[Max Well]

I've had trouble finding information showing how wheel size and tire combinations affect aerodynamics and rolling resistance at speed, so I was hoping we can collect data from our forum's experience. As city driving styles and conditions vary those values won't help for this question. But there should be reasonable consistency at highway speeds with cruise control if the grade is reasonably level during the sampling. So if anyone has a chance to gather the information needed in the chart below we might be able develop a better understanding. Ideally the grade should average out to be relatively level over the course of the route, and longer distances should even out elevation changes.

[Bimar]

Following..

[Max Well]

Originally Posted by X4guy

Following..

Thanks, X4guy. I really hope we can get some contributions from all over as it'll be easy for me to make additions and then simply edit my opening post so we always have the most recent compilation at the beginning. With Excel I can keep the models grouped separately (30d, 30i, M40i, ...) and then subgroup based on tire sizes and then sort to speed as well. Should prove to be very interesting, and the more data contributions the more sound the analysis.

[ZHPsedan]

Originally Posted by Max Well

Thanks, X4guy. I really hope we can get some contributions from all over as it'll be easy for me to make additions and then simply edit my opening post so we always have the most recent compilation at the beginning. With Excel I can keep the models grouped separately (30d, 30i, M40i, ...) and then subgroup based on tire sizes and then sort to speed as well. Should prove to be very interesting, and the more data contributions the more sound the analysis.

We'll have around 900 miles to report on with 19" summer run-flats once we take delivery at Performance Center in early January.

[bwcooper]

Originally Posted by Max Well

Having no luck finding definitive information on the affects of wheel size and tire combinations on aerodynamics and rolling resistance at speed, I was hoping we might be able to collect data from our forum's experience. This is not an attempt to necessarily laud the G01's fuel efficiency as it is a performance vehicle (although commendable, the thought of this tends to rub some the wrong way), but rather to find how different tire sizes can affect aerodynamics and rolling friction. As city driving styles and conditions vary so much that won't help for this question. But there should be some reasonable consistency with highway speeds set at cruise control if the overall grade is reasonably level during the sampling. So if anyone has a chance to clear the mpg gauge at the start of a trip, one would just need to share overall hwy mpg (or whatever units you use where you are), hwy miles travelled, average hwy speed, and the X3 model type and wheels/tire information. Ideally the grade should average out to be relatively level over the course of the route (as going consistently up or down a grade for 20 miles will skew the data).

A rather crude analysis (doesn't take into account vehicle weight, for example) but it might be able to shed some light on how wheel/tire sizes affect aerodynamics and roll resistance.

Great idea.

I assume you expect folks to just report the numbers the car reports on the trip status vs needing to check their math by refilling gas tank and computing mileage from that--the way we had to do it before all this instrumentation. I've had some cars that seemed to come up with different numbers than I ever got. With only one fill up so far, I've not had a chance to see how accurate my M40i trip computer is, though given the advanced monitoring they have nowadays, I'd be surprised if it isn't right on.

[skyblue2002]

Originally Posted by Max Well

Having no luck finding definitive information on the affects of wheel size and tire combinations on aerodynamics and rolling resistance at speed, I was hoping we might be able to collect data from our forum's experience. This is not an attempt to necessarily laud the G01's fuel efficiency as it is a performance vehicle (although commendable, the thought of this tends to rub some the wrong way), but rather to find how different tire sizes can affect aerodynamics and rolling friction. As city driving styles and conditions vary so much that won't help for this question. But there should be some reasonable consistency with highway speeds set at cruise control if the overall grade is reasonably level during the sampling. So if anyone has a chance to clear the mpg gauge at the start of a trip, one would just need to share overall hwy mpg (or whatever units you use where you are), hwy miles travelled, average hwy speed, and the X3 model type and wheels/tire information. Ideally the grade should average out to be relatively level over the course of the route (as going consistently up or down a grade for 20 miles will skew the data).

A rather crude analysis (doesn't take into account vehicle weight, for example) but it might be able to shed some light on how wheel/tire sizes affect aerodynamics and roll resistance.

I would also like to know the weights of the different OEM wheel/tire options. I assume the 21” package is the heaviest and the 19” the lightest. The lower rotational inertia of the lighter set-up should contribute to better economy and performance independant of rolling resistance and/or aerodynamics.

[Max Well]

Originally Posted by ZHPsedan

We'll have around 900 miles to report on with 19" summer run-flats once we take delivery in early January.

Sounds great, ZHPsedan - know you're looking forward to it!

Originally Posted by bwcooper

Great idea.

I assume you expect folks to just report the numbers the car reports on the trip status vs needing to check their math by refilling gas tank and computing mileage from that--the way we had to do it before all this instrumentation. I've had some cars that seemed to come up with different numbers than I ever got. With only one fill up so far, I've not had a chance to see how accurate my M40i trip computer is, though given the advanced monitoring they have nowadays, I'd be surprised if it isn't right on.

