Does the speedmeter on cars actually show it a little bit above what you are actually going or did my dad just want me to shut up?
Posted by AuroraKivi@reddit | askcarguys | View on Reddit | 41 comments
ly5ergic@reddit
I've noticed, in many cars I've been in, show 1-4 mph faster than GPS says.
If the tires are worn or low on air it should make the speedometer run a little high. I dont know how large that would be in reality though.
Banishedandbackagain@reddit
Most of the ones I've driven in Australia are below GPS speed.
Yesterday I drove a Mazda BT50 2015 and it was showing 117 and actually doing 110
shortedaman1@reddit
My car has 2 speedometers. Normal with needle and an actual speed digital one. They're always about 3 mph apart.
BoondockUSA@reddit
Yes, the vast majority read faster than your actual speed. SAE specs for speedometers (which many auto manufacturers try to comply with) has a higher tolerance for reading too fast than reading too slow. I forget the exact SAE spec but it’s something like -2% to +5% (but don’t quote me on that).
Power-sport vehicles like motorcycles are especially bad at reading faster than actual speed. One of my bikes was a full 10% off. My current bike is better at 5% off.
Police vehicles were the only ones that I’ve experienced that read accurate to true speed, and if one was off, it was only by 1 mph.
My disclaimer is that is with vehicles with stock tire sizes. Start messing with tire sizes or gear ratios and the speedometer will be more inaccurate.
Beartato4772@reddit
It is very, very difficult, in fact impossible to make an entirely accurate speedo for a car, not least the tyres you put on it or aftermarket wheels both change the speed a car will read.
It's illegal for a speedo to read under, for reasons that should be obvious.
Therefore they read slightly to a surprising amount over.
Also he wants you to shut up.
SavageCabbage11@reddit
I dont think theyre 100% accurate. I think its within a couple miles per hour. i dont really know
Live_Bug_1045@reddit
It does read a bit higher, on my 2007 around 5kph at 50 (55 on the Speedo is 50 on GPS) and on the other car a 2025 is 1kph above GPS. Cars read high, otherwise they risk themselves to legal actions.
Willing_Sink_3623@reddit
been a dealer tech for several years across many different makes. can confirm that they are designed to read high via the numerous service bulletins i have read on the topic. a good rule of thumb when getting a new to you vehicle is to verify speed with gps for a week or so to get a feel for how off it is, usually a percentage. my 1995 3 series is off 12%, my 2012 toyota is off 7%, 1995 skyline which has been converted to mph but still displays on a kmh gauge is 0%(the odo is comical), my motorcycles are all hopeless. a unique feature on the nissan altima is that it doesnt display a speed, they use a faster than you gauge, industry first.
loweexclamationpoint@reddit
What's that mean about the Altima?
hatred-shapped@reddit
Maybe 15 or so years ago, but modern cars are pretty accurate. More so if it has a screen for the gauges. They run off of the GPS build into the Sat Nav.
EuroCanadian2@reddit
I am 99.5% sure that's not the case. They use wheel speed sensors (which are there for ABS anyway) or more commonly transmission output speed sensors. In the old days, it was mechanical - a spinning cable inside a sheath.
If you spin a tire on ice, the speedometer goes up even though the car doesn't move.
If it used GPS, it would quit working without access to GPS signal, like in a longer tunnel.
WhatsMyNameAGlen@reddit
This is correct
throwaway17717@reddit
I don't think this is accurate, at least where I live. Whenever I go through like a school zone radar, it is ALWAYS 3mph less than my speedo on the display. Unless the reader things always show 3mph higher than I'm actually going to slow you down, which I suppose is equally plausible.
lowfreq33@reddit
From the passenger seat your visual perspective would be different than the driver (on a physical speedometer with a needle) and it would actually appear that the car is traveling slightly slower. There are factors that can affect the speedometer’s accuracy like tire pressure, improper calibration, wrong tire size, but it’s fairly likely your dad did just want you to shut up.
Totally_NotaBot735@reddit
I think what they’re asking is if the speedometer shows the true speed or if it’s calibrated to be out, so if you were sitting on 80 you’re actually doing 76.
tandyzmills@reddit
the speedometer vs. what? the viewing angle doesnt change a thing.
migorengbaby@reddit
Absolutely it does. Lean left and right and watch where the tip of the needle points. It will appear to move higher and lower depending on the angle you’re looking at it.
chris14020@reddit
The needle sits above the actual lines on a mechanical gauge, causing a parallax error - you may see the needle appearing to sit somewhere different when viewed at an angle than when viewed perfectly head on.
