640d stock tyres 275-30-20 rear, and 245-35-20 front.,,changing rear profile to match

[rasa bmw]

not to clued up on tyres,but i intend to buy some uniroyal RS5,,but cannot get the 30 profile for the rear,only 35 profile ,which is same as front,i know it aint out by much and speedo out a fraction..but are there any other things to look into,very rare i post about such things ha ha.ps,they also not got 245-40-20,otherwise i would have done it that way etc,,,they do sell 265-30-20,another possibility with 245-35-20 on front ?they also do not do 235-35-20,wats up with stupid companies not doing certain tyre sizes ?so i would like 275-35-20 and 245-35-20,,which they do sell,will this be ok or not ?

[Opie55]

Your post is a bit confusing so I can't tell of you are confused about how tire sizes work. You seem to be trying to find tires where the middle number - in your case the 35 - "match" because you want the profile to be the same. The profile will not be the same if you get middle numbers that are the same.

The first number, the 275 and 245, are the width of the tread in millimeters. But the second number, which is the height of the sidewall, or "profile" as you are calling it, is not a measurement. The second number is called an aspect ratio. It is a percentage of the first number. 35% of 275 is 96mm, but 35% of 245 is 86mm. So if you buy tires with matching aspect ratios your rear tires will have a bit less than half an inch taller profile. So unless all four tires on your car are the same width, trying to match the middle number is pointless.

[Qsilver7]

Originally Posted by Opie55

…So unless all four tires on your car are the same width, trying to match the middle number is pointless.

Ditto Opie55…you have staggered wheel widths so a tire mounted to a wider wheel will be stretched…so even if the tire’s aspect ratio is the same…the sidewall height will not be the same if mounted to a wheel with a different width.


Can’t tell from your car info is your 640d is xDrive or not…but if it is, you need to be in approx 2% rolling diameter between the front and rear axle. There a lots of wheel/tire calculators that can help you find compatible tires that would fall within spec.

Good luck.

[Opie55]

Q, I think you may have the confused the issue even more. He’s not dealing with stretch because he’s getting tires of the correct widths, it’s just the sidewall height at issue.

[rasa bmw]

yeh,,i cannot get 275-30-20 uniroyal tyre as they do not make them,but they do make a 275-35-20,,,so was thinking of going 275-35-20 and 245-35-20,,,also they do not make a 245-40-20,,so cannot keep rolling circumference normal like
bmw intended,,,just want to know if ahavng same profile on staggered tyres will be ok

[rasa bmw]

Originally Posted by Qsilver7

Ditto Opie55…you have staggered wheel widths so a tire mounted to a wider wheel will be stretched…so even if the tire’s aspect ratio is the same…the sidewall height will not be the same if mounted to a wheel with a different width.


Can’t tell from your car info is your 640d is xDrive or not…but if it is, you need to be in approx 2% rolling diameter between the front and rear axle. There a lots of wheel/tire calculators that can help you find compatible tires that would fall within spec.

Good luck.

bmw did not do x drive on 6 series in england..

[rasa bmw]

Originally Posted by Opie55

Your post is a bit confusing so I can't tell of you are confused about how tire sizes work. You seem to be trying to find tires where the middle number - in your case the 35 - "match" because you want the profile to be the same. The profile will not be the same if you get middle numbers that are the same.

The first number, the 275 and 245, are the width of the tread in millimeters. But the second number, which is the height of the sidewall, or "profile" as you are calling it, is not a measurement. The second number is called an aspect ratio. It is a percentage of the first number. 35% of 275 is 96mm, but 35% of 245 is 86mm. So if you buy tires with matching aspect ratios your rear tires will have a bit less than half an inch taller profile. So unless all four tires on your car are the same width, trying to match the middle number is pointless.

yup pointless,if i was asking that,i was asking if it would be ok to run same profile,as i cannot get the 30 profile for the rear in uniroyal rainsport 5

[Opie55]

Originally Posted by rasa bmw

yup pointless,if i was asking that,i was asking if it would be ok to run same profile,as i cannot get the 30 profile for the rear in uniroyal rainsport 5

Like I said, your post was unclear because you referred to the profiles "matching", or in the case of this reply, being the "same profile." They are not the same profile. They are the same aspect ratio, which is just a number and means nothing by itself. Profile describes the actual height of a sidewall. Having said that, whether it's ok or not depends on you. If the car is not XDrive, then the diameter of the tires between axles is not really important. It's not going to rub or anything. But your rear tires will have nearly 1/2" smaller sidewalls, which means its overall diameter will be nearly 1" smaller. That's enough to notice, and I think having smaller rear tires would look weird. But its' up to you if you care about that.

[D'Mann]

You do not have an X-drive, so the front and rear do not need to have the same profile...

I think OP is correct on the profile term vs. aspect term meanings.

If you have an X-drive you would want the 275x35 on the back, and 245x40 on the front to keep the profile the same. The 35% aspect on the rear is close to the same height as the 40% aspect on the front. That would keep the rolling circumference of the front and rear close enough to prevent binding on the transfer case.

Since you do not have an X-drive, there is no transfer case to worry about, so you can safely run any size tyre front to rear that you want.

