im just newbie in car stuff. i'm wondrin' i have a 2009 lancer GT. while im cruising on the 401 i'm running 120 kph on my speedometer but when i looked on my gps it says my speed was about 115/116 kph.. is there anyone who noticed this too?

 

That's pretty normal... those two speed readings are being determined in two different ways... hence the couple km/h difference.

 

Your speedo would be the most accurate.

 

I'd say the GPS is more accurate. Manufacturers tend to make the speedo read just under what you're actually doing. Kind of an active approach to stop people saying that the speedo said the car was under the speed limit when they get pulled over. My speedo has been like this on all the cars I've owned.

 

speedo could be off just by having different sized tires

 

GPS could be off becasuse its using a few different satellites to pin point your position within a 5m radius. Tire sizes etc also throw off your speedo off as well.

 

GPS can not compensate for vertical changes in elevation. Thats one factor, Tire ratios are another

 

i have a magellan and accuracy is pretty good. maybe off by 2km at most.

 

that's good thing to know that im not the only one noticing this. but when i ran 130 the gps said 125.. 5 kph off??? maybe its my tires?? cause i have a GT which 16" compare to GTS? what you guys think about this?

 

It has nothing to do with tire sizes. The GPS just isn't that accurate. If your GPS has the ability...look at it's accuracy radius... at the best it will probably say 5m.

 

Not to be a dick or anything, but why is this so important? At the end of the day, something that is mechanically attached to your car will most likely more accurate than big brother.

Odometer gives you your instantaneous velocity at a given time, GPS will give you an average velocity over the time it took you to go from point a to point b, which is the time it takes you to make ur gps 'tick'

The most accurate way to get a speed from an observer would be from a cops laser gun, and if you're doing 130+, it doesn't matter +/-5 kph.

 

OP, its normal for the speedo or GPS to be off, at the end of the day, they are both calibrated systems, and they both have a margin of error. some cars are better calibrated than others.

how does elevation effect speedometer reading?

 

it differs, imo.
some GPS systems are quite accurate, some aren't..

 

Elevation doesn't affect speedometer reading, but potentially can affect GPS reading.

Maps are dealt with in 2 dimensions, X-Y.
And when it calculates your avg speed like I posted earlier, it will only use XY to do so.

Driving up a very steep mountain, you are primarily moving in the Z axis. Say you are going 100km/h.

In the XY plane you are only going potentially 20-30 km/h. And it is what the GPS wil tell you.

 

He means the GPS will see a car travelling Side B and Side C in the same amount of time as having the same speed. When in reality... the car travelling on Side C is going faster (since it's covering more distance in the same amount of time).