Timmarino@reddit
It depends. Is the vehicle stock? Same rims, tires, transmission, rear ends if applicable ect. In my pickup my truck came with 373 but I had the rear end go out I had a 242 axle sitting here at the house and now mine is about 13-15 mph off. If you put 22 inch rims and it come with say 17s it will affect it. But if bare bones stock most have an electronic speed sensor these days and are pretty accurate old cable ones were slightly off but close
ScrapYard101@reddit
Depends on the car, and even more so, it depend on the wheel size.
NoWastegate@reddit
I believe it is a requirement to show speed slightly above. My Porsche Macan was +2 and my Lexus GS is +3
Grand_Combination_19@reddit
It's a law that speedometer cant under report speed so it over reports
Draaxikas@reddit
I drive variety of cars because of my job, and often compare their speedometer readings with actual gps (phone, waze).
General rule is that cars show higher speed around 4-5 km/h (about 3mph) when moving at road speed (90-100 km/h or 55-60mph range).
When you talk about getting a ticket for speeding, you have to understand that all measuring devices have margin of errors, that are taken into account.
In the country where I live general speed limit on road is 90 km/h. But because when issuing a fine, possible errors of your car speedometer and speed camera are both taken into account, so cameras register speeds from 97 km/h, in which case you'll receiva a ticket for speeding at 94 km/h (because 3km/h is counted as possible error of your speedometer).
But because car speedometers are generally adjusted to show speed higher than actual speed, it leads to situations where in reality you can drive at 98-99 km/h by your speedometer (meaning 8-9 km/h or 5mph) over the speed limit without actually receiving any fines, which in my eyes is a significant difference.
Dedward5@reddit
They did and some still do. They used to be “designed” to under read because manufacturing/operating tollerence meant that if you designed them to be “spot on” any errors could put you over the speed limit.
As they got better eg moved from mechanical to electric and then digital, the tolerances are better so they an be designed to read much closer, or even bang on.
ScallionSmooth5925@reddit
Eu registration reqires it to show +4 ±10%
AC-burg@reddit
They are still crap
databeestjenl@reddit
Dutch law states that a Speedo may never under report. This automatically makes it over report, but this varies wildly by manufacturer, model, winter tires etc.
SneakerTreater@reddit
A little from column A, a little from column B. Source: am dad and am car guy
Bobloblaw_333@reddit
Ding, ding, ding!! This is the answer!
QueasyParamedic6783@reddit
My digital speedometer is 2mph faster than my gps and radar sign I pass everyday.
Cornelius-Figgle@reddit
They do. It's illegal for it to underread so most manufacturers make them overread for tolerances.
Mine is a fairly consistent 2mph above the actual speed
Brilliant-Onion2129@reddit
Speedometers do vary from car to car. Very few roll out of the factory dead on. Toyota for example read a little faster than what you are actually traveling. Get a speedometer app on your phone.
Hotdog_disposal_unit@reddit
Ever since I’ve had gps speed available it’s always shown my cars speedometer to say I’m going faster than the gps
tandyzmills@reddit
Assuming the car has OEM size tires and wheels, they are generally pretty accurate, within 1 MPH or so.
AC-burg@reddit
Infiniti and Honda are known to 2 to 3 miles an hour over what you are actually going. I assumed it was for leasing purposes. Steams me when you are the end owner though.
chris14020@reddit
It's for liability reasons. Gauges are allowed to read a little bit too fast (actual 55/reading 57, for instance), but cannot under-report (actual 57/reading 55). So, they build in a little room for error (equipment inaccuracies, tire tread depth changes, etc.)
tandyzmills@reddit
I drive a honda and its pretty accurate, according to GPS.
Admirable-Composer22@reddit
Compare it with your phone. Apple and Google Maps both have speedometers.
AuroraKivi@reddit (OP)
too late unfortunately. This was just a random thought that popped up thinking about past events from years ago
boringcarenthusiast@reddit
Some cars are calibrated to show a bit over the car’s actual speed, like BMWs. Actual speed and displayed speed can also vary depending on tire/wheel size (too big, too small, etc.)
Juan_Bot@reddit
It does