[Sams335]

I wasn't sure what the best forum would be to ask this, but here goes...

The Car:
2009 335 xDrive Sedan 6mt with Cobb Accessport (custom dynotune). Went from stock 161 17x7.5s to Avante Garde M310 19" staggered setup.

I used to average between 19-20 mpg per tank both before and after my access port and dynotune (done in April 2012), depending on mix of city and highway.

After I had my new rims and tired installed, i've dipped to 17mpg, which has me a bit concerned. Would larger diameter and width rims cause this drop? I'm very cognizant of my driving habits, and nothing has changed over my last two tanks of gas other than the wheels.

Any thoughts? Anyone have experience with this? Thanks in advance for your help guys.

[avocet]

my mileage diff on a 328i e90 is around 6% between my 17's winter setup and 19's summer.
225's square on the 17's, 225/255 on the 19's.

[Sams335]

Ok glad I'm not the only one. Although mine looks to be more like 10-15%...

Thanks for the info.

Anyone else?

[ChasVS]

Calculate the rolling distance of the 19's vs the 17's and see how much farther you are actually traveling per revolution. The difference will probably be the amount you are seeing. Unless you can recalibrate the Odometer, you'll always be off. The Odometer thinks you have traveled X feet per revolution and you actually traveled Y. The difference between X & Y is the change you are experiencing all other things being equal.

[Sams335]

Originally Posted by ChasVS

Calculate the rolling distance of the 19's vs the 17's and see how much farther you are actually traveling per revolution. The difference will probably be the amount you are seeing. Unless you can recalibrate the Odometer, you'll always be off. The Odometer thinks you have traveled X feet per revolution and you actually traveled Y. The difference between X & Y is the change you are experiencing all other things being equal.

Makes sense, but when I used the wheel calculator on this site I was looking at under a 2% difference. So I'd expect the difference to be 2% or less...

[galahad05]

How much heavier are the 19s?

[Sams335]

Not sure what the weight difference is. I'm thinking it's probably a few lbs per corner, but under 5.

[ChasVS]

Originally Posted by Sams335

Makes sense, but when I used the wheel calculator on this site I was looking at under a 2% difference. So I'd expect the difference to be 2% or less...

That's likely a diameter difference, not the circumference difference.

To get the Circumference. Multiply the tire & wheel diameter by pi ( 3.14 ). This will give you the circumference.

Diameter = 20 in
20 x 3.14 = 62.8
Circumference = 62.8 inch

Compare that to the Circumference of your original wheels & tires.

[CUTigers]

Are you doing your own math after a fill up? Or just going off what the car says? I'm sure the cars math would be off with diff spec wheels. Try it on your own next fill up

[Jace_91]

I think the 2% is the actual loss of mileage, since there is more contact with the pavement there is more friction so there is probably a bit of real mileage loss, as well as the bit of added weight. the 10% you are seeing is probably the car doing the math based off of 17" wheels.

[FRRYY]

I switched to 19"s as well and my numbers match yours.

[Sams335]

Thanks all... I'm going to run some numbers and I'll report back soon.

[Darius]

It should be so.
1. You added more metal than rubber/air. So more weight and there is a formula:
T= F*L, here T-torque, F-force came from engine(driveshaft), L- length from the center of the wheen to tire surface. So the bigger diameter therefore more toque is needed.
2. If new tires ar more wide-you have more contact with pavement, therefore more grip and friction.

[Bimmerlover76]

Originally Posted by ChasVS

Calculate the rolling distance of the 19's vs the 17's and see how much farther you are actually traveling per revolution. The difference will probably be the amount you are seeing. Unless you can recalibrate the Odometer, you'll always be off. The Odometer thinks you have traveled X feet per revolution and you actually traveled Y. The difference between X & Y is the change you are experiencing all other things being equal.

Sounds about right. Also, what's the weight difference. It may be negligible but contributory.

[wooddog44]

Originally Posted by Sams335

Thanks all... I'm going to run some numbers and I'll report back soon.

Found out the hard way -the trooper who pulled me over told me - on a vehicle I had that this also affects your speedometer which will read out slower than you are actually traveling.

