Speedometer Calibration

[GT4GT]

I own a 2020 CX-9 with stock 20" tires. On a recent road trip, I noticed the speedometer was off by 2 mph when driving above 60 mph. In other words, when the speedometer read 70 mph, I was really going 72 mph. I based my actual speed on Waze and a few speed detection signs I passed.

Is this considered within tolerance? Perhaps, I've been lucky but every vehicle I've owned for the last 30 years has been spot on.

 

[sm1ke]

I don't think I would be confident in relying on an app to display my current speed accurately - I would consider it more of an approximation.

IME, the speed detection signs I've come across very in accuracy. Some are dead on, some report my speed as too slow, some too fast. Most of them just don't work at all though

All that said, if you feel your speedometer is off, swing by the dealer and ask them to recalibrate it for you. I don't know if there is a cost involved, but I'm pretty sure they can assist.

 

[richabrams6@]

I have a 2019 and noticed the exact same thing with my speedo. It's consistently off and I've checked it against 3 different apps that provide gps speed and they all show the speedo is off. However in my car, the speedo reads high, so it says I'm going 65 but I'm going 63 per the gps apps.

 

[youri]

Speed detection signs, altough in theory capable of being precise, don’t really work that well because their reading is over a very very short period of time

GPS signal is not precise enough to provide accurate speed values, no matter which app, so i wouldn’t rely on those too much.

That being said, check your tire pressure. Under or overinflated tire will change the speed reading. Tire wear will also affect it. You can check out this article: https://www.google.ca/amp/s/newyorkspeedingfines.com/tread-wear-tire-pressure-affects-speed/amp/

All that being said, please do mention it to your dealer in case it is a malfunction, but i think 2 miles per hour at 70mph is within acceptable norm.

 

[richabrams6@]

Turns out my recall of the readings was backwards! When the speedo says 65, the multiple gps apps indicate 67. Same as the OP. 3% error seems way high. My tires have 5K miles and are inflated exactly to the Mazda specs. GPS speed measurements are usually more accurate than speedos, assuming clear reception.

Here's what the US government says:

"The government provides the GPS signal in space with a global average user range rate error (URRE) of ≤0.006 m/sec".

I find the error to be consistent under many different driving conditions including wide open roads, so the reception is good. Since I see the error with different gps apps, I believe the speedo is off.

 

[youri]

the US government says:
"The government provides the GPS signal in space with a global average user range rate error (URRE) of ≤0.006 m/sec".

Thats the accuracy of the signal rate from the satellite to the receiver, not the measured travelling speed by the GPS device.

Regardless, let us know what the dealer says, i am curious to see what the result will be and if there are cases of poorly calibrated speedometer.

 

[singlemalt_18]

Thats the accuracy of the signal rate from the satellite to the receiver, not the measured travelling speed by the GPS device.

Yes, that accuracy rate is the error rate of all the moving parts ie the satellite & the planet, adjusted for the time delay of data transmission. Under optimal conditions, precise location can be calculated to about 4 inches for highest resolution devices. Calculating speed is an entirely different thing.

 

[richabrams6@]

I checked the Waze speed against my highly accurate sophisticated GPS system on my boat, which tracks 13+ satellites. I found the speed matched at speeds up to 35 mph. Just another data point.

 

[jefferson88]

I doubt a dealer would calibrate a speedometer (for free) for a 2mph difference. Many dealers would consider this within tolerance, especially because the diameter of the tire changes as the tread wears. In [the OP's] case, as the tires get smaller the speedometer will become more accurate (the tires will need to rotate more as one complete revolution will cover a shorter distance than they do now).

It makes sense that each vehicle model leaves the factory with a calibration set to a general tire circumference. But not every model leaves the factory with the same "stock" tire because these cars can be equipped with different rim sizes (based on trim level) and ship to different regions with different climates, meaning different tire tread patterns.

I know Jeep dealers charge around $125 for speedo recalibration for those folks who modify their Wranglers with big offroad tires. Call your dealer and see what they can do, just don't expect it to be free because it's not an error on their part. Also keep in mind the speedometer will be incorrect again in a few thousand miles when your tires wear down.

 

[richabrams6@]

Not to beat a dead horse, but for the 20in OEM tires, if they wear out complete, the revs per mile from new would only change 1% (8mm change on a 763mm diameter).

 

[jal142]

The rules for accuracy vary around the world, but a 3% error in the US is not in violation of any law or regulation I am aware of. Other countries (in Europe) would take exception because your speedometer is underreporting and that is against the rules there.

GPS speeds are actually pretty good, within a few tenths of a mile per hour assuming you have a clear view of the sky and a good receiver.

