[MTCD Android Head Units General] doitright:
Who said anything about ECU? I said SOFTWARE and CAR'S COMPUTER.

Cars have multiple computers, you know...

Could be the software running on a microcontroller that is built into the gauge cluster.

This is what you need to know; *regardless* of where the sensor is or how many sensors there are, if it is a SENSOR, it is using digital logic (math) to CALCULATE the speed. If it has an old fashioned mechanical cable, then its obviously AMERICAN. Though honestly, I would be surprised if even a mid-late 90's dorF had a mechanical speedo cable.

Funny thing about the accuracy rules. I *think* I recall reading that the speedo reading had to be within +/- 10%. Most cars INTENTIONALLY target a speed reading that is about 5% above reality (make the driver think they're going fast, reduce chances of speeding tickets, etc.). Typically, the ODO will still be dead on. OFTEN, you will even see a difference between the gauge and the OBD readout!

About that subaru... apparently you don't know much about tire sizing.

"Subaru Impreza line up for 2004(first year they offered the full line up in the us) has a variance of 10% (205, 215, 225) this may be an out of the norm but its a car line I know best."

You see those numbers? Those are WIDTHS, in millimeters.

So those numbers... 225/45R17 (WRX STI)

205/55R16 or 215/45R17 (WRX sedan and wagon)

So first off, I wouldn't consider the STI to even BE the same car as sedan or wagon. Its more of a suicide car.

The sedan and wagon have two available sizes. Lets break them down.

First number: width, in millimeters.

Second number: sidewall height, in PERCENT of the width.

Third number: rim diameter, in INCHES. Yeah, I know. Mixing metric and imperial measurement. Pretty wacky, right?

205/55R16:

The tire's diameter is 2*(0.55*205)mm + 16 inches. That's 8.88+16 inches = 24.88, and *pi=78.16 inch

215/45R17:

2*(0.45*215)mm + 17 inches. That's 7.62+17 inches = 24.62, and *pi=77.34 inch

So, you can see, that they're actually pretty close together. They only vary by 0.8 inches, which is about 1%.

But there is more. You see, the 215 is a slightly wider tire, so when the vehicle is sitting on it, it will "sit in" a tiny bit less than the 205, bringing them to about ZERO difference.

Note that even without the difference between the two tires as the car sits on them, tread depth (and how the tires wear down over the tire's life) will account for FAR more than 1%.

The last one, the widowmaker edition.... 225/45R17

Yes, this one is a slightly larger tire.

2*(0.45*225)mm + 17 inches. That's 7.97+17 inches = 24.97, and *pi=78.44 inch.

78.44-78.16=0.28 inch, /78.16=0.36%.

Yeah, slightly larger. By LESS THAN HALF OF ONE PERCENT :eek:

Total spread is about 1.4%.

Maximum difference from the mean, 1.0%.

So AS I SAID, most of the time, cars come equipped with ONE size of tire. Those that have different tires are typically where you have choice between 4x4 and 4x2. Example: Toyota Tacoma < 2016 (in 2016, they did away with the smaller size option).

4x4 (and prerunner): 245/75R16 (and two other marginally different options along the same lines as subaru wrx)

4x2: 215/70R15

245/75R16: 2*(0.75*245)mm + 16 inch = 14.47+16 inch = 30.47, *pi=95.72

215/70R15: 2*(0.7*215)mm + 15 inch = 11.85 + 15 inch = 26.85, *pi=84.35

(95.72-84.35)/84.35=13.5% (how much bigger the big one is compared to the small one)