I replaced the battery on my little computer this morning, forgetting that this would erase not only the odometer reading (which wasn't that much, I only ran the thing for a week before the battery died!) but also the code that tells it what the wheel size is. So it defaulted back to 700 X 25C. I am running 26" wheels with 1-1/2" tires. How far off will my reading be, and is there a conversion factor I can use to find out how far I really rode today?

It's this computer:
https://www.filzer.com/products.php?id=4
User's manual: https://www.filzer.com/img/products/4-instructions1.pdf

I think the correct wheel factor is 2030... does that sound right?

I am running metric, by the way, except for tire size.

Thanks!

1985 would be a more realistic default value for 26" 1.5" tires.

But the best way to be sure is to do a "rollout" test. Take your bike out on your driveway or the street, and position the front valve stem at 6 o'clock (straight down). Have an assistant mark the position of the valve stem on the pavement with a pencil or chalk.

Now, while pressing down on your bars to simulate riding weight, roll the bike forward in a straight line. Ideally, you'll be able to roll through 3 or 4 revolutions. Mark the ending 6 o'clock valve stem position.

Measure the distance between the two marks (in mm), and divide by the number of revolutions. This will be the circumference of your tire.

Just wonder. Why don't you just measure the distance between skewer (axis of the wheel) to the floor (which would be the radius) while someone sitting on the bike. That way, you can get the "cleanest" measurement. Then use the C=2*pie*radius to find the circumference of your particular tire.

Probably harder and definitely less accurate than the rollout test.

Rollout with someone sitting on it will be most accurate. Or just do what I do, set it to 9999 and you'll be amazed at how fast you are!

it's 2010 according to cateye's cheat sheet plastered on my work bench!

Sheldon Brown (God Bless him) has a chart for every tire size and every computer on the market

Ditto the rollout... It takes just a few minutes, and it's the most accurate.

One thing I do with a new bike or new computer:

I used the GPS in my company van to measure one of my commute routes, and I noted mileage at various reference points in addition to the total.

I'll take that slip of paper with me on a commute and jot down bike comp. mileage next to the GPS numbers. Once I get home, I'll fudge the wheel diameter setting up or down a few mm to compensate. For instance, my road bike's wheel setting measured 2140mm, but it was far enough off of the GPS measurements that I had to reset it to 2150mm.

Anal retentive? You betcha. If I'm going to sweat for my miles, I want to log every d*mned inch.

One tip: Once you get the correct setting, trim and stick one of those little Brother P-Touch labels on the side of the computer body. Likewise, you can label the last known odometer reading as your zero reference... My road bike's label reads 0 = 38,463.2... Newer computers allow you to reinput your odo reading, but the early-90s Specialized SpeedZone I have doesn't.

If you download and read the PDF file, owners manual
it says 2030 for a 26X1.50 tire.

Just to explicitly state it, that "factor" is actually the circumference of the wheel as measured in milimeters. Do the rollout procedure described above, measure it in mm and enter that, it'll be super accurate.

A rollout is easy, and accurate. If your tape measure is in feet and inches, use google to convert it. For instance, if two revolutions measure 13 feet 4 1/8 inches:
do a google search for: (13 feet 4 1/8 inches)/2 in mm

It gives this answer:
(13 feet (4 1/8) inches) / 2 = 2 033.5875 millimeters

So that's 2034 mm

Google Calc is fun. You can do conversions like: 1/4 cup in teaspoons

Um, you mean the PDF owner's manual I linked to in my original post and where I got the 2030 figure? Thanks.

To clarify: I was wondering if there was a chart or conversion trick for figuring out the odometer reading corrected for the tire size. I figured it out on my own; by dividing kilometers by wheel circumference (I realized immediately after I posted that the "wheel factor" in the owner's manual was the circumference in millimetres, and that made it easy), I got the revolutions per kilometer travelled, then I calculated out the odometer reading from that.

For what it is worth, the 26" wheel with 1.5" tires (2030 mm circumference) covers 95.57% of the distance travelled on 700 x 25C tires in the same number of revolutions. In other words, I really rode 67 kilometres yesterday, not 70, and that was what I wanted to know.

(And I measure in metric, since I am in Canada - except for tire sizes which are still in US measurements here. So I don't need to convert feet and inches, thank you.)

I just loop a piece of string around the tire and came up with 2038 for my Serfas Drifter in 26x1.50".
A mapmyride.com route lists 10.35 Mi and my odometer read 10.26 Mi. Close enough for me.
Remember, if you're off by 20, that's only 1%.
Your car is a LOT worse than that!

You do realize that the cycle computer is probably more accurate, right?