Speedometer vs GPS

Early this spring I meet my wife in her home town 2 hours away from our home. She had driven her Mitsubishi Eclipse and I had the Tacoma. We both headed back home at the same time on the highway. I was following her and speaking to each other on our phones. She was checking out the adaptive cruise control system. She set her cruise for 120 km/h slightly over the posted limit of 110 km/h. I decided to do the same having the similar radar sensor for adaptive cruise control. My cruise control kept sensing her vehicle and slowing down to her speed. I backed off by slowing down and allowing her to lead far in the distance. I engaged the cruise at the same speed she was set for 120 km/h, we both have digital speedometer numbers to set exactly 120. Tacoma cruise control worked as expected and I did catch up with her again and started to slow to her speed. I like to call it tractor beam mode. We spoke again and I confirmed her set speed and decided to lead with my cruise on. As expected she slowly fell away into the distance. Both vehicles are new 2 months apart, she has a 2019 and mine is a 2020. One of our new vehicles was not accurate.
Our next highway trip was in her car and I brought the GPS to check her car speed. Here is a picture of of the GPS, HUD, analog and digital speedometer.

The 5 green leafs appear one at a time as you drive economically to the manufacturers standards.

Today we had the first oil change done and I wanted the speedometer issue addressed.

Besides having a safty recall for an ECU software update and oil change the dealer was aware of the speedometer issue I described and had the following printed document at the service counter.



I confirmed with the service desk that the safety recall would not address the speedometer issue and that I would need to tell my wife that no chage was made and the difference still exists.

Her vehicle has andoid auto but no navigation like the Tacoma where you can input tire size changes and create a future speedometer adjustment. I believe I go to settings within the Tacoma's navigation menu to make changes. This I have not done yet and I will get to do soon as I have new tires and wheels to install. My new KO2 tires are 265/70R17 from stock 265/70R16, so a 3% change difference.

If you have a Mitsubishi this probally the reason everyone is passing you because your not going the speed you think your are.

The tire calculator could be used to determine approximately how flat or over inflated a tire could be to make that much change in speed. Tire size is P225/55R18 and P215/70R16 winters were on at time of GPS picture. The car does have 2 TPMS tire ID selections available when changing tire sets but has no variable tire size input.

The final line says it is not a defect and safety design practice.

 

Wait wut? This is news to me unless its a new for '20 thing. Either way most vehicles' speedometers are not perfectly accurate and will vary a couple mph/kph from GPS. Doesn't seem to me to be a big deal, especially because you are highly unlikely to get rolled for doing 2 mph over the limit.

 

When I go 120 on my Taco, my GPS says I'm going 117. It's pretty normal to have a slight variance from vehicle to vehicle.

 

Maybe the percentage is higher than expected. Odometers are very accurate and speedometers lie apparently.

 

Wait until he finds out alignment specs change when you sit in the vehicle.

 

From "Car & Driver"

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