[newbiebmwx5]

The factory size is 275/45/20, would it be ok to get 20 inch rims
From tire rack and put on 275 40 20 winter tires on it for my second set? Thanks!

[mmm50i]

Yes, of course. Try to pick a set of wheels with the same or slightly lower offset.

For reference, my winter wheels are BMW’s 748M wheels, 20x9” +35m.

[Orient330iNYC]

Originally Posted by newbiebmwx5

The factory size is 275/45/20, would it be ok to get 20 inch rims
From tire rack and put on 275 40 20 winter tires on it for my second set? Thanks!

it wont break anything as long as its the same all around, but why would you go down in size?
-less sidewall, so less protection against road hazard damage
-your odometer will add on more mileage than you actually travel ( ~4%) (so you'll tack on an extra 4 miles that you didnt travel for every 100 miles)
- your speedometer will read even higher than before over actual speed ( ~5-6mph faster than reality vs 3-4mph)

[newbiebmwx5]

Originally Posted by Orient330iNYC

it wont break anything as long as its the same all around, but why would you go down in size?
-less sidewall, so less protection against road hazard damage
-your odometer will add on more mileage than you actually travel ( ~4%) (so you'll tack on an extra 4 miles that you didnt travel for every 100 miles)
- your speedometer will read even higher than before over actual speed ( ~5-6mph faster than reality vs 3-4mph)

I didn’t know that’s why I was asking. This would happen even if it’s on its own rims and not on the stock rims that came with car?

[Orient330iNYC]

Originally Posted by newbiebmwx5

I didn’t know that’s why I was asking. This would happen even if it’s on its own rims and not on the stock rims that came with car?

yeah, its the tire size that matters.
40 aspect ratio tires have shorter sidewalls, which makes the diameter smaller
roughly:
275 X 0.4 = 110mm
275 X 0.45 =123.75mm

diameter of the full assembly = 2 x sidewall height + rim diameter
rim is 508mm (20")

220 (sidewall height) + 508 = 728mm for 275/40/20
247.5 (sidewall height) + 508 = 755.5mm for 275/45/20

the smaller the diameter, the more revs per mile, so you will end up with a speedometer and odometer going farther and faster than you are really going.

[MADBMWX5MC]

One of my favorite sites for tire/size comparisons: https://tiresize.com/comparison/

[newbiebmwx5]

Originally Posted by Orient330iNYC

yeah, its the tire size that matters.
40 aspect ratio tires have shorter sidewalls, which makes the diameter smaller
roughly:
275 X 0.4 = 110mm
275 X 0.45 =123.75mm

diameter of the full assembly = 2 x sidewall height + rim diameter
rim is 508mm (20")

220 (sidewall height) + 508 = 728mm for 275/40/20
247.5 (sidewall height) + 508 = 755.5mm for 275/45/20

the smaller the diameter, the more revs per mile, so you will end up with a speedometer and odometer going farther and faster than you are really going.

Will the same issue happen if I go smaller size like the 19 inch oem size on rims or 18inch ?

MINUS WHEEL DIAMETER: 18" Smaller Wheel Diameter Than Factory Size Learn More


18" Recommended
255/60-18
FACTORY SIZE WHEEL DIAMETER: 19"


19" Recommended
265/50-19

19" Alternate
255/55-19

[Orient330iNYC]

those are all within 1% of the factory diameter:
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
275/45-20 4.9in 14.9in 29.7in 93.4in 678 0.0%
255/60-18 6.0in 15.0in 30.0in 94.4in 671 1.0%
265/50-19 5.2in 14.7in 29.4in 92.5in 685 -1.0%
255/55-19 5.5in 15.0in 30.0in 94.4in 671 1.0%

18" may not clear if you have MSport brakes, be sure to select that option for your car on the wheel finder on tire rack if you have msport.