2008 335XI

2008 335XI Auto Tran
Car came standard with 17" wheels. For summer driving I am considering going to a larger diameter wheel. Can I go all the way to a 20"? What are the ramifications to this kind of upsizing? I am talking about the mechanics of the all wheel drive, speedometer etcl. I know I have to look out for all around clearance. I have plenty of acceleration but I would like better mileage. I will stick with the run flat tire, so comfort will never be achieved but I do want control and road-ability.

Al Gordon
Denver, CO

Not really sure, maybe - You can use the tirerack.com wheel selector to see what diameter wheels it shows will fit your car.

The question is, why would you want to go to a 20" wheel? They will be much, much heavier, increase your unsprung weight, and reduce your acceleration if your rolling diameter is larger than stock - if you choose a tire size that maintains original diameter, then you'll be running such a low-profile tire that you run a much-increased risk of killing the tires or the rims, as you will have extremely little in the way of sidewall height. IF you do virtually _only_ highway driving AND reallllly minimize your throttle inputs, yes, perhaps you'd see some increase in mpg, but, I'm gonna guess perhaps not enough to even remotely begin to offer payback on what the cost of 20" rims (big $$$) and 20" tires (also big $$$) will be. A downside of larger, heavier, rims is that they will cost you mpg. gains due to their weight, inertial-resistance, etc. If you opt for a stickier hi-performance summer tire, you'll also have more rolling resistance, so those hoped-for mpg gains would likely end up a remote hope if not an impossibility.

Control & road-ability, need more specific terms for what you'd like to get; if by road-ability you mean ride-quality, going to much larger dia. wheels and ultra-low profile tires will degrade your ride quality. The increased unsprung weight means the suspension is less-able to respond to road imperfections, bumps, thumps, etc., and the ultra-low-sidewall-height tires will be passing more of that through to the chassis, which you'll be feeling more. Lower-profile tires can yield more immediate steering responsiveness, due to less sidewall flex, if that's what you mean by control. Tread design, tread-depth, and tread compound can also contribute to a feeling of "control" & are elements to consider in choosing tires - racing slicks, for instance, have no tread-blocks that will squirm and deform as they're being run on or turned, which is one reason why the steering response of race cars might seem lightning-quick compared to a street car running new street tires with 10/32 of tread depth.

As far as the awd system, I believe the main point is to not have substantial diameter differences front to rear or side-to-side - doing so can make the car's wheel speed monitoring & compensation systems think one wheel or another is slipping constantly, and it might be constantly trying to compensate by applying the brake too much for that wheel, or kicking in other parts of the traction/stability control systems. The computer system is calibrated to what comes stock and expects only so much variation before kicking in. That doesn't mean you can't run plus-sized wheel & tire setups, but they need to be appropriately sized, particularly for the awd models. Speedometer variation may vary depending on what you do - if you run a wheel/tire setup larger dia. than stock, then your speedo will read low compared to your actual speed; also, accounting that BMW speedometers typically are setup optimistic, indicating you're a few mph higher than actual speed - which means larger-than-stock rolling diameter setups might have the speedo showing closer to your actual speed. Smaller diameter than stock will have the speedo indicating higher than actual speed, which might be compounded if your speedo reading is optimistic now. Some people seem to be overly concerned about speedometer accuracy, but having the speedo reading higher or lower than actual speed isn't going to harm the electronics or anything, at least as far as I know.

RFT's are progressing, you may find the current offerings are better than what came on yours. You might want to consider a less-radical change, I'd tend to think you might stand a better chance of improving your avg. mpg by opting for a higher treadwear-rated tire than what came originally, which typically have quite sticky near-race-tire 140 treadwear ratings - fun, lots of lateral grip, but they wear out more quickly, and, being kinda sticky, will have increased rolling resistance. Something like an a/s (all-season) runflat, perhaps a different brand than what you started with (comments on tirerack.com aren't particularly favorable to the Bridgeston rft's, for instance, so perhaps the Conti's or others), run your tire pressures 2-6 psi high (less rolling resistance, although perhaps at the cost of wearing out the middles prematurely), and try not to drive it like ya stole it. One trade-off, tires that last longer (ie, higher treadwear-rating numbers) won't be as sticky, so, as far as ultimate performance goes, you're trading off some ultimate lateral grip (maybe no big deal if you don't take turns so much like you stole it), and stopping distance may be somewhat increased over the sticky oem tires (less straight-line grip too). If you want to keep the wheel & tire costs a bit more in line, with less risk of damage to wheel and tire, less negative impact on ride-quality, less negatives from increasing unsprung weight, you might consider a more moderate plus-one setup, opting for 18" wheels, ideally as light as you can get them for whatever you're willing to pay for 'em, paired with tires that won't cost you mpg due to being sticky & similar to what came on the car. If maintaining ride-quality is a primary issue, then sticking with 17's and avoiding the low-profile tires gives you the best chance for that.

Here are some useful comments from an old thread - different car under consideration, but pertinent to the discussion -

Welcome to the club. Not sure if 20" will fit, me personally I would not do it especially to an xdrive since it could cause more wear on the clutch in the transfer case and that can get expensive.