[lkstaack]

Considering the number of people who post on this board, I was surprised at the lack of consensus for proper F3x tire sizes. So, I analyzed my situation based upon information I’ve gained from this and other sites to make a tentative tire size decision. Now I understand why there isn’t the perfect set of tires for us. Please fact-check my analysis and tell me if you come to a different conclusion. This is a lengthy post, so feel free to skip to the bottom to read what I have come up with.

I recently purchased larger width rims to replace the 19x8 and 19x8.5 Type 403 wheels on my 335i M Sport with Dinan springs (about .5” lower than M-Sport suspension). My new 305Forged wheels have the specifications below.
Front: 19x8.5 ET 32, 21.6 lbs
Rear: 19x9.5 ET 40, 20 lbs

The qualities I’m looking for in tires are indicated below by order of importance. In other words, I want the widest tires that will fit without rubbing or interfering with my speedometer reading.

1. Maintains accurate speedometer reading
2. No rubbing
3. Large width
4. Comfort

There seems to be a general consensus on:

• Keep tire size close to 1% of stock to maintain an accurate speedometer reading **EDIT** I learned that the speedometer receives information from the left rear wheel. So, while it remains important for AWD to stay w/i 1% for all wheels, it isn't important that front tires are within 1% for RWD bimmers.
• Stock speedometers usually report a 1% slower speed. So, a + speedo error is preferable to a - one.
• A larger aspect ratio helps prevent wheel damage and provides a more comfortable ride
• Camber increases a little when lowering suspension
• A larger wheel may cause tire stretch and a smaller wheel a bulge, depending on tire width

There is less consensus on:

• A larger tire width provides better traction
• Road handling is improved with a smaller rim (i.e. 18”) due to less unsprung weight
• Despite the increased camber, lowered bimmers will rub more with large widths as compared to those with the stock height
• Some people think the “bulge” caused by rims that are narrower than the tire width is not aesthetically pleasing (I don’t necessarily agree with this)

I placed my front and rear wheels on ramps and attempted to measure the clearance from the tire to shocks and tire to fender. While I tried to get accurate measurements, I ran into a problem: suspension clearance gaps differ as suspension compresses in real-world driving, as does fender clearance gaps. Gaps in the rear were particularly problematic because the 12’oclock position has a lot more potential travel/clearance then the 3 and 9 o’clock positions. I had to eyeball and estimate these gaps, but I wouldn’t place a high level of confidence on their accuracy. Also, I couldn’t check on suspension clearances while turning.


Clearances:

Front
-Inside: 19mm
-Outside: 19mm
Rear
-Inside: 18-20mm
-Outside: 18-25mm


I used the websites below to calculate the height, suspension clearance, fender clearance, wheel diameter difference, likely speedometer error, and how much (or little) the tire will protrude from the rim as compared to the stock tire and rim.

rimsntires.com
tiresize.com

I considered many different tire sizes

Front Wheels


Rear Wheels



The charts show that there is no perfect tire size that matches every specification of the stock tires. But based upon my analysis, I am inclined to choose 255/35 x 19 for the front and 265/35 x 19 for the rear, even though there are problems with these sizes. This is my reasoning.

Front: 255/35

• Height and Diameter Difference from Stock. This tire diameter is virtually identical to stock.
• Speedometer Error. This is my top concern. But, this tire size has the least amount of error in sizes that definitely won’t rub.
• Suspension and Fender Clearance. This tire should clear, but it will be very close. I could get rubbing if my measurement estimations are off.
• Tire Bulge. This tire will stick out 17mm more than the stock front tire. As a comparison, the stock tire size in the rear sticks out 17mm more than the stock front tire. I should have gotten a 9” rim.
• Analysis: This tire has many of the same specifications as stock. However, it may rub and it will stick out 17mm more than stock. Should I accept the risk that it may rub? Or, should I play it safe with 245/35 tire, even though it will induce significant speedometer error?

Rear: 265/35

• Height and Diameter Difference from Stock. This tire diameter is .3” (1.2%) larger than stock. This may cause a little speedometer error. I really want the larger 275/30 tire, but I think that the 1.8% speedo error compounded with inherent BMW speedometer error will cause problems.
• Speedometer Error. This is my top concern. There will be a little error, but the 1.2% error caused by the larger diameter may cancel out speedometer error inherent to BMWs.
• Suspension and Fender Clearance. This tire should clear without problem. However, my desired 275/30 tire may rub.
• Tire Bulge. This tire will stick out 15mm less than the stock rear tire. It is only 24mm wider than the rim.
• Analysis: This tire is the safest bet out of the bunch. It isn’t as wide as I would like it, but it will cause the least amount of speedometer error.

