# Wheel diameter + speed calculation

#### earth

##### Well-Known Member
When I changed from GP400S tires to ProRace3 I was very impressed by the extra speed from them. It was probably partly new kit adrenalin and maybe lower rolling resistance. But I noticed recently that they are maybe 1mm less diameter that the GP4K's and I have not adjusted my cycle computer at all.

How much difference might this make? Less than 1mph?

#### RecordAceFromNew

##### Swinging Member
For 700c wheels the diameter at the bead seat is 622mm. Assuming the tyre adds 20mm each side to the rim to make up the wheel diameter then a 1mm difference in tyre diameter will make a difference in computer calibration constant of:
(1+1)/(622+20+20) or 0.3%.

#### Norm

##### Guest
Are you sure that the diameters are that close?

#### slowmotion

Ball park....but here goes...

The circumference of both tyres is roughly 2100 mm. If one is 1 mm bigger or smaller than the other (in terms of diameter) it will be about 3mm different in circumference.

That is 3/2100=0.00143.

Just increase of decrease the computer reading by that amount.

For example, if your computer reads 100 mph (plenty of peeps claim such figures...),
you might be out by 0.143 mph.

A 1mm difference in diameter represents an entirely trivial speed reading error...IMVHO

Edit. I used the roll-out circumference, not the bead seat stuff

OP
E

#### earth

##### Well-Known Member
Thanks that settles it. The speed increase was down to a faster tire and excitement.

#### montage

##### God Almighty
balls. just deleted my post.
Basically using 2100mm as your initial circumference, then increasing the radius by 1 mm caused a 6mm change in circumference.

Little effect on average speed tbh