Bike Computers and Tire Size

CharleyFarley

Well-Known Member
Location
Florida, USA
From what I've seen, Garmin is a popular bike computer but I bought two other computers before I knew about Garmin. Both of them are Cateye Padrones and are excellent computers if all you want to know is speed, distance, average speed, maximum speed and time actually riding.

They come with instructions for different size wheels, but I found they are not accurate if accuracy is something you desire. For instance, I have a beach cruiser I bought in January 2020. I soon swapped the tires for whitewalls. The instructions have a chart to tell you what number of millimeters to set it on, but this isn't accurate. So I took a soft flexible tape measure and attached it to the middle of the tire all the way around. I got a reading of 2038mm. That was then set in the computer.

A few months ago I decided to go back to black tires, but I didn't measure them for the purpose of resetting the computer. Today, I decided it really ought to be measured. Attaching the tape measure around the wheel, I got a reading of 2110mm, a difference of 72mm, or 2.38 inches. So I've been losing that much with every revolution of the wheel. It might not amount to much on a short ride but over several months of 7km every day, with a loss of 72mm with each revolution of the wheel, I'd say it was a considerable loss.

The chart lists a wheel size of 26" x 1.75" as 2023mm but the actual measurement of the wheel was 2038. That was the whitewalls.
The chart lists a wheel size of 26" x 2.10" as 2068mm, but the actual measurement is 2110mm. That's the new black tires.
Maybe I'm pumping the tires up a bit too much? :laugh:

Anyway, I was wondering if the Garmins have to be set to tire sizes. How else will they know to measure the distance travelled? If they do, and a chart is provided as is with the Cateyes, they may not be giving accurate mileage.
 

Joffey

Guru
Location
Yorkshire
From what I've seen, Garmin is a popular bike computer but I bought two other computers before I knew about Garmin. Both of them are Cateye Padrones and are excellent computers if all you want to know is speed, distance, average speed, maximum speed and time actually riding.

They come with instructions for different size wheels, but I found they are not accurate if accuracy is something you desire. For instance, I have a beach cruiser I bought in January 2020. I soon swapped the tires for whitewalls. The instructions have a chart to tell you what number of millimeters to set it on, but this isn't accurate. So I took a soft flexible tape measure and attached it to the middle of the tire all the way around. I got a reading of 2038mm. That was then set in the computer.

A few months ago I decided to go back to black tires, but I didn't measure them for the purpose of resetting the computer. Today, I decided it really ought to be measured. Attaching the tape measure around the wheel, I got a reading of 2110mm, a difference of 72mm, or 2.38 inches. So I've been losing that much with every revolution of the wheel. It might not amount to much on a short ride but over several months of 7km every day, with a loss of 72mm with each revolution of the wheel, I'd say it was a considerable loss.

The chart lists a wheel size of 26" x 1.75" as 2023mm but the actual measurement of the wheel was 2038. That was the whitewalls.
The chart lists a wheel size of 26" x 2.10" as 2068mm, but the actual measurement is 2110mm. That's the new black tires.
Maybe I'm pumping the tires up a bit too much? :laugh:

Anyway, I was wondering if the Garmins have to be set to tire sizes. How else will they know to measure the distance travelled? If they do, and a chart is provided as is with the Cateyes, they may not be giving accurate mileage.
Garmin would use GPS 👍
 

Darius_Jedburgh

Looking for the lost chord.
Old technology uses tyre sizes. If you must use this never rely on a chart. Inflate tyres to your preferred setting. Find a flat piece of road. Sit on the bike and measure one revolution. Put the valve at the bottom and mark where it comes back to the bottom.
Your weight will reduce the circumference. You must always keep the same pressure, which will change with atmospheric conditions beyond your control.

Finally, ditch the unit and get a GPS one. So much easier.
 

