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Briefly

I'm reaching the conclusion that the different trim levels offered for the same model of the same car brand do not have the same rolling diameter (and hence their Revolutions Per Mile—or rpm, the other rpm—is different). This is wild to find out, because it means that unless calibration is done from one trim to the next (and that's highly unlikely), the speedometer will be giving just a rough estimate of the speed.

Confirming that different car trims have different rolling diameters

Extended Version

Let's look at this through an example. One model of one brand has multiple trim levels (like most). (I selected a particularly gigantic car to get numbers that are spread further.)

One of the differences between these trims are wheels of a different size. Instead of 265/50R20 for the base trim, the highest trim gets 275/40R22 wheels.

Calculating the rolling diameter should be pretty elementary, and so we need not go to a "tire size calculator" web site to do this.

265/50R20 diameter = 2 * 265 * 0.50 + 20 * 25.4 = 773.0 mm

275/40R22 diameter = 2 * 275 * 0.40 + 22 * 25.4 = 778.8 mm

This means that the higher trim has 5.5 mm larger rolling diameter than the lower one.

My real objective is the fourth question below, but feel free to comment on any—including simply the one in the subject line (the one "official" question).

  1. Which of the two is taken into account by the speedometer? Is it just an average, and the speed indicated is anyway just a rough (+/-2%) measure?
  2. Do (modern) cars have any way of determining the wheels installed? (The pressure is, after all, transmitted wirelessly.)
  3. Is this entered at some stage of calibration and before delivery?
  4. Does this mean that the end user who selects rims and wheels of a different size needs to go back to the dealership to recalibrate the speedometer?

Related

Chronologically:

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    Speedometer calibration on the assembly line is specific to the vehicle as manufactured including trim level and tire options. See this post for more info: mechanics.stackexchange.com/a/93418/44030
    – MTA
    Commented Jan 31 at 17:30
  • There are also different overall final drive gear ratios for one vehicle type. For example there might be 4 variants from 2 engine sizes and 2 final drive ratios to suit different tastes. It's not always obvious: you have to dive into the specs. Commented Jan 31 at 20:08

1 Answer 1

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Which of the two is taken into account by the speedometer? Is it just an average, and the speed indicated is anyway just a rough (+/-2%) measure?

It really depends on how the manufacturers/engineers set things up. Some vehicles use the wheel speed sensors to get an overall speed of the vehicle. Others use the speed of the output shaft on the transmission (or transfer case if it has one). This input is used to calculate the speed of the vehicle by the ECU, and is then fed to the speedometer so you can view your speed. The speedometer itself is just a dumb piece of equipment which reports what the ECU is telling it to.

So, is the speed indicated a rough measure? Not really. It is strictly an output of a mathematical equation taking factors into account. It takes the speed of its input and calculates what is going to be presented on the speedo. Some manufacturers may set that indication a little faster than is actual and some may set it a little lower. Others may have it exactly right. Different manufacturers have their own reasons for doing what they do.

Do (modern) cars have any way of determining the wheels installed? (The pressure is, after all, transmitted wirelessly.)

No. They do not. The best they can do is understand how many rotations there are over a period of time. This would be done by the tone ring used by the ABS system. This is just the rotations, but it doesn't directly know what the size of the tire is by that measurement.

Is this entered at some stage of calibration and before delivery?

Absolutely. The ECU has a table which tells it an X input speed equates to a Y MPH, then tells the speedo what that number is to present it to the user. To calibrate for different tire sizes, you just have to adjust the constant in the table to make the speedo depict the correct speed.

A much easier way for manufactures to deal with this considering different wheel/tire combos for different trim levels is to ensure the rolling distance of both tire sizes are the same or very nearly the same. In the States, tire manufacturers call this measurement RPM or rotations per mile. It's a parameter you can find on most tire resellers sites online.

NOTE: Per your calculations, you have the diameter, but what you're really looking for is the rolling distance of the tire, which is where the acronym RPM comes into play here (not to be confused with other RPM or rotations per minute).

Does this mean that the end user who selects rims and wheels of a different size needs to go back to the dealership to recalibrate the speedometer?

Getting the speedo calibrated can be done in a lot of places, the last of which I'd take my car to is the dealership (AKA: steelership). IOW: You don't have to take it to the dealership to get this done. I would bet a lot of tire shops could get it done for you.

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