If you really want to be certain, go find a flat stretch of interstate, set the cruise control at 60mph, and use the mile markers to see if you are really going a mile a minute.

 

[Mazdiod2]

A Scangauge or Ultragauge is able to display MPH as interpreted by the computer.

I check this number against my dash speedo as well as my Garmin GPS.

Fortunately on my cx5 they all match evenly as well as I can determine but the numbers are usually moving as it's difficult to maintain an exactly constant speed.

Other cars I've owned haven't been as accurate and were often off by 2-3 mph at highway speeds. Some have been closer at 30 mph but not all.

I've had the notion that the GPS reading is accurate unless you're traveling on a significant grade, which would alter the reading because of the vertical component in your speed, which wouldn't be included.

 

[GT4GT]

OP here. I took my 2020 CX-9 into the dealer yesterday for the power liftgate recall and mentioned the speedometer issue. Keep in mind this service advisor is one of the least helpful service advisors I have every dealt with, but he said there was no way for them to test or calibrate the speedometer.

I have owned many cars in my lifetime including a 26 year old one that I still own and a 2008 Mazda CX-9 GT. I check their speedometers against Waze, Speed Monitoring signs, and standalone GPS devices. I am always amazed at how accurate every other car's speedometer has been. My 2020 Mazda CX-9 is the first car with this issue. So either this this car's speedometer is off or every car I previously owned has been off.

It's an annoyance but "within tolerance". It just means that when I'm doing 75 MPH, the CHP will clock me at 78 MPH. Probably better for my driving record if it were the other way around.

Some CX-9s have 18" wheels vs 20" wheels. If my speedometer was calibrated for an 18" wheel, but I have 20" wheels, could that cause the speedometer to be off?

 

[sm1ke]

Some CX-9s have 18" wheels vs 20" wheels. If my speedometer was calibrated for an 18" wheel, but I have 20" wheels, could that cause the speedometer to be off?

OEM 18" wheel and tire setup is 18x8 +45, with a 255/60R18 tire. The OEM 20" wheel and tire setup is 20x8.5 +45, with a 255/50R20 tire. Both setups result in almost the exact same total wheel diameter (763.2 mm vs 763 mm), so there should be no specific calibration for OEM 18s or OEM 20s.

 

[GT4GT]

OEM 18" wheel and tire setup is 18x8 +45, with a 255/60R18 tire. The OEM 20" wheel and tire setup is 20x8.5 +45, with a 255/50R20 tire. Both setups result in almost the exact same total wheel diameter (763.2 mm vs 763 mm), so there should be no specific calibration for OEM 18s or OEM 20s.

Thanks.

 

[Odrapnew]

I just have to chuckle. You should see my motorcycle speedo error.
At ground speed of 70mph, my speedo reads about 80mph.
This is a bone stock 06 GSXR750. Very common to be off 10-12% straight from the factory.
For the longest time, I thought everyone was driving extremely fast.
It does make me wonder if my speedo is off by that much, is my odometer clocking 10% more miles than actually traveled? Both are digital, so I would expect if one is off, so is the other.

 

[Duke Yugo]

I have been looking for an answer for months for my 2016 CX-9 GT with the same speedometer problem where it would show 73 when in fact the vehicle is doing 75 MPH. I was hoping that this could be resolved at the Mazda service but now reading this thread my hopes are gone.

 

[sm1ke]

I have been looking for an answer for months for my 2016 CX-9 GT with the same speedometer problem where it would show 73 when in fact the vehicle is doing 75 MPH. I was hoping that this could be resolved at the Mazda service but now reading this thread my hopes are gone.

Have you checked your tire pressures and tires for wear/damage? If so, check with your dealership anyway. The OP mentioned that his particular dealer wasn't very helpful, but that doesn't mean that yours will be just as unhelpful. It doesn't hurt to try.

 

[Duke Yugo]

Mystery solved!
Last year I changed tires on my CX-9 and for some unknown reason, I ordered 255/55R20 to be installed.
OEM tires are 255/50R20. In the size 55, the speedometer reading is about 3% off showing slower speeds. That is exactly the "issue" I had all along. I don't know why I made the mistake I did, but others should also check on the size of their tires if they have any speed reading problems.
I can't believe it took me 6 months to figure this one out.

 

[Corvetteboy]

Duke, I am intentional think of installing 255/55R20's on my 2018 CX-9 Sig.
How do they compare to the original size (other than the speedo being off), any issue with rubbing in the front wheel wells when turning, how is the ride and handling?
Can you share what specific tire (brand and model)?
Thanks