Increased wheel circumference's impact on auto tranny.

[paul mc11s]

I am toying with the idea of putting 255/55/18's on my 2011 F10 520d.

Doing so will increase the circumference of the wheel to 10% more than what BMW's recommended wheel size (either 245/45/18 or 225/55/17) is.

Rubbing etc aside, will the increased circumference be a problem for the gearbox/engine/car's brains? The car is an auto.

I have thoughts of the car calculating a combination of speed (as measured at the wheels), revs and gear and that the 10% might throw a spanner in those works.

Any thoughts?

 

[Maljan]

I assume that you want a higher aspect ratio for better ride quality and pothole survivability. I also assume that your car is currently fitted with 245/45R18 tyres on 18x8.0 inch wheels.

Have you considered going for narrower tyres? A 225/50R18 will be 0.7% bigger in circumference and a 225/55R18 will be up by 3.7%. That is a lot better than 10%.

AFAIK a 225 tyre will mount on wheels that are 6 inches to 8 inches wide, so you should be OK there.

 

[paul mc11s]

Thanks for your reply Maljan,

Yes I want higher aspect ratio for better ride quality. Yes I have the 245/45/18's on there now.

I think the width of the tyre doesn't impact the diameter and consequently the circumference of the wheel, but I may be wrong.

This site: http://www.errolstyres.co.za/content/tyre-overall-rolling-diameter

says that a 245/45/18 has a rolling circumference of 2017mm.
A 255/55/18 has a rolling circumference of 2230mm.

That means that the second tyre has a the same circumference as tyre one + 10%.

So the car will think it has travelled 2017mm with one revolution but will in fact have travelled 2230mm. This will be apparent on the speedo but I am okay with that.


My question is more about the dependance of the car to know the speed in relation to the engine revs at the time of changing gear.

I mean is there something inside the car that says for example at 2000 rpm on the engine in 1st gear the car should be travelling a particular speed and if not, that there is a problem?

 

[DieselFan]

Why don't you just fit non runflats? It will trump any other changes you're thinking of making. My change from 17inch rfts to 19 inch normal tyres was a drastic improvement on ride comfort.

 

[Ratslaaf]

I had 19" RFTs and ride comfort was truly excellent on my F10.

 

[paul mc11s]

Why don't you just fit non runflats? It will trump any other changes you're thinking of making. My change from 17inch rfts to 19 inch normal tyres was a drastic improvement on ride comfort.

Thanks DieselFan.

I have the wheels and tyres in my garage and quite like them.

I hear what you are saying on the non rfts. That may be a route worth considering.

You don't still have your 17's lying around do you?

Paul.

---

I had 19" RFTs and ride comfort was truly excellent on my F10.

Thanks Ratslaaf. What profile tyres did you have with that setup?

Paul.

 

[DieselFan]

They went on my e46. I have my e46s rims but they're not in great condition. Some curb rash etc. Just sell the tyres on the forum. If they're on good nic you should get decent money for them.

If it was me. I would fit normal tyres to the rims you like and sell the tyres which are on the rims.

 

[Maljan]

I think 10% more may be just too much. The speedo and odo will be out. The engine and gearbox computers need the road for determining when to shift, how much fuel to feed etc. Even the ABS disconnects at low speed AFAIK. All of this will now use wrong data. You know what they say about computers - garbage in, garbage out. It might even affect your motorplan.

 

[sash]

I think the width of the tyre doesn't impact the diameter and consequently the circumference of the wheel, but I may be wrong.

This site: http://www.errolstyres.co.za/content/tyre-overall-rolling-diameter

says that a 245/45/18 has a rolling circumference of 2017mm.
A 255/55/18 has a rolling circumference of 2230mm.

That means that the second tyre has a the same circumference as tyre one + 10%.

So the car will think it has travelled 2017mm with one revolution but will in fact have travelled 2230mm. This will be apparent on the speedo but I am okay with that.

slight correction, the width of the tyre does affect the diameter....but not as much as changing the profile

The sidewall of a tyre is calculated using the width of the tyre and the sidewall affects the diameter of the tyre.
circumference of 245/45/18 = (sidewall + rim diameter ) * pi
sidewall = 245mm * 45% = 110.25mm
rim diameter = 18 inch or 457.2mm
circumference = (220.5+457.2)*(22/7)
= 2129mm , errols doesnt seem too accurate

circumference of 255/45/18 = (sidewall + rim diameter ) * pi
sidewall = 255mm * 45% = 114.75mm *2(top and bottom)
rim diameter = 18 inch or 457.2mm
circumference = (229.5+457.2)*(22/7)
= 2158.2mm

which is just over 1% difference in this example

I agree with Maljans suggestion

 

[paul mc11s]

They went on my e46. I have my e46s rims but they're not in great condition. Some curb rash etc. Just sell the tyres on the forum. If they're on good nic you should get decent money for them.

If it was me. I would fit normal tyres to the rims you like and sell the tyres which are on the rims.

Thanks. I think I will sell the wheels and tyres which are all in great condition. I'm not sure I am allowed to advertise them on here yet though, since I don't have 50 posts under my belt yet.

Paul.

 

[DieselFan]

You're almost there dude just contribute a bit more to the forum and you'll be there in no time

 

[Ratslaaf]

Why don't you just fit non runflats? It will trump any other changes you're thinking of making. My change from 17inch rfts to 19 inch normal tyres was a drastic improvement on ride comfort.

I had 19" RFTs and ride comfort was truly excellent on my F10.

Thanks Ratslaaf. What profile tyres did you have with that setup?

Paul.

Hmmm, I think the back ones were 275/35/19 and the front ones were 235/40/19. But I'm going off memory here, it's the standard profiles on the MSport package with 19" wheels. They were Michelins though, really really good tyres

 

[Maljan]

I think the width of the tyre doesn't impact the diameter and consequently the circumference of the wheel, but I may be wrong.

Well, you are, but I will not hold it against you. :rollsmile:

BTW, the suggestion to switch to non-runflats in the same size look like a winner to me. Finding any 225 in 18 inch off the shelf might not be easy. Just because one can calcaulate a tyre size doesn't mean that somebody actually stocks it over here.

---

Update: you do get tyres in 225/55R18:

http://www.tirerack.com/tires/TireS...h=255/&rearRatio=40&rearDiameter=17&zip-code=

The problem is that they only go up to a V speed (240km/h) rating. All the 245/45R18 tyres have W (270 km/h) or Y (300 km/h) ratings. Not good.

Things look better for the 225/50R18 size:

http://www.tirerack.com/tires/TireS...h=255/&rearRatio=40&rearDiameter=17&zip-code=

But will a change from 45 to 50 make much of a difference in ride?

As far as load rating goes, it varies from 95 to 100 for the three sizes, depending on manufacturer and model. That should be not be a problem.

The load rating is the number that come after the size and the letter after that is the speed rating, e.g.
245/45R18 96W
But I suppose everybody knows this.