed will get a a very accurate reading in regards of speed. So you can go with that.
Set the cyclo-computer to whatever circumference.. let's say 2200mm and ride in a straight line with constant higher speed.. let's say above 25km/h. Check that speed is stable as both the speedo and the gps speed should not fluctuate. Take note of those two values. So the gps was saying 26.4km/h and the speedo said 29.3km/h with the initial value of 2200mm in the speedo for circumference. So this number must be shaved down by 26.4/29.3=90.1024% of the intial value.. so 0.901024*2200=1928.25mm

For all intents the last method is by far the easiest and one of the most accurate for a bike.

To me, it's much more important that the trip distance is accurate rather than the speed to 0.1mph precision. So with a new computer, I use the default setting for my wheel and ride a known distance of more than a mile, and then adjust my wheel circumference input to match that. Measuring the actual wheel data never seems to get me that close.

They have direct implication one over the other. If one is set right then the other will be right. What known distance you have? how do you measure that known distance? Sure the longer the distance the better the result, but road markings are sometime inaccurate. GPS? google maps from known locations?

If I define accurate to mean that it matches with Google maps, or the mileage markers on a measured route, then that's the definition of accurate - that's what I care about. And if a perfect measurement of my wheel would report something different by 2%... it might be more correct, but it's not useful information.

Geez, REALLY!
When I cared, I would place the tire stem down directly over a line crossing my garage, then roll out, perpendicluar to the line, while leaning over the handlebars to weight the tire. Go two or three revolutions, mark the distance on the floor, and measure. Inches, mm, doesn't matter. Conversion from inches to mm is EXACT (to the limits of your measurers resolution, stretch, and thermal stability) at 25.4 mm per inch (per the international metric system conversions, see NIST, for example). And, yes, those distances were often different from the numbers included in with the cyclometer.
If I get into Rando, or similar, I may go back to that. Otherwise, when I care, an estimate from Google Maps is good enough.
As to GPS, don't neglect the uncertainties there. Mentioned above is the periodic distances between discrete points. Others include: the inherent uncertainty of the geographic fix (up to 10 METERS old school ((~33 feet), around a meter now) provided you've got a good satellite constellation, often questionable around tall buildings, deep forests, and percipitous mountains. I've seen errors over a 1/4 MILE under such conditions with survey-grade units! It's those uncertainties that make geocaching so much fun!
Cheers!

My computer uses the GPS distance over a period of about a mile and calculates the circumference and then resets the speedometer off the sensor. But whether it uses GPS distance or wheel distance once it is calibrated I still don't know. I can switch my sensor between my 700x23 road bike and 26x1.125 commuter and they always read the same since my computer auto senses., And the thing is my old wired Cateye used my input which I always set to default, and those distance all match up with what I experience today with GPS, to a tenth of a mile on a 50 mile ride. Over the years I've done all the common rides and have all the distances based on my cateye, they all still hold as accurate.

Bottom line is to not worry about it.

GPS is very good for determining speed. A dedicated unit for such task like V-Box samples speed with 0.01km/h accuracy at 10ms intervals (done by some prediction and integration). Mainly used to measure some dynamics performances for vehicles.
Sure it might have some error is the road is not flat and the HDOP is high (btw there is an app to measure the HDOP for gps). But generally if the speed indicator on the average phone gps is not jumping at constant speed on a level road (phone gps recomputes speed every second or two.. so to ensure a constant speed look at the cycocomputer.. not calibrated yet but whatever speed is showing must be constant).. then that value is bang on to about plus/minus 0.1km/h good enough and if higher speed is involved the better. Because computing speed averages several points in time and gives an average over that time. since speed was constant during that time that average is about what it should be. Position wise it has more error like 10-15-100m even more if weird reflections from buildings are involved, but speed is computed and averaged from more points so in average is more reliable.

To calibrate over distance can be a better approach but it needs a reliable source of distance traveled. (gps can go haywire to display trip distance.. it jumps back and forth on different roads, it's a mess; road indicators and google maps indications are rougly but not necesarly for the path of the bike.. the bike may do some waves from a straight line and who knows if a turn is involved what line is considered for that distance) - either are good enough, but leaves a question mark that for the sake of science it can be done better.
For the most of us, any method by measuring roll-out, or comparing distances on road markings are good enough.

In the 5th wheel device, accuracy is very high (around 2600impulses/ revolution of the wheel - it has 1000 imp/rev at the encoder side and it has a gear multiplier in between of 1:2.6 - in contrast to one impulse/revolution for cyclocomputer), and precision is given by us by correctly determining the circumference of the wheel to the fraction of millimeter. This sort of accuracy is needed at high speed since the car can reach 250km/h with this measuring device and if a millimeter is off then all performances will be off. Not acceptable. Standardized tests for measuring key performances like acceleration time to 100km/h has to be done to the hundredth's of a second. The speed indicator also have to have a high degree of accuracy of at least 0.01km/h at maximum speed. (5th wheel as a measuring device is very good since the bike wheel has a constan load from some springs, it has very low deformation in contrast to car tyre, is not braked nor powered so it's not affected by those regimes, etc. - old fashioned equipment that still works with high accuracy and precision if one takes care to measure the calibration factor by some precise method above)

So this measuring device actually uses a bicycle wheel. Here is the best example where measuring the circumference of a bike wheel is serious business.