Speedometer is off by 6-8% but VSS over OBD2 is deadon

[TridenT]

Thoughts about how to go about fixing this? Speedometer is about 6-8% fast. It's not a huge issue. If anything, it keeps me going slower because I think I'm going quick. And the reason I know this is because I hooked up an OBD2 scanner with my laptop and then read the readout while driving and checking my speed via Waze. Waze GPS speed and VSS over OBD2 were the same but my speedometer is reading 6-8% faster.

It's not a big deal but I wouldn't mind fixing it if it was a simple issue.

 

[DeviousTrap]

If VSS is correct, it's likely by design. Many car speedometers overestimate your actual speed as that's safer than if they were off in the opposite direction (showing slower than you're actually going).

 

[Mark R]

This is deliberate. Many jurisdictions place strict limits on speedometer accuracy - but the range is not symmetric typically the legal range for the speedometer display is 100-110% of actual speed.

This can vary with vehicle type, e.g. if a particular vehicle is required to have a calibrated speedometer, then a different range may apply - e.g. 98-102%

 

[bruceb]

In my car, 1999 Grand Prix, the speedometer and the HUD digital display, match up. Also when I put my GPS (Magellan) in the car, it can also show vehicle speed and it agrees with my HUD says. So I would say it is accurate (at least in my car). Not worth trying to get yours fixed, but a transmission shop can probably do something (or a shop that does nothing but calibrations, like where the police go to get their speedos calibrated) If yours has a pointer, you could just move it a teeny bit, but you need the cluster out to do it.

 

[tortillasoup]

This is normal but bothered me enough on my civic to actually remove the speedometer needle and install it back to the actual speed. I actually did this while driving 60mph as I had the cruise control set to 60mph according to OBD-II, had the cover for the gauge cluster removed, pulled out the needle and re-installed it in the correct position. You can't remove and re-install the needle into the correct position unless the cluster is operating i.e. the wheels are spinning and or you can send a VSS signal to the cluster to show an actual speed. If you have the center needle removed and you turn on the cluster, the cluster will get confused as it does a resistance check to gauge the resting position of the needle.

 

[Brian Stirling]

I've owned mostly Japanese cars over the last 35 years and most of them read high by upwards of about 3-4 mph. My current car, a 2012 Ford Focus, actually indicates about 1 mph lower than actual.

I have also hooked up an OBDII dongle to log data and have noticed the GPS speeds are slower by this same amount and matches what my handheld GPS says. In short, my ODO under reports mileage by almost exactly 1%.


Brian

 

[Zenmervolt]

This is normal but bothered me enough on my civic to actually remove the speedometer needle and install it back to the actual speed. I actually did this while driving 60mph as I had the cruise control set to 60mph according to OBD-II, had the cover for the gauge cluster removed, pulled out the needle and re-installed it in the correct position. You can't remove and re-install the needle into the correct position unless the cluster is operating i.e. the wheels are spinning and or you can send a VSS signal to the cluster to show an actual speed. If you have the center needle removed and you turn on the cluster, the cluster will get confused as it does a resistance check to gauge the resting position of the needle.

Repositioning the needle will not make the speedometer accurate if it's off by a percentage.

What you describe only works if the speedometer is off by a fixed amount.

When the speedometer is off by, say, 10%, it will read 33 mph at 30, but 66 mph at 60. If you reposition the needle to read correctly at 60, the gauge will read 27 mph when you're going 30.

Basically, what you're recommending will not fix the issue the OP described and, frankly, is ridiculous.

ZV

 

[Brian Stirling]

Repositioning the needle will not make the speedometer accurate if it's off by a percentage.

What you describe only works if the speedometer is off by a fixed amount.

When the speedometer is off by, say, 10%, it will read 33 mph at 30, but 66 mph at 60. If you reposition the needle to read correctly at 60, the gauge will read 27 mph when you're going 30.

Basically, what you're recommending will not fix the issue the OP described and, frankly, is ridiculous.

ZV

True, but the Japanese cars I've owned that read high tended to be high by a pretty much fixed amount so moving the needle would help.

