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Tire sizing question for full size spare for MY using M3 19" wheel

I have spare set of Model 3 19" Sport wheels and I want to make a full size spare for the MY. the specs on the M3 wheel are 19 x 8.5 wide, offset 40mm. Considering the wheel size for the MY are 9.5" wide, I don't want to assume the OEM 19" MY Tire size would be the same for the 8.5" width M3 rim. I need a tire sizing guru to tell me what 19" tire size I should use on the 8.5" wide wheel to get closest to the overall diameter of the MY OEM tires. According to Modern Spare, the MY OEM tire diameter is 27.7". Thanks for the input.
 

jcanoe

Active Member
Oct 2, 2020
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4,503
Maryland
The M3 OE 19" tire size is 235/40R-19, Extra Load (XL), Load Index 96 (1565 lbs maximum per tire), Speed Rating W (168 MPH)

The MY OE 19" tire size is 255/45R-19, XL, 104 (1984 lbs maximum per tire) Speed rating Y (186 MPH)

The M3 OE 19" tire Load Index rating is 419 lbs per tire below under the Load Index for the Model Y with 19" wheels, i.e. not safe. (You can't just multiply the Load Index number by 4 and compare this with the Gross Vehicle Weight Rating for the vehicle. The Load Index specified by the vehicle manufacturer takes into the maximum dynamic load when starting, stopping, cornering, changing lanes abruptly and towing.)

According to Tire Size the diameter of the MY OE 19" (tire size 255/45R-19) is 28" and the diameter of the M3 OE 19" tire (size 235/40R-19) is 26.4". If instead you use a tire on the M3 wheel with dimensions 235/45R19 the diameter is 27.3" (2.5% smaller than the MY OE tire.)

One possible tire would be the Vredestein Quatrac Pro in size 235/45R-19, XL, with a Load Index of 99 (1709 lbs maximum load per tire) and a Speed Rating of W. The Load Index is still incorrect for the Model Y but not as grossly mismatched as the Load Index rating the M3 OE 19" tire for use on the Model Y.

Another possible tire would be the Michelin CrossClimate2 size 235/45R-19, XL, 99, V (149 MPH)

If you use a tire with a width of 245mm instead of 235mm; i.e. size 245/45R-19 there are multiple tires available with a Load Index of 102 (1874 lbs maximum load per tire), Speed Rating of V or Y. The diameter is 27.7" which is what you stated Modern Spare quotes for the MY spare. (A Load Index of 102 is in the range that Tesla specifies for the Model Y tires (101 for the 20" wheels, 103 (rear tires) for the 21" wheels; and 104 for the 19" wheels.

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35
 
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The M3 OE 19" tire size is 235/40R-19, Extra Load (XL), Load Index 96 (1565 lbs maximum per tire), Speed Rating W (168 MPH)

The MY OE 19" tire size is 255/45R-19, XL, 104 (1984 lbs maximum per tire) Speed rating Y (186 MPH)

The M3 OE 19" tire Load Index rating is 419 lbs per tire below under the Load Index for the Model Y with 19" wheels, i.e. not safe. (You can't just multiply the Load Index number by 4 and compare this with the Gross Vehicle Weight Rating for the vehicle. The Load Index specified by the vehicle manufacturer takes into the maximum dynamic load when starting, stopping, cornering, changing lanes abruptly and towing.)

According to Tire Size the diameter of the MY OE 19" (tire size 255/45R-19) is 28" and the diameter of the M3 OE 19" tire (size 235/40R-19) is 26.4". If instead you use a tire on the M3 wheel with dimensions 235/45R19 the diameter is 27.3" (2.5% smaller than the MY OE tire.)

One possible tire would be the Vredestein Quatrac Pro in size 235/45R-19, XL, with a Load Index of 99 (1709 lbs maximum load per tire) and a Speed Rating of W. The Load Index is still incorrect for the Model Y but not as grossly mismatched as the Load Index rating the M3 OE 19" tire for use on the Model Y.

Another possible tire would be the Michelin CrossClimate2 size 235/45R-19, XL, 99, V (149 MPH)

If you use a tire with a width of 245mm instead of 235mm; i.e. size 245/45R-19 there are multiple tires available with a Load Index of 102 (1874 lbs maximum load per tire), Speed Rating of V or Y. The diameter is 27.7" which is what you stated Modern Spare quotes for the MY spare. (A Load Index of 102 is in the range that Tesla specifies for the Model Y tires (101 for the 20" wheels, 103 (rear tires) for the 21" wheels; and 104 for the 19" wheels.

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35
thanks for the reply.
 

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