Do you agree with my analysis? Why or why not?


**EDIT** I ordered Michelin PS4S 245/35x19 tires for the front and 264/35x19 in the rear. I chose not to get the 255s for the front because 1) I learned that matching diameter sizes isn't as important in the front, and 2) I don't want to risk rubbing during turns.


**EDIT** I've been driving with my new wheels and tires for about two weeks now. Because the diameters of the old and new wheels are almost identical, there is very little difference in the stance or wheel gap of the car.





The 245/35s are much wider than the 225/40 and I haven't encountered any rubbing.



I have driven with four passengers without any rubbing in the rear or front. Wheel gap remains the same with Dinan springs as with my stock 403M wheels.





I must say that I am very pleased with new wheels and tires with the sizes that I chose.

[alohasurftoad]

glad to see you're detailed. i'm pretty detailed myself but you're next level.

from the factory bmw speedos tend to read high. indicated speed is higher than actual speed. i believe the speedo gets it's signal from the rear wheels.
a larger than oem diameter in the rear will cause the speedo to read closer to actual.

255/35/19, 265/35/19 will work, tires will have slight bulge up front, perfect rear (no bulge or stretch)

245/35/19, 265/35/19 perfect fit on both but front diameter and sidewall height is less than rear. bmw M-cars sometimes have staggered diameters like this causing somewhat of a foward rake.

a typical set up for 19x8.5, 19x9.5 is 235/40/19(F), 265/35/19(R), 10mm stretch up front, perfect fit at rear. personally this is what i'd get

[lkstaack]

Originally Posted by alohasurftoad

i believe the speedo gets it's signal from the rear wheels.

This is an important consideration if true. If so, I can safely select the 245 size without speedometer consideration. Can anyone confirm that the speedometer is driven only from the rear wheels?

[jnecr]

Power brake and do a burnout, you shall find out..

[lkstaack]

Originally Posted by alohasurftoad

a typical set up for 19x8.5, 19x9.5 is 235/40/19(F), 265/35/19(R), 10mm stretch up front, perfect fit at rear. personally this is what i'd get

So, why would you opt for a front tire narrower than a 245? And, why would you prefer a stretched front tire?

[alohasurftoad]

Originally Posted by lkstaack

So, why would you opt for a front tire narrower than a 245? And, why would you prefer a stretched front tire?

245/35 fits an 8.5" rim perfectly and the wider you go upfront helps reduce understeer however the wheel gap will be increased and the car may sit with a slight forward rake. if you have adjustable height coilovers you may have to lower the front to have the same wheel gap as the rear, for aesthetics.

235/40 is 10mm undersized (stretched) for a 8.5" width (barely noticeable) however it is 0.6" or 15mm taller than the 245/35 so the fenderwell gap is reduced and the front and rear is diameter is the same 26.4F/26.3R.

you could also do 255/265, 255/275 or i've seen 245/40, 275/35 .

personally i don't like stretched tires. 235 is more for looks and 245 is performance. it's really close so either way will work okay.

http://f30.bimmerpost.com/forums/sho...+265%2F35%2F19

http://f30.bimmerpost.com/forums/sho...+265%2F35%2F19

[kern417]

I usually try to reference performance companies that build aftermarket wheels. Dinan and APEX for us helps understand the best wheel/tire fitment for performance. Of course you can make a lot of things fit with stretched tires and/or an aggressive alignment. But for a good street car with less than 2* of camber, they seem to have very good options with more than enough tire.

The only reason i'd say more tire isn't always better is because a wider, stickier tire will also increase your rolling resistance. It's kind of like performance aero (wings, canards, etc). It can help, but only add it if you need it. Otherwise the penalty isn't worth the "improvement" if any is even there.

And clearance to your suspension should no decrease under compression. as the damper moves, so does the hub/wheel.

[terryd5150]

Originally Posted by lkstaack

So, why would you opt for a front tire narrower than a 245? And, why would you prefer a stretched front tire?

I like your attention to detail.

I currently have 235 on an 8.5" wheel and my wheels have paid the price for it.

Do not go lower than a 245 on an 8.5" wheel.

I'll be getting my wheels reconditioned and moving to a 245.

[floydarogers]

Originally Posted by lkstaack

This is an important consideration if true. If so, I can safely select the 245 size without speedometer consideration. Can anyone confirm that the speedometer is driven only from the rear wheels?

