My attempt to re-calibrate the speedometer

Here in EU we have Regulation No 39 and we have those rules:

The production shall be deemed to conform to this Regulation if the following relationship between the speed
indicated on the display of the speedometer (V1) and the actual speed (V2) is observed:
in the case of vehicles of categories M and N:
0 ≤ (V1 – V2) ≤ 0,1 V2 + 6 km/h;
in the case of vehicles of categories L3, L4 and L5:
0 ≤ (V1 – V2) ≤ 0,1 V2 + 8 km/h;
in the case of vehicles of categories L1 and L2:
0 ≤ (V1 – V2) ≤ 0,1 V2 + 4 km/h.

Long story short it's by design.

 

To make the car so it may under report speed could be illegal??

this seems a Byzantine solution, to a questionable “problem”. If it bugs you, just speed a little?

 

i wouldn’t trust gps as an accurate measurement of speed. Find the nearest “your speed” electronic sign like a school and you’ll get a much more accurate speed measurement.

 

Complete accuracy is never possible. Example:

I once detailed a steel railing (in a public concourse) requiring rail spacing to not exceed 4”. I dutifully spec’d 4” spacing. General contractor called to explain: inspector will show up with close tolerance machined 4” dia ball, and if he can push it through the rail, anywhere, he will fail it.

that was on my conscience for years lol. I think they just let it slide, or we got lucky with the tolerances.

just food for though lol. But seriously, the mandate is ultimately, they don’t want people to be going FASTER than they think they’re going. Ever.

 

someone once told me on here “off topic” even though it was related to the exact idea.

 

If I go 50 km/h in my '15 Prius, the electronic speed signs will say 48-49 km/h.

It's pretty much across ALL cars manufacturers to be slightly off. My '05 Cooper S doesn't show the same speed as the same electronic sign either. Honestly, I don't know why you're willing to mess with this. As someone else said, it's by design.

If you mess with the speed sensor, it will probably code and unless you can mess with the ecu coding to fix it on the other end, that's way too much work for a 1-2 km/h difference. What tells you that your GPS isn't off?

 

ALL speedometers from ALL manufacturers are designed to read slightly (1-2 mph) higher the actual speed. Just another area where the lawyers want to minimize liability.

 

My 2002 Town&Country Speedometer is 1mph slower.

 

In order to have the speedometer read 100% accurate, the diameter of the tire tread has to match the original design. When the post said that you think it's the right tire, that hints that you did not check this important parameter.

As everyone knows, the tire diameter gets smaller as it wears. That means that the distance traveled per revolution will be less with tires that are close to replacing. It's against the law in most states to fiddle around with that number.

When you buy tires you can look for the tread diameter and try to find one that matches your car's design. If you have 2 vettes and a stang that are all 100% accurate you are quite lucky, especially considering the propensity for adding odd sized aftermarket tires to muscle cars.

 

As I’ve mentioned before, there haven’t been government accuracy requirements for passenger car speedometers in the U.S. since 1982. The speedometer adjustment is a commercial decision by Toyota.

A method of adjusting the speedometer reading that also causes the odometer to register less distance than actually driven would violate the federal and state criminal laws on odometer tampering, 49 U.S.C. § 32703 and (for example) La. R.S. 32:726.1.

The more practical problem, on third-generation Prius cars, is that the speed pulse signal present at the combination meter’s SI input terminal isn’t used by the speedometer; it’s just distributed to the other ECUs connected to the +S output terminal. As the Repair Manual (more info) states: “This signal is not used for combination meter assembly [ . . . ] operation. Combination meter assembly components such as the speedometer operate using data received via CAN communication,” from the skid control ECU, which needs accurate wheel speed readings for ABS/VSC/TRAC to work properly.

 

The high readings are not mandated by "the government". It (or rather, some jurisdictions falling under that broad label) and separate liability issues only address low reading. If the speedometer read dead-nuts-on, there is no fuss.

The high readings are driven by private corporations trying to build in some margin against getting themselves into troubles. Normal industry practices were established back in the old days when analog meters had significant accuracy problems. Today's digital measuring systems have narrower error bands, which has allowed the industry to shift to narrowed margins, but the common practice of building in some margin, persists.

Try rechecking your calculation.

Many tires start with about 10/32"nds of tread (some have even more), and remain legal down to 1/32" in a few states. While most set a limit of 2/32", certain dry southern states allow thinner.

That means the tire can lose 9/32"nds of radius, or 18/32" of diameter, or more than 1.4cm of diameter, not the 0.5 cm of your claim.

Note: Actual change of effective rolling circumference is a bit more complicated than this, and smaller than one would expect purely from changes of radius. Remember that the contact patch as flat, not circular, so the normal circle equations don't work out right.

 

My 195-65-15 tires are reportedly 831 revs/mile, so 31 cm (effective) radius, including 11/32" of original tread. So wear off 9/32" of that, or .71 cm, we're down to 30.29 cm. My 60 mph speed would be inflated to 61.48 at that point.

Still sounds like something I could live with.

 

Go bigger on the tires, 205/70R15? Went from 215/50R17 to 225/50R17 per @Airboss confirmation of no rubbing on our v and am much happier...

moto g(7) power ?

 

It's not really a myth if it's true, and your math says that it is true.

BTW, it's the circumference that matters, not the diameter. I'm too tired
to do the math.