Speedometer Accuracy

The EPA tests are run on dynos, so the car's speedo error is irrelevant.

These speedo offsets have been standard industry practice since before EPA ratings existed. But with analog meters now gone, the typical error spread is very sharply reduced

Posted via the PriusChat mobile app.

 

Yeah I guess no one got too stressed about accuracy, looking at this:

 

Believe any conspiracy theory you want.
I believe the Toyota Legal Department controls it.

 

When they do the testing on the dynamometer, the rollers on the device are indicating/controlling the speed, and not the speedometer in the car. This eliminates inaccuracy between vehicles.

 

On an analog meter, 2-3% error is unnoticeable. Heck, after driving Prius so song, I don't know how fast I am going on this hybrid car anymore.

 

Up here in Canada (posted speeds in kilometers per hour), all the Budget rental trucks are equipped with with mph-only speedos. I believe there's a little decal on the dash, showing conversions. Good fun.

 

IIRC, Gen3 switch from mph to kmh was a single button very accessible right on the dash near the instrumental panel. I think I have to go through the menu to change it on the PRIME (and Gen4)???

 

I thought that would be illegal by now.
Back when the country first converted you could get stickers to put behind the analog meters showing both values. I did that when my car at the time had km/h in blue letters with blue dash lighting to make them invisible. The sticker had those in a bright yellow.

 

Yup, one dedicated button, up high on the dash just to the right of the steering wheel.

Yeah I was thinking along similar lines. Most speedos have one system in large text around the perimeter, and smaller number further in. But as far as I can remember I had to resort to guessing. I remember thinking "there's gotta be kmh on this speedo too", then noticing the decal. I'm always as stressed as a cat in a room full of rocking chairs to begin with, driving a big truck, so...

 

Telematics tracking systems on cars default to using GPS for speed, they only use the speedo if they cant get a good GPS signal. GPS is accurate as long as you're driving in a straight line for a decent amount of time.

 

quick google search found several interesting articles

Vehicle Speedometer Accuracy | How Fast Are We Really Going | OEC

Speedometer Scandal!

and one of my favorite youtubers had this (warning: adult language)

 

"It's my guess they do it this way so they meet both US and UK regulations with one standard."

Based on various numbers previously posted here, it seems they don't necessarily use the same offsets in the US and UK. Many UK and other Euro members have reported higher differences than reported by North American members.

 

On any car equipped with a GPS/Nav system, why don't they just display the actual speed on the NAV system? I check my GPS on my phone to set my speed accurately when using cruise control.

 

I believe that’s so; in the various editions of the Repair Manual (more info), there are at least three different versions of the table showing the intended error: the U.S. one, which I posted before, and two versions for Europe, one of which is also used in Japan. For an actual speed of 120 km/h, for example, these say the acceptable range of displays is 122 to 124, 123 to 125, or 125 to 127 km/h, respectively.

I believe the speedometer regulations, in countries outside the U.S. where they exist, would apply to any speed display, including one shown by a navigation system.

On aircraft, by the way, this is exactly what’s done: the primary flight display shows indicated airspeed from the pitot-static system, and the navigation display shows groundspeed from GPS or inertial navigation.

 

Airspeed is critical on an airplane, since this controls flight parameters. Speed over ground is less important. For my car, speed over ground is the only thing I care about.

 

When I drove on a small street, the Radar Speed Control Sign reading is 28 while 30 is showing on my Prime.

 

Good to see this got discussed a lot. I’ve noticed a 5.5-6% difference in speed with my prime. It’s constant between my all season and winter tires as well. When compared to GPS, when showing 100kph, gps bounces from 94-95kph. On radar signs showing a posted speed limit of 50km/h, it shows me doing 47. I haven’t compared distance though.

With my truck, I use an app and obd unit to change the factory tires to adjust the Speedo. The truck was off by about 10-11% when the garage installed my winter tires (tires were smaller diameter, thus why the extreme difference).