google map speed or odometer

I assume you’re taking about the speedometer, rather than the odometer (which measures the distance you’ve travelled), since you’re asking about speed accuracy, but actually the same principles apply to both.

The speedometer calculates your speed by counting the rotations of the driveshaft, axle or wheel in a given period of time and multiplying it out to give a speed in km/h or mph. However, there are variable factors that will affect the accuracy of that calculation, such as tyre wear and pressure - meaning that it’s impossible for the calculated speed to be accurate all the time. So the international regulations mandate that it’s not allowed underestimate your speed at all (so it should never tell you you’re going slower than you actually are) - but is allowed overestimate your speed by up to 110% plus 10 km/h. It doesn’t mean that they always overestimate by that factor - it means that’s the maximum amount of overestimating legally allowed.

GPS can be much more accurate, but that accuracy also depends on uncontrollable variables, such as the number of satellites you can get a reading from, signal reflection from buildings, etc. The global average “user range rate error” of GPS is ≤0.006 m/sec over any 3-second interval, with 95% probability, meaning it’s very accurate (for use by the general public at least) under ideal conditions.

So in general and on average, GPS is much more accurate than your speedo, but there’s places and times GPS will loose accuracy (or stop working altogether) - such as tunnels or “urban canyons” (roadways with, high buildings each side) , whereas your speedo should always operate as long as the wheels are turning.

I go with the speedo. It gives you a bit of leeway so if i do breeze past a speed camera van at 125 or even 130 indicates speed in a 120km/h zone for example i know i'm actually safe enough. If you slip over to 121 on your precise GPS reading you are fooked.

Does GPS take slope/elevation change into account? Assuming the speed is calculation is based on the time taken to travel between two points, then would someone drive downhill at X kph appear to be at a lower speed than someone travelling the same distance on level road?

The GPS doesnt measure speed directly, it measures your position which will include an altitude component, or being pedantic it doesn't measure position either it measures the time difference between signals from the various satelites to calculate position. It will be up to whatever software is taking that data to calculate the speed using whatever algorithms they have.

No, it makes perfect sense.

I stick to 120 generally speaking, or whatever the limit is. If i slip a few kms over the cars under estimated reading while overtaking or getting carried away, lapse in concentration or whatever and i see a speed camera van, i know i'm probably ok.

If i stick to 120 with an accurate GPS reading, if i go 1km/ h over, i'm done.

I dont see the sense in going out of the way go get a GPS reading to reduce my margin for error for the sake of gaining 2 or 3km/h progress.