Tachometer vs GPS

I want to know if there is major difference between the speed of a vehicle obtained by a tachometer and GPS device. If there is a significant difference, which one is better and why?

Your question makes no sense.
A tachometer in a car measures the engine RPM.
To then determine the cars velocity requires additional input, such as the relationship between the
engines RPM and the wheel speed.
The cars velocity then requires information about the tyre circumferance.
How accurate the speed is , depends on the accuracy of all this information.

Considering that GPS will give your position to a tolerance of around 10 metres and the car's odometer (that's the thing that shows distance traveled) to within 0.1 units (be they miles or kilometres) then you are comparing 10 metres against 100 metres (or 176 yards) so the GPS wins hands down.

However GPS was (and might still be) a military system and they did not warrant 100% coverage nor 100% up-time. You pays your money and takes your chance.

Why would this question "make no sense" ??
You have 2 different devices bringing up to you the same information, it seems normal to wonder which ones gives the best one :wink:

keep in mind that the tachometer in a car indicates allways about 10% above the true speed. Not because it is not accurate, but because car manufacturers don't want to be responsible if you drive beyond the authorized limit.
GPS gives the speed over ground it measures, if the signal is good the value is accurate. Sometimes, though, with some navigation softwares, in weak signal conditions, the speed is displayed with a delay of a few seconds, which makes the information useless :wink:

A tachometer measures engine RPM.
Its not a speedo or odometer.
Even a speedo relies on a fixed known relationship between wheel speed and tyre size.

ok, you are right, tachometer was not the right word, the relationship between RPM and speed includes tyres size and which gear is used