Speedometer incorrect and Cruise Control Wrong

I think it's more than just whim too: there's legislation, UN accords on this?

Belabouring a dead equine, but: I find it hard to get lathered up about this: if your speedometer says you're going 80, and you're actually going 78, this is a big problem?

 

OCD.

 

Yes, there is. I looked it up a while ago but couldn't find it recently, but there is definitely at least one UN or international agreement related to speedometer accuracy - and it errs on the conservative side (e.g., if the car is actually in motion at 55 MPH the speedometer is going to read high, say...57 MPH.)

 

I think He is dyslexic.

 

Note that the UN document linked here repeatedly in the past, is mostly an EU market standard with a fair number of Asian markets added in. None of the covered countries are in the (North, Central, South) Americas.

 

I have finally found the US CFR that covers speedometers. As I expected, it doesn't solve the mystery of why all speedometers measure your speed as higher than actual. Speculation will have to rule the day.

49 CFR 393.82 - Speedometer. | US Law | LII / Legal Information Institute

"§ 393.82 Speedometer.
Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph).

[ 70 FR 48054, Aug. 15, 2005]"

 

Plus or minus, that's interesting, depending how strict the speeding laws are, you could be breaking the law without knowing.

 

And I suspect, though cannot prove, that is why car OEMs have the speedos set up to report a couple of mph faster than actual speed.

 

The expression "each bus, truck, and truck-tractor" is puzzling. Also puzzling, the statement that speedometers can be inaccurate by up to 10%, in either direction. This runs contrary to every thing I've heard/read previously, and just seems irresponsible. Personally, I would much rather have a vehicle overestimate than underestimate speed.

 

That's not what Click and Clack say:

Blog Post | How much does tread wear affect the speedometer reading? | Car Talk

Posted via the PriusChat mobile app.

 

They go off the rails a bit, with this bit (highlighted in yellow:



That typical new 25" diameter tire likely has tread depth around 10/32", and when down to the wear bars (legal mininum) has tread depth 2/32", which is an 8/32" difference, which is 1/4", not 1/2". Doing the math with that difference, their 4% becomes 2%:



Also, how likely is the down-to-the-wear-bar scenario. Personally, 4/32's is the lowest I would go, more likely 5/32". And that measurement is likely taken at the inside tread edge, which tends to wear faster than outer edge, due to negative rear camber. Add all these factors, and the 2% dwindles to maybe 1%.

 

At the risk of prolonging an already-tedious thread, the section @bbald123 kindly cited comes from the Federal Motor Carrier Safety Regulations. See 49 CFR § 393.1, which explains that the rules in 49 CFR Part 393 apply only to commercial motor vehicles, as defined in 49 CFR § 390.5 by gross weight, number of passengers, or use to transport hazardous materials in quantity. Those regulations don’t apply to private passenger cars, for which there has been no Federal speedometer standard since the former FMVSS No. 127 was rescinded in 1982.

There are still mandatory standards in Japan, however. Article 46 of the Safety Standards for Road Transport Vehicles (in Japanese) and its Attachment 88, Technical Standards for Speedometers (PDF in Japanese), requires that, at each required test speed:

0 ≦ V₁ − V₂ ≦ V₂/10 + 4​

where V₁ is the indicated speed and V₂ is the actual speed, both in km/h. English translations of these documents are available in the Automobile Type Approval Handbook for Japanese Certification, published by JASIC.

I’ll let members in other countries comment on their own national standards, such as Regulation No. 39 (click the 39 tab on that page for links) promulgated by the United Nations Economic Commission for Europe under the 1958 Agreement, to which I note that neither the United States nor Canada is a signatory.

 

That column betrays nothing to indicate that they thought about this topic more than a minute, nor ever compared it to actual on-the-road results. Such as, shouldn't I be seeing a noticeable shift in my car's speedometer (compared to GPS) and odometer (compared to GPS and roadside mileposts and fixed routes repeated many years) readings as my tires wear down?

And how does this fit in with the class action lawsuits about cars with odometers that read too high, shortchanging drivers on warranty coverage? If tire wear was a significant contributing factor, it should have been mentioned.

Why did my attempts to match up engine RPM, manual transmission ratios, final driver ratio, car speed, and tire size, produce a quite noticeably incorrect answer when I used the basic circle equations from the tire size specs? Calculations came much closer to reality when I abandoned those circle equations, and instead used the tire RPM (revolutions per mile) spec.

Based on my experience, and on past discussions about this with others here on PriusChat who have looked at similar things, I must dismiss this particular answer from Tom & Ray, until more formal detail is produced.

That is because you live in a wetter climate, where tread depth is important. Many other folks live in drier climates down south -- it seems that California requires only 1/32" of tread -- and a significant chunk of drivers are cheapskates when it comes to tires. So there are a fair number of folks who actually do run down that low.

 

That, and with our low mileage, our tires are "aging out", before wearing out, lol.

 

Excellent clarification! Thanks!

 

"0 ≦ V₁ − V₂ ≦ V₂/10 + 4"

This bit says, in a very cryptic/obtuse way, that your actual speed should be less than or equal to the displayed speed.

"0 ≦ V₁ − V₂ ≦ V₂/10 + 4"

This bit, I think, is an even more cryptic/obtuse way of setting a limit on how high the speedometer can read, compared to actual speed? But my brain is full, lol.

 

I put over 50,000 miles on my OEM tires with no noticeable change in indicated vs actual speed with using my GPS as a standard. My car indicates between 1 and 2 mph faster than I am really driving.

I just replaced the tires with the same and observed no change.

 

My 1–2 mph offset is exactly the same as yours, even after 20 000 miles (32 000 km).

 

I have an additional encouragement to remain at or lower than the posted speed limit. This vehicle has Road Sign Assist which reads and displays speed limit, yield, stop, and do-not-enter signs on the dashboard and in the HUD. If I set the cruise for 65 mph it will hold speed at 64 mph on the level and going downhill, and will usually accelerate that 1 mph to 65 when going uphill. The speed is displayed in whole mph but I think internally there is a tenths column. At 66 mph or more the little speed limit sign on dashboard and HUD will blink and a yellow border will appear around the sign. Sometimes it does this with the mph still reading 65, maybe internally 65.6. I can't confirm but I think at 10 mph over the posted speed limit the display will blink continuously and produce an audible beep.