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2Addicts | BMW 2-Series forum Technical Topics Wheels and Tires -- Sponsored by The Tire Rack Can I switch from 245/35 18 to 245/40 18?

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      12-06-2020, 01:27 PM   #1
helloelectro
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Can I switch from 245/35 18 to 245/40 18?

There's more tire options at the 40 sidewall, can I safely do this? Really want to get some r1r Proxes on the car. I'm on billstein b14 coilovers but at a raked setup so even with the car lowered my rears have some gap still.

Appreciate the input!!
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      12-08-2020, 09:52 AM   #2
TrboMike
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This will close your gap a quarter of an inch. The tire will raise the height of the car half an inch.

Is this just your rears or are all four going to be changed? If you are AWD you will be changing the speed difference between fronts and rears by 3.75% which the center diff should be able to handle but is not ideal.

If you are also doing your fronts, this could cause rubbing (the tire will be taller but slightly less wide).

Your speedo will be off by 4% (when you're actually going 60 your speedo will show 57mph).

Otherwise I cant think of any other reasons why not.
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      12-08-2020, 11:20 PM   #3
helloelectro
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Quote:
Originally Posted by TrboMike View Post
This will close your gap a quarter of an inch. The tire will raise the height of the car half an inch.

Is this just your rears or are all four going to be changed? If you are AWD you will be changing the speed difference between fronts and rears by 3.75% which the center diff should be able to handle but is not ideal.

If you are also doing your fronts, this could cause rubbing (the tire will be taller but slightly less wide).

Your speedo will be off by 4% (when you're actually going 60 your speedo will show 57mph).

Otherwise I cant think of any other reasons why not.
That's all? Hehe. Yeah I stepped back from this and went a different route. I started digging in the speed variations and it became a worse and worse idea lol.
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      12-12-2020, 10:41 PM   #4
bthomas518
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Quote:
Originally Posted by helloelectro View Post
That's all? Hehe. Yeah I stepped back from this and went a different route. I started digging in the speed variations and it became a worse and worse idea lol.
Sort of a bonus, your odometer will record fewer miles.
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      12-13-2020, 08:15 AM   #5
helloelectro
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Quote:
Originally Posted by bthomas518 View Post
Quote:
Originally Posted by helloelectro View Post
That's all? Hehe. Yeah I stepped back from this and went a different route. I started digging in the speed variations and it became a worse and worse idea lol.
Sort of a bonus, your odometer will record fewer miles.
True. Haha. Doesn't too much of a deviation mess with the computers too? Or is that more of an AWD related issue?
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      03-02-2021, 11:04 AM   #6
TrboMike
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You never answered if you're staggered or square (all 4 tires same size) or what offset your wheels are (how close to suspension and fender). Did you buy anything yet?

I'm about to buy new summer rims and thinking about 245/40-18 instead of 245/35-18 like the OP. Yes I'll take a hit in handling with the taller sidewall, but I'll have more cushion for potholes and the tires are much cheaper in that size. And the tire will fill the gap more as OP said. Now I just wonder if the 5mm taller tire will rub.

Speedo difference vs OEM tires:
245/35-18 is 1.35% shorter than OEM so at 60mph my speedo will think I'm 60.81mph
245/40-18 is 2.45% taller so at 60mph my speedo will show 58.53

If I can get it to fit with the offset wheels I'm getting, I think I'm going 245/40
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      03-03-2021, 06:36 AM   #7
Heitzer
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For the MP4S summer tires the revs per mile change from 840 (35 aspect ratio) to 809 (40). That's a 3.8% difference. Your "thrust" will be that much lower so something to think about. The 40 tire is a bit heavier too.

I have done the switch to 245/40 in the rear for winter tires because there are no 245/35s I like. I can tell the change in thrust/acceleration.
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      03-03-2021, 03:57 PM   #8
TrboMike
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If you consider the OEM tire size "base" or "zero":

245/35-18 is -1.35% (or smaller) than OEM
245/40-18 is +2.45% (or larger) than OEM

If you're square (all tires the same size) like we're talking about, you're not 3.8% different from stock. 35 ratio is 1.35% off in one direction and 45 is 2.45% off in the other direction. Thats a 1.1% difference when you're choosing between the two.

If you were doing staggered and chose 245/35-18 front and 245/45-18 rear then yes you would be 3.8% different front to rear. Wont matter much if you're FWD but I wouldnt do that to the center for an AWD.
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