T4tomo

Guru
Garmin would use GPS 👍
indeed and doesn't it auto-reset the non GPS measure based on the wheel speed sensor so it matches your GPS, for the times when GPS drops out or you're on turbo.

btw CharleyFarley a no need to bend your tape round a tyre, make a mark on the floor when your valve (or a chalk mark on the tyre) is at the bottom and another after one revolution, then measure (more accurately) with a steel tape measure, the distance between the two marks, assuming its important to know accurately how inaccurate you have been:okay:
 

rogerzilla

Legendary Member
Tyre size is 100% accurate if you set it properly. This is tedious to do but I have done it:

1. Make a paint or chalk line across the tyre. Use something washable.
2. Ride the bike in as straight a line as you can to leave as many marks on the road/patio/path as you can.
3. Measure the total distance between marks and work out the average distance between two marks.
4. This can get you within 0.1%. For the ultimate accuracy, check against a measured 10 mile TT course - if you know it is a real 10 miles, as some are slightly off to allow a start and finish at convenient lay-bys. My local course has no such issues and the start is a line on a kerb stone. I rode it and the computer said 9.99 miles (176 yards off), so I increased the circumference setting by 10/9.99 to get it dead-on.
5. You have to repeat the process if you change tyre size!
 

dave r

Dunking Diddy Dave Pedalling Pensioner
Old technology uses tyre sizes. If you must use this never rely on a chart. Inflate tyres to your preferred setting. Find a flat piece of road. Sit on the bike and measure one revolution. Put the valve at the bottom and mark where it comes back to the bottom.
Your weight will reduce the circumference. You must always keep the same pressure, which will change with atmospheric conditions beyond your control.

Finally, ditch the unit and get a GPS one. So much easier.
Thats how I set mine, have done for years, if I use the phone to record a ride the two are very close. At the moment I've no interest in a GPS device, all my rides are on roads I know and most of the time the computer gives me all the information I need.
 
OP
CharleyFarley

CharleyFarley

Well-Known Member
Location
Florida, USA
Garmin would use GPS 👍
Thanks, Joffey. I read somewhere else that the GPS is less accurate. It really doesn't matter much, though. My Cateye can be very frustrating to set it up. Then when the batteries need replacing, it's a problem, again, to set it up and program it with the mileage that was on it. I searched for better instructions a couple of years ago, and found quite a few cyclists were having problems setting it up. But I got it figured out and have written my own instructions on a file I keep for my bikes. The instructions that came with it, are not very clear.
 

dave r

Dunking Diddy Dave Pedalling Pensioner
Thanks, Joffey. I read somewhere else that the GPS is less accurate. It really doesn't matter much, though. My Cateye can be very frustrating to set it up. Then when the batteries need replacing, it's a problem, again, to set it up and program it with the mileage that was on it. I searched for better instructions a couple of years ago, and found quite a few cyclists were having problems setting it up. But I got it figured out and have written my own instructions on a file I keep for my bikes. The instructions that came with it, are not very clear.
Batteries last years, my current one is years old and still on the original battery.
 

dave r

Dunking Diddy Dave Pedalling Pensioner
More obsolete technology. Garmin/Wahoo/any other has rechargeable batteries. Plug in as you would a phone.
No disrespect intended but progress happens all the time and makes life easier - unless you are an ostrich.
Its down to personal choice, if you want a GPS device use one, if you want to use something else use that, or combine different devices if you want, theres no compulsion to use GPS.
 

Joffey

Guru
Location
Yorkshire
Thanks, Joffey. I read somewhere else that the GPS is less accurate. It really doesn't matter much, though. My Cateye can be very frustrating to set it up. Then when the batteries need replacing, it's a problem, again, to set it up and program it with the mileage that was on it. I searched for better instructions a couple of years ago, and found quite a few cyclists were having problems setting it up. But I got it figured out and have written my own instructions on a file I keep for my bikes. The instructions that came with it, are not very clear.
They are accurate enough for our needs. Happy shopping and happy riding!
 
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