Speedometer Observation

[Jeremy556]

I noticed today, while passing one of those radar speed limit signs, that it displayed 1-2mph less than my speedo (at 65), which is exactly the same as it displayed before changing from 3.73 to 4.56 gearing. I then confirmed the same with GPS speed measurement.

This was interesting to me, as most vehicles use a speed sensor mounted to the transmission (or countershaft sprocket) and require recalibration when changing final drive gearing. The A750E/F transmissions have a speed sensor, which would be representing 75.9 mph based on stock size tires and 3.73 gear ratio.

When new (stock), I noticed the speedo read about 5-8% higher than actual GPS measured speed, while odometer mileage was accurate with GPS mileage. This seems to be some kind of industry standard, as I have compared a couple Toyota cars, Honda cars, and Honda motorcycles, they all display about 8% above actual speed on speedo, but are mileage correct on the odo.

Another observation: the ODB2 speed readings exactly match the odometer, and are lower than what is displayed on the speedometer.

 

[rigtec]

Hi [MENTION=3743]Jeremy[/MENTION]

Did you upgrade your tires to a larger size? Larger will cause the speedometer to measure a (nominal) lower number while a smaller tire will cause a higher reading; likely 3-4 mph, 'depending on how much of an increase of tire size, and at what speed you're traveling'.

Cheers bro!

 

[NoDak]

most factory ones are 1.6 mph (1km) higher than actual due to liability (lawyers).

unless its certified (cop cars) you wont find one showing actual speed.

No speedometer can be 100 percent accurate. In fact, most manufacturers build speedometers so they fall within a fairly narrow tolerance range, no more than 1 percent to 5 percent too slow or too fast. As long as a car is maintained at factory specs, its speedometer should continue to register vehicle speed within this range. But, if a car is modified, its speedometer may need to be recalibrated

But, generally, most speedometers exaggerate speeds, says Wall Street Journal columnist Jonathan Welsh. “Industry officials have said speedometers are set ‘optimistically’ to give drivers an inflated sense of speed (and help them avoid tickets).” Welsh says. “GPS devices record distance over time without the effects of mechanical variables and thus tend to be more accurate.”

The difference between actual speed and the speed dial tends to get noticed more with German cars because their speedometers are designed to never report a speed lower than actual speed. European law (ECE-R39) says speedometers cannot show speeds less than the actual speed, and they must never show more than the 110 per cent of actual speed plus 4 km/h. So, under those rules, a car could be moving at 100 km/h, but the speedometer could legally display as high as 114 km/h.

 

[JET4]

most factory ones are 1.6 mph (1km) higher than actual due to liability (lawyers).

This is true.I changed the factory size tires to new A/T tires size 275/70/17 and then I GPS the truck to see how much the speeometer was off. The speedometer was right according to the GPS. I checked it two or three times to make sure it was right, and it was.

 

[Zeder]

Hey Jeremy,

I have noticed this error as well, and I am guessing its in the instrument cluster for the from the following observation:

I have a ScanGauge, Nuvi GPS, Ipad and a Galaxy S4 on my dash when travelling/working. All of them display the same speed while the factory speedometer reads higher than the rest of the electronics. When all the electronics display exactly 100km/h the speedometer reads 106-107 km/h.

I have stock size DuraTracs 265/70/17 and initially assumed the error was created from the different rubber but it can't be with the ScanGauge matching external GPS values (3 different sources).

I keep forgetting contact the dealer about this. I just never go there. Ha, which is a good thing! 2 years, +60000 km like new, going strong!

 

[03_4x4Runner]

My 14 Limited reads dead on on both the digital and analog speedometer according to 2 different Garmin gps. 4runner is completely stock.

 

[BlackWorksInc]

I have a SpeedoHealer coming on in the next day or two so I can correct the 7% difference from upsizing to the 285/70R17's from the factory 265/65R17's.

 

[cleveland48]

Mine with stock tires reads 1mph faster than the Garmin GPS