Speedometer Error on Purpose?

[TallPaul]

I get the impression that a typical car or light truck speedometer will read a higher speed than actual. Seems I have to go about 72 mph to be going 70. This was crudely checked by mile markers over a fair distance, knowing the markers are sometimes not right on. Anyway, the question:

Assuming it's true that OEM speedometers read slightly higher than actual speed, why? I can think of two reasons:

1. Help consumer avoid speeding tickets. Seems like a weak reason and why would the automaker care? Insurance company pushed for it--much more likely.

2. Increase corporate average fuel economy (CAFE) numbers for reduced fines. Now this seems very likely, unless EPA already precluded such shenanigans.

 

[crazyoildude]

do you own a Honda?
Also some gps units are extremely accurate

 

[d00df00d]

Maybe they know there will be error no matter what they do, so they intentionally skew it high to be safe.

 

[ThirdeYe]

My Acura is like that. I have to go about 73-74 to do 70 as verified by my GPS and those little police radar speed trap machines. Also I should note that the tires I'm using are slightly smaller than OEM because they don't seem to carry the OEM size anywhere around here.

 

[Quattro Pete]

Originally Posted By: TallPaul

1. Help consumer avoid speeding tickets. Seems like a weak reason and why would the automaker care? Insurance company pushed for it--much more likely.

Yup. It's so you can't possibly sue the auto maker for getting a speeding ticket, or for losing control of the vehicle because you were going faster than actual.

In EU, there is actually a directive that mandates that a speedo must read between "0 and 10% + 4km/h" above the actual speed.

http://en.wikipedia.org/wiki/Speedometer#Error

 

[Shannow]

Yep, there will always be an error, even a reasonable difference between new and old tyres.

So they make sure that there's no (or some tiny percentage, in line with statistical analysis) that read under your true speed.

 

[silverrat]

When It shows 140kmh on my dial it shows 128kmh on the gps.

 

[Virtuoso]

I dunno.

Every cop that's given me a ticket has been pretty spot on!

 

[Big_Ed]

My brother asked his mechanic about this and he said the manufacturers purposely make the speedometers off by 5-10%.

I'm not sure it's true, but many of the cars I've owned seemed to be off by a few mph. In order to actually go 55mph, I had to drive so my speedometer read about 58. I checked it with my GPS, and it seems to be true. Anyone else compare their speedometer with the speed listed on a GPS?

 

[crazyoildude]

i heard there was a class action lawsuit against that car company because their speedometers were a little on the fast side therefor piling up more mileage and warrantees were expiring quicker and more money was made when people went over their mileage on leases..Also it made the appearance that the cars got better MPG'S
Look it up i think it was 2002-2006 models but i could be wrong
On the other hand speedometers are not 100% accurate unless they are calibrated and certified like on most police cars and most of the time it says that on the speedometer

 

[XS650]

2005 VW Jetta reads 4 mph fast at 70 mph
2006 Acura TL reads 2 mph fast at 70 mph
2006 Corvette it dead nuts right on at 70 mph.

Odometers in all three cars are very close to correct. Much closer then their speedometers.

I have a Scangauge 2 in my Jetta that reads right on for speed, so the car OBD-2 computer knows how fast the car is going but the speedometer lies a bit.

 

[dwendt44]

By law/regulation speedometers aren't allowed to run slow; show a slower speed than actual.
So to be safe, speedos are usually fast.
A very few are dead on from the factory.

 

[Shaman]

really off on motorcycles, understandably so.

 

[97f150]

My Dodge is 2 mph above actual speed. My F150 is 1.

 

[chevrofreak]

The speedometer and odometer in my moms 2002 Outback lie. I calibrated the Scangauge II using a GPS, and it shows that her speedometer and odometer both read about 3% high.

My 96 Crown Victoria Police Interceptor is off slightly at lower speeds, but above 35mph is dead accurate.

My 86 Honda VFR700F is off by about 8%

 

[crinkles]

when i compare the needle to GPS, no appreciable difference.

when i drove across australia, 4,500 km and tracked everything on GPS, I determined a 1.8% difference between Odo and GPS mileage. pretty good.

 

[Colt45ws]

My Cavalier is dead on.
I have larger tires on my Crown Vic than factory, 255/45ZR18 instead of 225/60R16. So it reads about 3 low at 70.

 

[mpvue]

no conspiracy. its a mechanical thing, inaccuracy from car to car is to be expected. those work zone speed signs are usually pretty close to my speedo, and on my recent road trip, my phone GPS was showing the same as well.
tire wear and inflation will skew the numbers as well.

 

[JHZR2]

Ive actually found that domestic cars are more accurate than foreign ones. My old MBs tend to be off a bit, though less than 10%, but they are purely mechanical.

My 91 BMW is within 1MPH

 

[ARCOgraphite]

Originally Posted By: crazyoildude
do you own a Honda?
Also some gps units are extremely accurate
You beat me to the Honda thing. A great way for a manufacturer to boost its fuel milage! Then again, I would assume the feds test dont utilise the car speedo in its CAFE test though, so it would only be a customer perceived false gain. Sneeky!