How to Calibrate your Speedo Correctly

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Derks420

Derks420

New Member
May 30, 2010
114
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Central New Jersey
Hey guys, I see alot of people making speed claims that are CRAZY! I am positive that may have not set up their speedo right. Here is the best way to find the size of your tire.
1. Take a piece of rope and tape the end on the tire.
2. Place tape every 8 to 10 inches until you have wrapped the ENTIRE TIRE (circumferance) once.
3. Measure the length of the string in inches.
4. Multiply your length by 25.4 ( 25.4mm=1 inch)
5. When you program your speedo, put the number that you got in as the wheel size in mm.


Example.
My tire is 77 inches in circumferance. So I multiply 77x54.4 which is 1955.8mm. When I programed my speedo, I put it in as 1956. Now acurate within 1/2 mph.

Good Luck
 
Kevlarr

Kevlarr

New Member
Jul 22, 2009
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Mi
Easier way.

Put a drop of any kind of liquid on your tire (3&1 oil, grease, maple syrup, etc), get on your bike and roll it forward with your weight on it. Measure the distance between the dots on the ground, convert to mm and enter that number in as the wheel value.
 
Derks420

Derks420

New Member
May 30, 2010
114
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Central New Jersey
Then you would need a clean surface to roll on, and should prob do 2 or 3 revolutions. then measure and use the average cuz oils smears and wouldnt be as accurate as the string...
But Either way will do
 
scotto-

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
6,505
24
38
Ridin' inSane Diego, CA.
Yes, or mark your tire with chalk and chalk the pavement at the same point and with the help of a friend have them steady you and roll forward until one revoluton and mark the pavement then measure the distance between the two marks. The rolling circumferance in inches or millimeter and input for the calibration. Same, same and same. All the methods work.
 
P

Patr1ck

New Member
Apr 15, 2010
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Tempe, AZ
Would having the weight of the rider on the bike when doing these methods be more accurate, since the overall diameter of the tire would be less than unloaded?

Pat
 
Bikeguy Joe

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,837
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up north now
scotto- said:
Yes, or mark your tire with chalk and chalk the pavement at the same point and with the help of a friend have them steady you and roll forward until one revoluton and mark the pavement then measure the distance between the two marks. The rolling circumferance in inches or millimeter and input for the calibration. Same, same and same. All the methods work.
Click to expand...
This is the best and most accurate way. Oil will spread, not to mention what it will do to your tires, string can be stretched.
 
AslansMonkey

AslansMonkey

Member
Oct 2, 2008
194
1
16
Good suggestions. Personally I just lay a tape measure on the ground and line up the stem on the front tire so that it is perpendicular to the ground and at the beginning of the tape measure. Then I roll out one revolution and get the reading right off the tape measure.

But the idea is to enter the actual tire circumference in millimeters into your cycle computer. Not all tires are created equal so using a general "guide" isn't always accurate.

I suspect some of these speed claims are made with mechanical speedo's which aren't even close to accurate.
 
AslansMonkey

AslansMonkey

Member
Oct 2, 2008
194
1
16
severedcorps said:
on my speedo it only asks me for the wheel size, in my case 24 inches..

my speeds seem accurate..
Click to expand...
It's possible that it's close. The faster you go the more you'd notice any discrepancies. Because not all 24" tires (or any size for that matter) are created equal, however, the stock tire size entry in a cycle computer is a shot in the dark at best. The most accurate way to set a cycle computer is using the roll out method mentioned above.
 
S

severedcorps

New Member
Jul 13, 2010
54
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Vancouver, BC
This is good info.. I'll recalibrate when I get my new wheels, but until then I'm leaving it where it is.

1916 is default for 24 inch tires on my instructions
 
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