# tire diameter?

#### Ultima

##### Platinum Member
So when my bike tire says its 26x2.125, does that mean it'll fit 26 inch rims (like car tires), or that its actual diameter is 26 inches? Just wondering so I can calibrate my speedometer

#### sohcrates

##### Diamond Member
26 inches is the diameter, yes. (i've only seen car rims that go up to 24 inches though, sorry )

#### Ultima

##### Platinum Member
k, cause on car tires its just the rim diameter right? Like you can have 15,16,17,18inch tires but their overall diameter is around the same cause the 17 and 18 inches are much thinner.
So with the bike tire, the 26 inch is the actual diameter of the tire and not just the rim?

#### sohcrates

##### Diamond Member
ahhh..now i get what your asking

in that case, i do believe that the rim itself is 26 inches...with tire it would be a little more

#### amnesiac

##### Lifer
Try using a ruler?

#### Ultima

##### Platinum Member
hmm
actually the rim isn't 26 inches. The entire tire is 26 inches in diameter, rather it's slightly less cause the circumference appears to be 204-205 centimeters which is 25.7 inches in diameter
So I set my speedometer to 205 centimeters instead of the 207 that 26 inches would be

#### Ultima

##### Platinum Member
But the difference isn't that much. It's a dif of say going 29.5km/h or going 29.8km/h.
Or a diff of having travelled 2.1km as opposed to ... 2.1km.

So use the 207 if you wanna feel like you're going a little bit faster than you actually are. I remember setting it to the highest i could once and it said I was going 80mph when I was going maybe 20. hehe...

#### DAPUNISHER

##### Super Moderator CPU Forum Mod and Elite Member
Super Moderator
How many PSI are you riding your tires at? What is the PSI min/max rating on that set you have?

#### Squisher

##### Lifer
You also have to figure in the fact that your weight will be deforming the tire. Sounds hard but it's not.

Draw a straight line on the cement, mark a line on the front tire even with a mark on the ground, then roll the bike with you on it along the line until you've made one revolution exactly, make another mark and measure.

Helps to have a couple of friends helping hold you up and watching for and making the marks.

#### Ultima

##### Platinum Member

<< How many PSI are you riding your tires at? What is the PSI min/max rating on that set you have? >>

40-85. Dunno what they're inflated at, probably 85? My old tires were max 45.. those were more offroad tires.
Yeah, that's true about the deforming. Hmm..