Are speedometers always off by percentages?

Sukhoi

Elite Member
Dec 5, 1999
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After several conviently placed police radar trailers the last few days, I have determined when my speedometer says I'm doing 45 MPH I'm actually doing 39-41 MPH. Is it really possible that my speedometer would be 5 MPH fast at all speeds? Or is it just 11% fast? Thus if it said I'm doing 65 MPH in reality I would be doing 58 MPH.
 
Apr 5, 2000
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Speedos can be come uncalibrated for various reasons. My father's car is 5-7mph off based off those trailers (I can definitely feel that it's going slower however)
 

Mallow

Diamond Member
Jul 25, 2001
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I have more faith in my car's speedometer than that damn speed trailer.
 

Sukhoi

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Dec 5, 1999
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Originally posted by: Angrymarshmello
Speedos can be come uncalibrated for various reasons. My father's car is 5-7mph off based off those trailers (I can definitely feel that it's going slower however)

Right, but is there any known problem that would cause a speedometer to always be off by a constant 5 MPH? Or are speedometers always off by percentages because of the way they work?

The reason I'm asking is because this becomes a big difference at highway speeds. If my speedometer says I'm doing 80 MPH and it's 5 MPH fast then I'm actually doing 75 MPH. But if it says I'm doing 80 MPH and it's 11% fast then I'm actually doing 71 MPH. Quite a difference when you get around cops that like to give tickets for 10 MPH over in a 65 MPH zone.
 

notfred

Lifer
Feb 12, 2001
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It's off by a percentage, not a set amount (unless it shows 5mph when you're not moving, then maybe the needle got bent or something).

My car is within one percent of accurate, based on mile markers on the freeway. Drive 10 miles, measured by mile marker, and also measure your odometer readings at the start and end. It should be 10.0 miles. if it's 10.1 miles, you're off by about 1%.
 

Sukhoi

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Dec 5, 1999
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Originally posted by: Mallow
I have more faith in my car's speedometer than that damn speed trailer.

Unless the cities rent the trailers to each other then I've done it on two different trailers in the last week. Besides, over Christmas break my dad was driving behind me and said I was doing 45 MPH in a 45 MPH zone when I know my speedometer said I was doing 50 MPH.
 

Sukhoi

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Dec 5, 1999
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Originally posted by: notfred
It's off by a percentage, not a set amount (unless it shows 5mph when you're not moving, then maybe the needle got bent or something).

My car is within one percent of accurate, based on mile markers on the freeway. Drive 10 miles, measured by mile marker, and also measure your odometer readings at the start and end. It should be 10.0 miles. if it's 10.1 miles, you're off by about 1%.

No, it shows zero when I'm not moving. As far as I know the needle is in fine shape.

That's a good idea on the odometer. I'll check that next time I'm on the freeway.
 

notfred

Lifer
Feb 12, 2001
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Originally posted by: Sukhoi
No, it shows zero when I'm not moving.

Then it's obviously not always off by 5mph :)

Mile markers tend to be pretty accurate in most places. The longer the test you do, the more accuracy you'll get.
 

T3C

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Jun 3, 2003
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Originally posted by: Sukhoi
Originally posted by: notfred
It's off by a percentage, not a set amount (unless it shows 5mph when you're not moving, then maybe the needle got bent or something).

My car is within one percent of accurate, based on mile markers on the freeway. Drive 10 miles, measured by mile marker, and also measure your odometer readings at the start and end. It should be 10.0 miles. if it's 10.1 miles, you're off by about 1%.

No, it shows zero when I'm not moving. As far as I know the needle is in fine shape.

That's a good idea on the odometer. I'll check that next time I'm on the freeway.

 

C'DaleRider

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Jan 13, 2000
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From Car and Driver, April 2002:


Speedometer Scandal!
Can you trust your most frequently consulted gauge?


Regular readers have probably noticed that when we describe a vehicle that really gets our juices flowing, we tend to hyperbolize about the accuracy and precision with which the steering wheel and pedals communicate exactly what is happening down where the rubber meets the road. It has recently come to our attention, however, that many of the cars we like best are surprisingly inaccurate about reporting the velocity with which the road is passing beneath the tires. Or, to put it another way, speedometers lie.

Yes, ladies and germs, we are scooping 20/20 and 60 Minutes with this scandal: Speedometers Lie! Okay, "exaggerate" may state it more aptly, if less provocatively.
When traveling at a true 70 mph, as indicated by our highly precise Datron optical fifth-wheel equipment, the average speedometer (based on more than 200 road-tested vehicles) reads 71.37 mph. Wait, wait! Before you roll your eyes and turn the page, let us dig just a bit deeper and reveal some dirt.

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. Are our cars trying to keep us out of traffic court?
To understand, let's first study the speedometer. In the good old days, plastic gears in the transmission spun a cable that turned a magnet, which imparted a rotational force to a metal cup attached to the needle. A return spring countered this force. Worn gears, kinked or improperly lubed cables, tired springs, vibrations, and countless other variables could affect these mechanical units.

