What is the difference between speed and speedo ??

[kkr2k2]

Originally Posted by sanket12

i think you have to deduct 10 to 12% to get the real speed

Why only deduct??, why not add to get the real speed?...
I meant how to decide, whether to deduct or add a percentage to get the actual speed of the car.
Can anyone answer my question without any assumptions or guessing?
Because, this would become important where speed limits are imposed.

Originally Posted by tsk1979

Get a GPS system. It will tell your actual speed after you are cruising at it for a few sec

This could be a stupid one, but.....
What distance the GPS measure to calculate the speed?
1. Is it the distance travelled by the car's Center of Gravity? (OR)
2. The displacement from point A to point B. (displacement refers to shortest distance between A and B) I asked so because, i guess GPS uses satelites.

[Ram]

Originally Posted by kkr2k2

Why only deduct??, why not add to get the real speed?...
I meant how to decide, whether to deduct or add a percentage to get the actual speed of the car.
Can anyone answer my question without any assumptions or guessing?
Because, this would become important where speed limits are imposed.

To get a 10-12% error in speed due to tire wear or oversize tires,
you have to have a 10-12% difference in rolling circumference,
which translates to 10-12% difference in overall wheel diameter.

This is IMHO a huge difference, which practically impossible in real life!
Variations in tire size for any specific car are not that wild!

With big cars it's impossible, but even if we calculate for the dimunitive Maruti 800,
This will mean that
for a positive error of 10%, the standard 145/70R12 tire tread is worn down by 2.5 cm which is preposterous!
and for a negative error of 10%, the standard 145/70R12 tire is changed to 145/88R12 which again is preposterous!

Originally Posted by kkr2k2

This could be a stupid one, but.....
What distance the GPS measure to calculate the speed?
1. Is it the distance travelled by the car's Center of Gravity? (OR)

All points on the car travel at the same speed unless your car has the ability to lengthen or shorten like an earthworm
or you're travelling close to the speed of light when the car length shortens (ref: Carl Sagan's TV show on Einstein's theory of relativity).

Originally Posted by kkr2k2

2. The displacement from point A to point B. (displacement refers to shortest distance between A and B) I asked so because, i guess GPS uses satelites.

It is indeed displacement from point A to point B.
This makes sense in two cases.
Case 1: You are travelling in an absolutely straight line between measurements. Then displacement is identical to distance travelled.
Case 2: When the displacements measured are infinitesimally small.
This will be the case if you have a high-end aircraft GPS receiver-computer, which costs as much as the car itself, or if the car is creeping at a speed below 5 km/h.

[0-100]

Is this error also found in imported cars or only in indian cars ?? And if yes den wat percent (like in indian cars it is usually 10-12%).
I owe a Toyota Corolla 1985 model. My speedo shows 160 on full acceleration, so wats my actual speed at that time ??
Plz Advise .