If you have a new account but are having problems posting or verifying your account, please email us on [email protected] for help. Thanks :)
Private profiles - please note that profiles marked as private will soon be public. This will facilitate moderation so mods can view users' warning histories. All of your posts across the site will appear on your profile page (including PI, RI). Groups posts will remain private except to users who have access to the same Groups as you. Thread here
Some important site news, please read here. Thanks!

google map speed or odometer

  • 16-11-2021 1:49pm
    Registered Users Posts: 9,647 ✭✭✭ cena

    Which is more accurate when it comes to matching the speed you do


  • Registered Users Posts: 50,124 ✭✭✭✭ bazz26

    Your odometer is intentionally calibrated to slightly overestimate your speed under EU regulations afaik so it will not be 100% accurate for sure. Any GPS based system will be accurate.

  • Registered Users Posts: 7,348 ✭✭✭ Gregor Samsa

    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.

  • Moderators, Business & Finance Moderators Posts: 17,104 Mod ✭✭✭✭ Henry Ford III

    GPS all the way. Speedos are a bit optimistic as Bazz already said.

  • Registered Users Posts: 12,943 ✭✭✭✭ Toyotafanboi

    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.

  • Registered Users Posts: 30,666 ✭✭✭✭ listermint

    GPS systems are not as accurate as wheel turns. Not sure where that information has come from ?

    They are accurate depending on make model and manufacture to the spec of the GPS systems they consume.

    Can they be more accurate? Yes they can of course.

    Is consumer stuff like your phone? No. They are dialled in to the nearest few meters no millimeters or inches in old money.

  • Advertisement
  • Moderators, Business & Finance Moderators Posts: 17,104 Mod ✭✭✭✭ Henry Ford III

    Makes no sense at all.

    Keeping under 120kph on GPS is as easy or otherwise as staying under 130kph on your speedo surely? I use cruise control a good bit on the motorway so speed control is a doddle.

  • Registered Users Posts: 5,748 ✭✭✭ SteM

    The Google maps speed would be based on the accuracy of the GPS on your phone. Some phones have accurate GPS and some don't. Depends on the phone I guess.

  • Registered Users Posts: 5,351 ✭✭✭ storker

    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?

  • Registered Users Posts: 30,666 ✭✭✭✭ listermint

  • Registered Users Posts: 796 ✭✭✭ brownej

    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.

  • Advertisement
  • Posts: 0 [Deleted User]

    Luckily we don't drive up Everest. The regular Joe elevation changes on normal roads over X distance contribute negligibly to the speed.

  • Registered Users Posts: 12,943 ✭✭✭✭ Toyotafanboi

    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.

  • Registered Users Posts: 20,913 ✭✭✭✭ Alun

    Actually, contrary to what most people think, GPS receivers calculate velocity by measuring the doppler shift in the frequency of the signals coming from the satellites. They have to do this anyway to compensate for the movement of the satellites relative to the receiver in their positional computations, so the information is there anyway, no additional computation needs to be done. It also is relatively a lot more accurate than the horizontal or vertical position, and isn't really affected much by non-optimal satellite constellations or urban canyon effects.