But today, nearly all speedometers are controlled electronically. Typically, they are driven by either the vehicle's wheel-speed sensors or, more commonly, by a "variable reluctance magnetic sensor" reading the speed of the passing teeth on a gear in the transmission. The sine-wave signal generated is converted to speed by a computer, and a stepper motor moves the needle with digital accuracy.
Variations in tire size and inflation levels are the sources of error these days. Normal wear and underinflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading. From full tread depth to baldness, speeds can vary by up to about two percent, or 1.4 mph at 70 mph. Lowering tire pressure 5 psi, or carrying a heavy load on the drive axle, can result in about half that difference. Overinflation or oversize tires slow down the speedometer. All our speed measurements were made on cars with new stock tires correctly inflated, but one might expect a manufacturer to account for wear and to bias the speed a bit low; results suggest that not to be the case.

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.

So there you have it: the raw, unvarnished truth about speedometers, laid bare without the underhanded aid of secret pyrotechnics. Readjust your comfortable indicated cruising speeds accordingly.


A link to comprehensive average speedometer deviaton, by testing done at Car and Driver, by manufacturer, car class (luxo, sports, etc.), continent..........HERE.
 

frostgiant

Senior member
Jun 19, 2000
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My speedometer is very accurate as I have verified it with a GPS.

There are a number of reasons accuracy can change, though. If you have the wrong size of tires, it will be off quite a bit.
 

C'DaleRider

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Jan 13, 2000
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Originally posted by: Electric Amish
It's an analog system. It could easily by out ± 2-5 mph

Very few, if any, cars built in the last 10 years are analog any longer........all electronic.
But today, nearly all speedometers are controlled electronically. Typically, they are driven by either the vehicle's wheel-speed sensors or, more commonly, by a "variable reluctance magnetic sensor" reading the speed of the passing teeth on a gear in the transmission. The sine-wave signal generated is converted to speed by a computer, and a stepper motor moves the needle with digital accuracy.
 

Electric Amish

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Oct 11, 1999
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Originally posted by: C'DaleRider
Originally posted by: Electric Amish
It's an analog system. It could easily by out ± 2-5 mph

Very few, if any, cars built in the last 10 years are analog any longer........all electronic.
But today, nearly all speedometers are controlled electronically. Typically, they are driven by either the vehicle's wheel-speed sensors or, more commonly, by a "variable reluctance magnetic sensor" reading the speed of the passing teeth on a gear in the transmission. The sine-wave signal generated is converted to speed by a computer, and a stepper motor moves the needle with digital accuracy.

Interesting.... but the dial numbers are still analog. ;)
 

PG

Diamond Member
Oct 25, 1999
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Notfred's idea is a good one and you can time yourself too. Keep is simple, 60 mph should be 60 seconds for every mile.

 

Eli

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Oct 9, 1999
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I imagine eventually all cars will come with GPS speedometers.. heh.
 

C'DaleRider

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Jan 13, 2000
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Originally posted by: propellerhead
All electronic does not mean not analog.

Thank you.........only an idiot nitpicker would say "But the numbers are analog"..........well, DUH! Surprise, we only read analog.......don't suppose you know anyone who can read pure digital information with their eyes, eh?
 

Demon-Xanth

Lifer
Feb 15, 2000
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Ways the speedometer can be off:
The mechanism can have an offset (ie: you're always 5MPH high)
The mechanism can have a scaling problem (ie: you're always 10% off)
The mechanism can have a linearity problem (ie: 2MPH off at 20, 5MPH off at 40, 2MPH off at 60)

Most likely are scaling errors, and given that your tires' circumference will differ based on air pressure, wear, and 100 other factors, it's unlikely your speedometer is ever within 0.1% accuracy.
 

Koing

Elite Member <br> Super Moderator<br> Health and F
Oct 11, 2000
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Originally posted by: C'DaleRider
Originally posted by: propellerhead
All electronic does not mean not analog.

Thank you.........only an idiot nitpicker would say "But the numbers are analog"..........well, DUH! Surprise, we only read analog.......don't suppose you know anyone who can read pure digital information with their eyes, eh?

Ditto with C'rider. No one reads bloody digital fluent!

 

desertdweller

Senior member
Jan 6, 2001
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Tire size will affect how far off your spedo is.

You might make sure your tires are the same size as
came from the factory.


DD
 

Sukhoi

Elite Member
Dec 5, 1999
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Originally posted by: Demon-Xanth
Ways the speedometer can be off:
The mechanism can have an offset (ie: you're always 5MPH high)
The mechanism can have a scaling problem (ie: you're always 10% off)
The mechanism can have a linearity problem (ie: 2MPH off at 20, 5MPH off at 40, 2MPH off at 60)

Most likely are scaling errors, and given that your tires' circumference will differ based on air pressure, wear, and 100 other factors, it's unlikely your speedometer is ever within 0.1% accuracy.

Until I get a chance to check it out at highway speeds I'm just going to hope that it's a scaling problem. :)

Tire size will affect how far off your spedo is.

You might make sure your tires are the same size as
came from the factory.

I'm pretty sure they're original size, but not positive. My dad put on the last set of tires and he's usually pretty careful to get the correct size.
 

biffbacon

Golden Member
Feb 22, 2003
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my bmw's speedo is off by like 5 mph. I always thought it was the radar signs things that were wrong until i got a gps, and when my car says im going 80, im really only doing like 75