[limeypride]

Hey guys,

The F90 allows us to configure what tires the car's running (it's under tire settings on the screen where you view tire pressure). The options include 19", 20", something to do with summer/winter and special authorization. Presumably, it uses this information for various things that I suppose includes calculating the car's speed. With the stock tires (Michelin PS4S 285/35/20 rear + 275/35/20 front), the car's speedo read roughly +1mph when compared side-by-side to my Dragy and/or my phone's GPS speed app.

The tirewalls of the stock tires are aesthetically too tall for my taste so I've replaced them with PS4S 295/30/20 rear + 285/30/20 front--aesthetic problem solved. But if I contrast the car's speedo now against my Dragy, the car understandably (see math below) reads +4mph higher. That led me to wonder, what tire setting should I be using now... and for that matter, when should I use "special" and is that something the dealer can precisely define.

Here's how the math breaks down:

Stock
Rear tires:" 285/35/20 // rolling circumference = 87.506"
Front tires: 275/35/20 // rolling circumference = 86.642"

Rolling circumference delta = 0.864"

New tires
Rear tires:" 295/30/20 // rolling circumference = 84.726"
Front tires: 285/30/20 // rolling circumference = 83.981"

Rolling circumference delta = 0.745"

… so

1. it would seem I've improved on the rolling circumference front-to-rear delta over stock
2. in the process, I've skewed the car's perception of speed

Does anyone know how to mitigate the niggling incorrect speed issue?

PS: if anyone's interested in my stock tires, let me know. I'm in and around the Redmond/Kirkland/Bellevue/Seattle/Woodinville/Everett/Snohomish areas.

[Nikolov]

The diameter of your new tires is smaller (by 22,5mm), which means they spin faster, hence the higher reading on the speedo. According to willtheyfit.com that's a 3.28% speedo difference, compared to the stock sizes. For example the 19" stock tires are only 4,9mm smaller than the stock 20" tires, which is a 0.7% difference.

I don't think the tyre settings in iDrive is related to anything else but the tire pressure. Also all cars show higher than actual speed, which is due to regulation. According to a quick Interwebs search the US regulation states that the speedo must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph). However the UK regulation states that the speedo cannot read slower than your current speed (I'm guessing it's most EU regulations), hence why all manufacturers add a 2-3% on top of the actual speed.

You can have the car coded to display the true GPS speed, I think that's the only fix if you have different tire sizes.