As to the speedometer, I went from the 30 to the 35% on the rear, and now my speedometer reads closer to accurate than it did with the factory size tyres on it.

[Walt White Coupe]

Doesn't differences in tire circumference also effect the stability control systems as well as the XDrive?

[D'Mann]

Originally Posted by Walt White Coupe

Doesn't differences in tire circumference also effect the stability control systems as well as the XDrive?

Not an xdrive

[rasa bmw]

this is one of reasons i was asking,ie any probs with dtc/abs etc,i have in the past ran a square tyre set up, on e46 330d.but not same profile(aspect ratio) on staggered tyres/wheels,,,,looks like i will be fine with 275-35-20 x 245-35-20..

[D'Mann]

Originally Posted by rasa bmw

this is one of reasons i was asking,ie any probs with dtc/abs etc,i have in the past ran a square tyre set up, on e46 330d.but not same profile(aspect ratio) on staggered tyres/wheels,,,,looks like i will be fine with 275-35-20 x 245-35-20..

I have been running that same tyre setup (245x35, and 275x35) on my 2015 650i Gran Coupe for over a year, and even had to use the antilock brakes yesterday. No issues what-so-ever. ~ 20k miles so far.

[rasa bmw]

Originally Posted by D'Mann

I have been running that same tyre setup (245x35, and 275x35) on my 2015 650i Gran Coupe for over a year, and even had to use the antilock brakes yesterday. No issues what-so-ever. ~ 20k miles so far.

cheers for that,i was 99% sure it would be fine,,but that 1 % was doing my nappa in,,mad how tyre company's can make a 275 tyre in all ratio but not a 30 profile..

[Walt White Coupe]

What I asked before:

Doesn't out of spec differences in tire circumference also effect the stability control systems mainly the DSC Dynamic Stability Control system?

DSC prevents traction loss in the driving
wheels when driving away and accelerating.

DSC also recognizes unstable vehicle condi‐
tions, such as fishtailing or nose-diving. Sub‐
ject to physical limits, DSC helps to keep the
vehicle on a steady course by reducing engine
speed and by applying brakes at individual
wheels.

[Qsilver7]

Originally Posted by Walt White Coupe

Doesn't out of spec differences in tire circumference also effect the stability control systems mainly the DSC Dynamic Stability Control system?

Yes, wheel specs does matter in regard to DSC...but it's tolerances are a bit more liberal than the xDrive transfer case. DSC tolerances can usually handle overall diameter/circumference difference up to around 3%.

When differences move higher than 3% on the same axle or different axle...this is when DSC can interpret that difference as a slipping wheel and then begin to either use ABS braking or engine throttle reduction to compensate for what it thinks is wheel slippage.

From what the OP seems to be doing in this scenario...I don't think they are approaching the limits that would cause DSC to start kicking in and causing issues.

If we are wrong...the OP will be back with the appropriate complaints!

[Walt White Coupe]

^^^^

Thanks for that information but just to clarify a little, we are not talking about energizing ABS.

i.e. DSC helps to keep the
vehicle on a steady course by reducing engine
speed and by applying brakes at individual
wheels.

[D'Mann]

Originally Posted by Walt White Coupe

What I asked before:

Doesn't out of spec differences in tire circumference also effect the stability control systems mainly the DSC Dynamic Stability Control system?

DSC prevents traction loss in the driving
wheels when driving away and accelerating.

DSC also recognizes unstable vehicle condi‐
tions, such as fishtailing or nose-diving. Sub‐
ject to physical limits, DSC helps to keep the
vehicle on a steady course by reducing engine
speed and by applying brakes at individual
wheels.

To individual "drive" wheels. DSC does not apply brakes to non-drive wheels - In the case the front tyres. It is very hard to have the front tyres (on a rear wheel drive) break traction because there is too much power.

[Walt White Coupe]

I don't see the description say individual "drive" wheels. It merely says individual wheels and the DSC will surely apply brakes to "individual" front wheels to keep the car under control if the car starts to get out-of-control.

[Dj650igcxdrive]

I have 245/35r20 front and 285/30r20 rear. 8.5” wide front and 9.5” wide rear. Rear offset is 36 i think. Just makes it.

[rasa bmw]

decided to just replace my rear tyres,also wheel refurb at same time,dark gloss bronze,picking it up in 1 hour

[rorz12]

Originally Posted by rasa bmw

cheers for that,i was 99% sure it would be fine,,but that 1 % was doing my nappa in,,mad how tyre company's can make a 275 tyre in all ratio but not a 30 profile..

Tell me about it.. I looked EVERYWHERE for 275/30R20 where I live. I was looking specifically for Michelin Pilot 4s to match my front wheels, and could only find really cheap brands or run-flats in 275/30. No-one seems to stock up on them (at least here) even though manufacturer makes them.

I ended up putting 295/30 on the rears, which I am not THAT happy about visually but they are actually a closer match with my front 245/35 in terms of circ, height etc. Haven't noticed any issues. Bit of a buldge on them but nothing crazy since my rear 356 style wheels are 9" wide.

p.s previous owner had 245/30 front 275/35 rear same as what you went with but the rear tire was noticeably bigger than the front since a 35 ratio on 275 tire is quite big compared to the front. There's also over 20mm height difference and +3% circ.. not ideal.