Viewing a single comment thread. View all comments

JackieMcFucknuckles t1_j1emxsz wrote

Reply to comment by egordoniv in Speedometer of a newer Mustang reads "Ground Speed" by egordoniv

GPS speed and Ground Speed don’t always match. That would be really the only other option, but it’s negligible.

Example: a car traveling over mountainous terrain will have a higher ground speed than gps speed since the gps speed is only accounting for position and not elevation. Traveling up a mountain and back down the other side is a longer ground distance (sum of 2 sides of a triangle is always longer than the length of the 3rd side) than the straight line distance if there were, say, a tunnel through the mountain.

It’s really not useful data though because it’s usually a negligible difference.

66

tonyswu t1_j1etj72 wrote

Why would a driver care about GPS speed though?

15

JackieMcFucknuckles t1_j1euwx8 wrote

Yeah, that’s kinda what I was saying. It is another option, but it would be mostly useless. I guess I just felt like it was a nerdy fun thing to share.

27

MeltsYourMinds t1_j1g2yuw wrote

On a flat track it’s more precise. Older speedos, due to the way they measure, get more imprecise as speed increases.

3

Full_Temperature_920 t1_j1ep0z7 wrote

Didn't even know GPS speed was a thing

4

A_Tad_Bit_Nefarious t1_j1g5vgl wrote

If you use navigation apps like Waze, Google Maps, or a dedicated unit like a Garmin, they typically have an option to tell you your speed. Sometimes they tell you the speed limit of the road you are on too so you can easily tell by how many mph you are breaking the law lol.

10

NemosGhost t1_j1g8g9s wrote

It is and it isn't.

GPS can measure speed/velocity and display it different references or units. It's a tool to measure, not the unit or datum itself.

Saying GPS speed is kinda like saying ruler length.

We do often say it out of habit though to compare it to air speed or boat speed which are the speed of the vessel moving through air or water that is also moving.

2

NemosGhost t1_j1g6ydm wrote

GPS measures position and therefore speed three dimensionally as long as it has at least 4 satellites or combination of references.

It's just a matter of what you want displayed. Modern units can do SOG or 3 dimensional speed.

4

hearnia_2k t1_j1evs25 wrote

But calling it ground speed is not exactly right either.... GPS speed in a straight line should be pretty accurate. Most systems use more accurate positioning satellites than GPS in addition, and they all offer altitude so that can easily be factored in.

Ground speed could be off if the car is drifting for example, since the wheels will be rotating at different speeds.

2

PhasmaFelis t1_j1gu6iq wrote

You're not wrong, but this amuses me because, in aircraft, "ground speed" means what you're calling GPS speed. Basically just the horizontal component of your speed. An aircraft in a vertical dive has a ground speed of zero even if it's close to breaking the sound barrier.

(For extra confusion, "air speed" is your speed relative to the air around you, and that air may also be moving. If you're in level flight, heading due north into a 50MPH wind blowing due south, you might have a 300MPH airspeed but only 250MPH ground speed.)

1