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What is the tallest sidewall tire that will work with the stock 18” wheel

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The rims are 19” x 8.5”.
Using willtheyfit.com, comparing to stock wheels, they should be similar in all dimensions, but 10mm closer to the car / strut.

If you have the non performance brakes, the stock offset is 40 mm not 35. The data that you're getting from the website probably assumes that you're talking about the performance brake car. The rotor hat on that is significantly thinner, moving the wheels in board almost 5 mm. So these wheels would actually move the wheel and tire assembly inboard 5 mm, if you have the non performance brakes again. You should be able to accommodate that, but it's not ideal. If on the other hand you you have the performance brakes, then you are talking about 10 mm closer to the strut which is cutting it close. It's still will probably fit but it may look a little funny with the wheels tucked in a full centimeter.
 
I am trying to keep the stock sport wheel geometry but with 245mm tyres.
Because: 1) More grip. 2) Protects the 8.5” rims a bit more. 3) I now own them.

My workings:
1. Stock Sport 19” x 8.5” with 235/40-19 +40 offset.
2. New wheels are 19” x 8.5” with 245/40-19 +45 offset.
3. The new rims will be 5mm in board compared to the Stock Sport.
4. The new tyre, outside, will be stock (-5mm + 10mm/2 = 0mm)
5. The new tyre, inside, will be 10mm inboard
=> Hopefully this will still clear the front strut & control arms when on hard lock...
 
I plan to install Continental Purecontat LS 245/50/18 on my Tesla M3 after my Primacy MXM4 wears out in 2-3 months.

Should I not do that size? I want extra cushioning for a quiet comfortable ride.



I am trying to keep the stock sport wheel geometry but with 245mm tyres.
Because: 1) More grip. 2) Protects the 8.5” rims a bit more. 3) I now own them.

My workings:
1. Stock Sport 19” x 8.5” with 235/40-19 +40 offset.
2. New wheels are 19” x 8.5” with 245/40-19 +45 offset.
3. The new rims will be 5mm in board compared to the Stock Sport.
4. The new tyre, outside, will be stock (-5mm + 10mm/2 = 0mm)
5. The new tyre, inside, will be 10mm inboard
=> Hopefully this will still clear the front strut & control arms when on hard lock...
 
Sorry to revive a old thread but my friend's LR dual motor came with 255/45-18 sized tires. I thought stock was 235/45-18.

You can get away with that and it is a possible way to upsize tires, but that's about the max rolling diameter and I suspect you've got less than 3-4 mm clearance with front steering knuckle. An additional upside of course is better protection of the rim, and your Speedo error is minimal because the speedometer is about 1% optimistic anyway.
 
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What has that been measured with?

Just my impression from the two cars that we have it actually isn't a constant percentage but it seems to be about a mile to a mile and a half per hour at any reasonable speed. That's from a GPS speed estimate like you would get on draggy. Haven't gone to a hundred to measure it but it's about a mile and a half an hour at highway speeds. So the 2% increase in Rolling diameter at least isn't added to an already overly conservative Speedo. In other words that just about balances out the optimism in the speedometer.
 
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Just my impression from the two cars that we have it actually isn't a constant percentage but it seems to be about a mile to a mile and a half per hour at any reasonable speed. That's from a GPS speed estimate like you would get on draggy. Haven't gone to a hundred to measure it but it's about a mile and a half an hour at highway speeds. So the 2% increase in Rolling diameter at least isn't added to an already overly conservative Speedo. In other words that just about balances out the optimism in the speedometer.
I was wondering because i’ve done 2 miles measured & marked @ 60mph & it was dead on 120 sec. That’s not with timing lights but even hand stopwatch will easily reveal 1% off.
 
I was wondering because i’ve done 2 miles measured & marked @ 60mph & it was dead on 120 sec. That’s not with timing lights but even hand stopwatch will easily reveal 1% off.

It's possible that our two cars are just outliers then. Or perhaps your car's the outlier and the optimistic speedometers are more common. Who knows. Only way to test that's really accurate is with a GPS system. All I know is that our cars are optimistic.
 
Only way to test that's really accurate is with a GPS system.
Accurate fixed distance with manual stopwatch time is comparable accuracy to dynamic consumer grade GPS measurement systems. Both can easily measure to sub-1% accuracy over distances of at least 1 mile (a few miles is better). Basically the GPS system is doing the same thing. The difference is the GPS's wiggle room is about +/-5m on distance, with amazing accuracy on time, where as the fixed distance wiggle room is on the time side. At 60mph 1/10th of a second, which is typical accuracy of using a stopwatch, is just under 3m distance. Basically it amounts to the same thing, although the GPS is more automated if the system is set up for it.
 
Tire wear and lower pressure would make speedometer show higher speed than actual. Every OEM sets speedometer to maximum pressure of largest OEM tires to avoid legal claims.

and to comply with federal regulations. They add a bit of margin too. Odometer readings have different regs as you can’t over report miles traveled.
 
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