Forum Mates: I'm planning to get a P85D in Anchorage, AK. I nearly have my P85 sold, and I'm excited about the AWD and ridiculous performance. The 21" tires are a concern, especially in the winter, so I'm going to get the 19" winter tires with rims for $2,500. Will the vehicle know which size wheels I have on the car at any given time so the speedometer and odometer reflect the correct speed and total mileage? Is there a setting to adjust accordingly? Thanks! Josh

I don't remember that when I reset my TPMS sensors. Why does the car need to know? The wheel radius is pretty much identical, so no recalibration of the speedo or air suspension is required.

I put on 20s today and it asked me that question. I put in 21 for the heck of it. With the diameter of the tire being relatively the same it should not matter .. except maybe the 21" call for a bit more air pressure. David

Josh, the diameter of the tires on the 21" wheels vs the 19" wheels are virtually identical. As such, switching from one to another should have virtually no effect on speed or milage display.

It really doesn't matter what the hell you do regarding wheel size w/ the S It takes it a few trips or maybe 30mi to completely forget the last setting and get on to the new one.(during the few trips the car seems to 'figure it out' after a little bit) not sure what the front does regarding SC but for the rear end: If you reduce the diameter of rears - the car will think the wheels are spinning (due to difference between f/r maybe?) and kick on TC limiting ~20kW power. drop hard regen and throttle a few times or N or P then back to D, or turn off TC and this problem goes away. note it will happen a few times if you are doing short trips, hence the 30mi(I think, TPMS sensors don't trip for about that long) If you increase the diameter of the rears- Regen is really weird. it will think you are loosing traction when you regen full. It will pull then drop off quickly until you are 'floating' at 0 regen(potentially dangerous if you don't expect it, not like you have no regen. it comes on strong then fades fast!). I think The TC light comes on too for this. Same case, few trips/30mi

TL;DR: Switching from OEM 19" to 21" wheel & tire set means you will actually be traveling at 65.17mph when your speedometer shows 65mph, and will be actually covering 100.3 miles for every 100 miles logged on your odometer. For anyone interested in more detail: The tire size for the 21" rims is 245/35-21 and the tire size for the 19" rims is 245/45-19. 245 indicates the width of the tire track in millimeters. The '35' and '45' numbers refer to the sidewall aspect ratio. '35' means that the height of the tire sidewall is 35% of the width of the tire track. For the 21" wheel the tire sidewall height is (245mm x 0.35) = 85.75mm. For the 19" wheel the tire sidewall height is (245mm x 0.45) = 110.25mm. To compute the diameter of the tire, we add the diameter of the wheel + 2x the height of the sidewall. This means that the diameter of the 21" tire is (21" x (25.4mm/1") + 2(85.75mm)) = 533.4mm + 171.5mm = 704.9mm. The diameter of the 19" tire is (19" x (25.4mm/1") + 2(110.25mm)) = (482.6mm + 220.5mm) = 703.1mm. This calculation yields a diameter difference of 1.8mm between the two tires (tire for 21" wheel is 1.8mm larger). To compute the actual speed vs displayed speed when switching between tires of different size, divide the diameter of new tire by the diameter of old tire (this is your conversion factor), then multiply by the target speed. So if your speedometer is calibrated for the 19" wheel & tire set at 65mph, switching to the 21" wheel & tire set means you are actually traveling at (65mph(704.9mm/703.1mm)) = (65mph * 1.003) = 65.17mph when your speedometer reads 65mph. To compute the actual distance traveled vs displayed milage, multiply your conversion factor by target milage. So for every 100 miles you log on your odometer on a car calibrated for 19" wheels that is actually equipped with 21" wheels, you've actually traveled (100mi * 1.003) = 100.3 mi.

Up in Alaska you might want to consider better winter tires than the Tesla set. Something like the Nokian R2. You can buy just the rims from Tesla.

Your entire point is moot because the tire diameters are so close together that when the tire treads wear the differences between the new worn diameters easily suppress that of the original 1mm difference.

So, ability to reset TMPS showed up last winter, probably because the SVC got tired of people coming to have their TPMS systems reset when they switch to winter tires (the 19s and 21s have different tire pressures). If you want the app, the display etc to show the proper wheel on your car, it still requires a trip to the SVC. O

Um yeah... the whole point is that the difference between the two is negligible, thereby answering OPs question

Thank you, all, for your thoughtful and detailed replies. I have the Nokian Hakkapeliitta 7s, and they performed beautifully last winter. My next question is: Will the 19" rims plus the TPMS sensors be less than $2,500, which is the cost of the winter tire/rim set purchased directly from Tesla?

Congrats, Josh! Of course, the AWD is perfect for Anchorage so of course you should trade up I just got mine one month ago, and I am happy with 19" - if I got a D it would probably be the straight 85D though. Besides, it doesn't snow very often in Hong Kong, like, once every 10 years on a deserted mountain top.

You are most welcome. Forget the "Not updated yet apparently" comment because it appears all four of those things listed are just "recommended".

That was an AWESOME explanation! I knew the answer from years of being a geek, but never could have explained to so well. Thank you!