Actually, we can confirm the opposite. It's an average of all 4 tire rates, pulled from the ABS sensors.

[lkstaack]

Originally Posted by terryd5150

I'll be getting my wheels reconditioned and moving to a 245.

Why not a 255?

[terryd5150]

Originally Posted by lkstaack

Why not a 255?

I don't want to rub on the inside.

[lkstaack]

Originally Posted by terryd5150

I don't want to rub on the inside.

So, you've confirmed that it will rub? I estimate 8mm clearance when the wheels are straight. Does it rub while turning?

[terryd5150]

Originally Posted by lkstaack

So, you've confirmed that it will rub? I estimate 8mm clearance when the wheels are straight. Does it rub while turning?

For clarification, I have not confirmed it will rub on a F3x; however, I can't find anyone that can confirm it will not rub, either.

[lkstaack]

Originally Posted by floydarogers

Actually, we can confirm the opposite. It's an average of all 4 tire rates, pulled from the ABS sensors.

That is very good to know. What is the source of this information?

So, if the fronts were 1% smaller than stock and the rears were 1% larger, would the speedometer interpret that as stock?

[floydarogers]

Originally Posted by lkstaack

That is very good to know. What is the source of this information? ...

The fact that there is no sensor on the transmission or differential, and nothing on the fronts (other than abs sensors.) BMW did this changeover some time in the late 90's or early 00's oughts. Eliminated several parts.

2% difference in wheel/tire sizes is probably too much - especially on an xDrive car. But it's probably some sort of average - after all, that's what DSC has to do to decide whether to brake a wheel.

[jnecr]

Originally Posted by floydarogers

The fact that there is no sensor on the transmission or differential, and nothing on the fronts (other than abs sensors.) BMW did this changeover some time in the late 90's or early 00's oughts. Eliminated several parts.

That's not convincing to me that it's an average of all 4.. They could still take it from one wheel or one axle.

[terryd5150]

Originally Posted by jnecr

That's not convincing to me that it's an average of all 4.. They could still take it from one wheel or one axle.

I agree with you on that; however, I had a 2009 335i that came with the sport package 18" wheels, 225/40 front, 255/35 rear.

I went to 19" 437M reps with 235/35 front and 275/30 rear; I noticed a distinct increase in the rate at which the odometer started adding up the miles.

10-minute drives around town (not on highways or interstates) would show I'd traveled 20 miles.

While not concrete evidence in itself, it does suggest this theory of "averages" may play into this equation somewhere.



Again, just pure speculation based on ancedotal evidence.

[jnecr]

Originally Posted by terryd5150

I went to 19" 437M reps with 235/35 front and 275/30 rear; I noticed a distinct increase in the rate at which the odometer started adding up the miles.

I dunno.. you're talking about a difference of 2% here. You're not going to notice an odometer difference unless you consistently drive the same long route (500+ miles) and notice that you're now traveling more/less miles.

[terryd5150]

Originally Posted by jnecr

I dunno.. you're talking about a difference of 2% here. You're not going to notice an odometer difference unless you consistently drive the same long route (500+ miles) and notice that you're now traveling more/less miles.

That would tend to make sense, at least that's what I would think.

I suspect the 235's being mounted on an 8.5" wheel caused them to be stretched and lose some height, thus throwing the equation off.

For comparison:

• 2008 335xi - I had this car for two years; put 5,500 miles on it
• 2008 335i - I've had this car for two years (still have it); I've put 5,000 miles on it
• 2009 335i - I had the car for 8 months; odometer says I put 6,000 miles on it - I put the 19"s on it after 3 months of ownership, that's when I started to notice an unusually high accumulation of miles.

All cars above saw the same daily drive, with comparable weekend driving; used a different car for long trips (good old 2006 E90 325i).

Again, just pure speculation.

[jnecr]

You're just driving a lot. You're talking about doing 2.5k miles/year vs 16k miles/year. This is in no way explained by tire size differences...

[kern417]

that's really bad lol. you probably either a. want to get that checked or b. put a dash cam in the car and see who's stealing it every night

[lkstaack]

Originally Posted by floydarogers

Actually, we can confirm the opposite. It's an average of all 4 tire rates, pulled from the ABS sensors.

I have researched multiple BMW related websites now. Those that address speedometer sensors point to one location. Even though ABS sensors calculate wheel speed for each wheel, only the speed from the left rear wheel is used for the speedometer. Can you provide information that disputes this?