Of course, most newer cars don't have mechanical speedos and instead have either a digital numerical display or a digital pointer -- both of which are merely the output from a computer of some type. Not much hope of re-calibrating them...


Brian

 

[TridenT]

True, but the Japanese cars I've owned that read high tended to be high by a pretty much fixed amount so moving the needle would help.

Of course, most newer cars don't have mechanical speedos and instead have either a digital numerical display or a digital pointer -- both of which are merely the output from a computer of some type. Not much hope of re-calibrating them...


Brian

It's still stupid and won't fix my issue.

 

[tortillasoup]

Repositioning the needle will not make the speedometer accurate if it's off by a percentage.

What you describe only works if the speedometer is off by a fixed amount.

When the speedometer is off by, say, 10%, it will read 33 mph at 30, but 66 mph at 60. If you reposition the needle to read correctly at 60, the gauge will read 27 mph when you're going 30.

Basically, what you're recommending will not fix the issue the OP described and, frankly, is ridiculous.

ZV

If OP talking in regards to his Honda civic which is the same model year I have, the issue is that @ 58mph according to the OBD, it's reading 60mph on the needle. This behavior of it reading faster than reality is the norm on these vehicles and you can change if it it bothers you like it bothered me.

 

[Smoove910]

do you have lower profile tires than what originally came with the car? This can cause a change in your speedo...

Just a thought...

 

[EightySix Four]

BMW is notorious for this, it's really annoying. As far as I can tell it is accurate up until around 35-40mph and then slowly the error percentage increases. It's not a fixed amount off at all.

 

[tortillasoup]

do you have lower profile tires than what originally came with the car? This can cause a change in your speedo...

Just a thought...

He's complaining about the discrepancy from what the computer is reading and what the gauge cluster says. What the computer reads is actually pretty accurate according to the GPS but what the cluster reads is off by a few mph.

 

[Meghan54]

BMW is notorious for this, it's really annoying. As far as I can tell it is accurate up until around 35-40mph and then slowly the error percentage increases. It's not a fixed amount off at all.

Here's an interesting read from over a decade ago done by Car and Driver about speedo accuracy. C&D uses what they describe as their "highly precise Datron optical fifth-wheel equipment" to assess speed during their vehicle testing and they test hundreds of vehicles.

http://www.caranddriver.com/features/speedometer-scandal

Some tidbits from the article:

Sorted by price, luxury cars are the least accurate, and cars costing less than $20,000 are the most accurate. By category, sports cars indicate higher speeds than sedans or trucks. Cars built in Europe exaggerate more than Japanese cars, which in turn fib more than North American ones. And by manufacturer, GM's domestic products are the most accurate, and BMW's are the least accurate by far. One other trend: Only 13 of our 200 test speedos registered below true 70 mph, and only three of those were below 69 mph, while 90 vehicles indicated higher than 71 mph.

So we sought out the rule book to find out just how much accuracy is mandated. In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph.

But wait, there's more. Driving in arctic or desert climates? You're allowed another plus-or-minus two percent near the extremes of 20-to-130-degrees Fahrenheit, and yet another plus-or-minus one percent if the gauge was ever exposed to minus-40 to plus-185 F. Alternator acting up? Take another plus-or-minus one percent if the operating voltage strays two volts above or below the normal rating. Tire error is excluded from the above, and odometer accuracy is more tightly controlled to plus-or-minus four percent of actual mileage.


The European regulation, ECE-R 39, is more concise, stating essentially that the speed indicated must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour (79.5 mph at a true 70). Never low. Not even if somebody swaps a big set of 285/35R-18s for stock 255/45R-16s.


There's your explanation of high-reading European speedometers, with the highest readings on Porsches and BMWs that are most likely to lure owners inclined to fool with tire sizes. Of course, only the speedometer must conform. Trip computers are free to report average speed honestly. Try setting your BMW or Porsche cruise control and then resetting the average-speed function. Unless you've screwed up the tires, the trip computer should show a nearly accurate reading. Even General Motors, whose domestic speedometers are the best, must skew its readings slightly high on vehicles exported to Europe.