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Ok to use a Model 3 Aero 18" wheel as a spare tire for MY LR AWD with 19" gemini?

yellowr6

Member
Sep 13, 2020
27
17
Bay Area, ca
hi guys

Planning a road trip so would be nice to have a spare tire with me. I have a MY LR AWD with 19" Gemini wheelset. I did some digging and some members here use the Model 3 Aero 18" wheel on their MY LR AWD (not performance model) with no issues.

So I end up picking up a used M3 18" aero single wheel for a backup spare tire. Got it for cheap from Ebay.


I install the single 18" on the rear passenger side today, just to test it out. Make sure everything is good go go before my trip. Fit nicely and no issues. I drove 5 miles on the street and another 15 miles on the freeway. While driving, I noticed I can't use AP. It would engage and disengage within a few second. Screen will tell you to adjust your speed and all kind of warning will pop on so I stop trying. I also noticed RE-GEN not working properly. As in, car don't want to slow down. I had to use my brake more often. I can feel the car having a little issues with braking.

Once I got home and park. I can smell the 18" wheel burning up a little. Once you touch the wheel, rotor, lug nuts, all of them are pretty "HOT". My other 19" is nice and cool.


What do you guys think it is? safe to drive? maybe for a few miles?

my guess, maybe set the RE-GEN to low instead of normal. I have no clue why the 18" wheel is so HOT. Feel liked that wheel is trying so hard to RE-GEN and slow down. Working a little extra for braking....



TIA. Really appreciate if someone can explain to me whats going on before my road trip next month. THX!
 

avs007

Member
May 14, 2021
382
276
PacNW
The M3 Aero wheel tire's circumference is 3% smaller than the OEM wheels on the MY. That means the M3 wheel will spin faster than the opposite wheel on the same axle. This can wreak havoc with the traction control system, as the Tesla will probably try to apply the brakes on the faster spinning wheel, to send torque to the opposite wheel on the axle. Your wheel is probably hot, because of this. This is why when you replace tires, you normally replace tires on both sides of the axle at the same time.
 

mba83

Member
Jan 10, 2019
111
47
South Carolina
Brakes are only applied when the vehicle is loosing control, which won't happen during normal driving. I think as a spare this wheel is better than nothing.
 

avs007

Member
May 14, 2021
382
276
PacNW
Brakes are only applied when the vehicle is loosing control, which won't happen during normal driving. I think as a spare this wheel is better than nothing.
That's not true. You are thinking stability control... I was talking traction control. The tesla has an open differential between each wheel on each axle. If one wheel is spinning faster than the other (particularly if it's spinning faster than normal, because the wheel is SMALLER than the other, the ECU can think it's slipping, and apply slight pressure to the wheel spinning faster. This is exactly how you can an open diff to behave like a mechanical limited slip one. But yes, in NORMAL situations this won't happen... But if you have a wheel that is smaller than normal on only one side of the axle, you can mess up the algorithms the ECU is using.
 
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avs007

Member
May 14, 2021
382
276
PacNW
You can do it but you'd be wise to change the tire on the 18" to match the diameter of the tires on the 19" wheels.
Exactly... Your closest bet for a decent price tire is 235/55-18. That is pretty close.... An exact match would be 255/50-18, but TireRack shows only one tire in that size, and it's > $500 each.
 

avs007

Member
May 14, 2021
382
276
PacNW
Also, if you do plan on using the 18" wheel with the original M3 tires, keep your speeds below 50mph. If you read up on temporary spare tires, you'll see one of the reasons you are supposed to limit driving to only a repair facility and to keep speeds < 50mph , is that the temporary spare will damage your differential with increased speeds/distances. Damage to your differential (or gearbox in Tesla's case), is NOT covered under warranty if you have mismatched tires on your axle.
 

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