[Turb0Surge]

Originally Posted by ChasVS

Calculate the rolling distance of the 19's vs the 17's and see how much farther you are actually traveling per revolution. The difference will probably be the amount you are seeing. Unless you can recalibrate the Odometer, you'll always be off. The Odometer thinks you have traveled X feet per revolution and you actually traveled Y. The difference between X & Y is the change you are experiencing all other things being equal.

^this = spot on.

[gatoracer]

I recently went from 17" square to 18" staggered. Not a huge jump, but bigger wheels. I manually calculated on my first fillup after install and saw a drop from ~23 mpg to ~21.5. That was about 3 weeks ago. On my last fillup avg about 22.5 mpg. I doubt the ECU "adapted" to the wheel size. Maybe my driving habits have tamed a bit. Either way I think you are going to lose a little mpg as the mass the vehicle is moving has increased.

[edirtaynine]

in addition to the rolling calculation noted above already, heavier wheel weight and possible increase in traction from wider tires also affect gas mileage.

[tullnd]

Did you say 5lbs per corner? If so, that's 20lbs of UNSPRUNG weight. That's pretty huge. BMW wheels are heavy. Ridiculously so. Coming from a world where most 18" high end rims were maybe 17lbs at the most(my 16" rims on my old car were under 16lbs), swallowing the weigh of these rims has been bitter medicine.

That said, to me, I can't deal with a weigh much over 20lbs on a 19" rim. Otherwise, I just see too much of a performance hit there. The comments above about rolling diameter are correct. Also, going to a wider and potentially stickier tire increases your rolling resistance too, which improves traction but at the cost of mileage. It's small, but all this stuff adds up. You add 20lbs of unsprung weight, that makes a difference too. It's more weight for your car to push forward(this is unsprung weight, not at all the same as what's riding on your suspension) and greatly affects handling.

All this added together could easily account for your mileage drop. Both perceived and not real(the rolling diameter) and the real(increased weight and rolling resistance).

[VMRWheels]

Generally, you want to keep your overall diameter within 3% of your OEM wheels. Even though your wheels are larger, the tires will compensate the difference in diameter. Just as long as you didn't go with a taller or shorter tire or something abnormally wide, you shouldn't see a HUGE difference in your MPG. The difference is mainly because your 19 inch wheels and tires are heavier than your stock 17's. Though it's only a few pounds, it's rotational weight.

www.1010tires.com/tiresizecalculator.asp <-- good tire calculator. Will show you overall diameter, circumference etc.

[e90-328i]

there are other posts that talk about when adding larger wheels that are heavier (more metal) and if you check tires, they are also heavier when going to 19" even though it looks like less rubber than 17"
that extra weight say 6-7 pounds extra on each wheel is 24-28 per set. but because its a rolling weight, it is like having over carrying over a 100 pounds extra in your car. i noticed a bit of drop in mpg in mine, but i really noticed the power difference. (no 328 jokes please-lol)

i ride road bicycles and the biggest difference you can make it so go to lighter wheels. a wheel that is 1/2 pound lighter will make it feel like you got a new pair of legs! more so then getting a 2 pound lighter bike.

[driverman]

To summarize many of the posts above, the difference in mileage could be caused by two things:

1. inaccurate odometer caused by new wheels/tires
2. heavier wheel/tire combination

The OP stated he had style 161 wheels. The style 161 tires/wheels on my 328i e90 with ZSP are staggered with 225/45/17 tires up front and 255/40/17 tires in the rear. The following wheel/tire combinations have approximately the same diameter, circumference and revs per mile:

225/45/17 255/40/17
225/40/18 255/35/18
225/35/19 255/30/19

So, if you have 225/35/19's up front and 255/30/19's in the rear, the odometer should be very close to accurate. If not, your tires are probably wider which means the diameter and circumference would go up and revs/mile would go down. In other words, you are going farther than your odometer indicates because of the larger tire/wheel combo, which would explain the APPARENT drop in gas mileage.

If you have 225/35/19's up front and 255/30/19's in the rear, then, as others have said, the drop in mileage (and performance) could be explained by heavier unsprung weight of the larger tire/wheel combo. Road & Track or Car & Driver did an article on this a few years ago.