# Distance mismatch.

#### Debian

##### New Member
My commute, as measured by my computer is a fraction over 7 miles each way.

I've recently started to use Google My Tracks and it measured my commute at nearly 5 1/2 miles.

Interestingly though, my computer and My Tracks agree on my average speed and journey time. One of them is wrong but which, and how?

#### gaz

##### Cycle Camera TV
Well if the avg speed and journey time is the same... then the distance is the same. Simple Maths... D=ST
D being distance
S being average speed
T being time

OP
OP

#### Debian

##### New Member
I know.

But which one is correct? And how to check it?

#### HLaB

##### Marie Attoinette Fan
Debian said:
I know.

But which one is correct? And how to check it?

Check the distance in another site like bikely or bikehike, etc

#### Moodyman

##### Guru
Doesn't Google use GPS, whereas most computers use the wheel rotation?

I read somewhere that GPS can be slightly inaccurate (not as much yours though) as it doesn't take into account the cyclists riding pattern.

#### HLaB

##### Marie Attoinette Fan
Moodyman said:
Doesn't Google use GPS, whereas most computers use the wheel rotation?

I read somewhere that GPS can be slightly inaccurate (not as much yours though) as it doesn't take into account the cyclists riding pattern.
Thats about right the gps draws up straight lines between points whereas the cyclist might have wobbled, etc but you're right that wouldn't cause such a large discrepency as posted.

#### Norm

##### Guest
Debs, you haven't changed tyres without changing the settings in the computer, have you? I might expect a difference of 1%-ish but 20% is too much to be just different cornering radii etc

#### gaz

##### Cycle Camera TV
Debian said:
I know.

But which one is correct? And how to check it?

plug the numbers into the formula and find out.

OP
OP

#### Debian

##### New Member
Gaz - both give the same average speed and journey time, just different distances.

EDIT - yes, sorry, woken up now. I'll do the calc.

#### Norm

##### Guest
It's not that tough to check it against another web site is it?

OP
OP

#### Debian

##### New Member
Norm said:
Debs, you haven't changed tyres without changing the settings in the computer, have you? I might expect a difference of 1%-ish but 20% is too much to be just different cornering radii etc

No Norm.

Computer was set up on current tyres and pressures by measuring distance travelled for five rotations.

#### beastie

##### Guru
What was the ave speed and time please

OP
OP

#### Debian

##### New Member
28 mins 31 secs at 14.65 mph.

Nearly 7 miles by my reckoning.

Correct

#### gaz

##### Cycle Camera TV
If you have the raw data and it equates to 7ish miles, then the one which is 5 miles is clearly wrong.