Thanks, bwcooper. I think for the purposed of this analysis just simply clearing the fuel econ gauge at the start of the hwy journey, then after x number of miles of relatively even grade (give or take) at set cruise control with minimal braking and passing acceleration events needed, just read off the number on the meter (green arrow in attached photo) as I did should be fine. Since I knew the RPM information I went ahead and added that to the chart which I'll re-post to first entry in a moment, as that might be interesting to see how the various engine sizes and fuel types require different RPMs at speed. Not a big deal if someone forgets to provide that. So it should be really easy. Only caveat being making sure roughly similar elevation points at start to finish. The longer the distance the better it would seem as that would iron out any hill climbs and/or passing/braking events. But even as little as 20-30 miles would at least be usable information it seems.

Originally Posted by skyblue2002

I’d also like to know the weights of the different OEM wheel/tire options. I assume the 21” package is the heaviest and the 19” the lightest. The lower rotational inertia of the lighter set-up should contribute to better economy and performance independant of rolling resistance and/or aerodynamics.

Also a good point, skyblue2002, so I have added that at the top of the spreadsheet. The yellow-highlighted cells represent information I don't have, so if our European colleagues can share the information on the 18" rim/tire sizes, and anyone can add the weights for each wheel/tire combo that would be great.

[ZHPsedan]

Only thing with these early numbers is that my Ecoboost F150's mileage increased significantly after 1000 or so miles. Went from 15 MPG to 18 or so avg after break-in.

[Sharpedge]

Originally Posted by Max Well

Sounds great, ZHPsedan - know you're looking forward to it!

Thanks, bwcooper. I think for the purposed of this analysis just simply clearing the fuel econ gauge at the start of the hwy journey, then after x number of miles of relatively even grade (give or take) at set cruise control with minimal braking and passing acceleration events needed, just read off the number on the meter (green arrow in attached photo) as I did should be fine. Since I knew the RPM information I went ahead and added that to the chart which I'll re-post to first entry in a moment, as that might be interesting to see how the various engine sizes and fuel types require different RPMs at speed. Not a big deal if someone forgets to provide that. So it should be really easy. Only caveat being making sure roughly similar elevation points at start to finish. The longer the distance the better it would seem as that would iron out any hill climbs and/or passing/braking events. But even as little as 20-30 miles would at least be usable information it seems.

Also a good point, skyblue2002, so I have added that at the top of the spreadsheet. The yellow-highlighted cells represent information I don't have, so if our European colleagues can share the information on the 18" rim/tire sizes, and anyone can add the weights for each wheel/tire combo that would be great.

I'll be happy to provide info as I do a 90mi commute 3 times a week. Half motorway and the other half on minor roads.

20d and 30d due to hit the streets in a few weeks but M40i is looking like it will be delivered in the new year. My order has been in for about 6 weeks and I still don't have a build slot.

[Max Well]

Originally Posted by ZHPsedan

Only thing with these early numbers is that my Ecoboost F150's mileage increased significantly after 1000 or so miles. Went from 15 MPG to 18 or so avg after break-in.

A good point to consider as well, ZHPsedan - this will be very easy for me to edit at any time, so additional break-in numbers might be useful to know as well. I'm thinking I might want to add Forum Member names as another column so one could cross-reference as time goes on...

[Max Well]

Originally Posted by Sharpedge

I'll be happy to provide info as I do a 90mi commute 3 times a week. Half motorway and the other half on minor roads. 20d and 30d due to hit the streets in a few weeks but M40i is looking like it will be delivered in the new year. My order has been in for about 6 weeks and I still don't have a build slot.

Thanks, Sharpedge, that'll be great - I hope we can get all models represented here.

[Max Well]

Using the website sor provided in his 20 Nov 2017 1014AM post in thread http://x3.xbimmers.com/forums/showth...1433123&page=5 , found at https://www.etkbmw.com/bmw/EN/search...3+M40iX/USA/36 , I was able to compile almost all of the wheel (not tire) weights. There were only five missing (in yellow) so hopefully someone can find those to allow chart completion. I also added a column to now allow wheel option to be shown with the individual data sets.

I spent a fair amount of time trying to do the same for tires but it's just too difficult to find exact weights since some of the tires seem custom to BMW and there are so many of them. If some can find definite weights for specific tires just post them here and I can compile as well.

So as an example, the 698M 19" total 107.6 lbs and the 699M 20" are 130.4 lbs total G01. Once the tire weights are known then maybe someone with knowledge (? skyblue2002) can share the rotational inertia figures and explain whether it would translate into a significant performance difference.

[sor]

It might be good enough to just come up with an average weight for the stock tire size. I know they're going to vary due to tread design, but perhaps a placeholder for now (or even calculating the highs and lows) will help determine whether the small differences between individual tire selection make any difference.

For example, looking at a variety of 245/50 R19 I see they're between 30-33 lbs. Looking at a variety of 245/45 R20 they're 28-30 lbs, and 245/40 R21are 27-30 lbs.

[Max Well]

Thanks, sor - that crossed my mind, but in seeing the fairly large spread on my worksheet (attached) even for the same tire just depending on brand, and then the error in guessing if it was close to BMW's specs, I was hoping someone with knowledge of BMW tire weights could share a definitive list. You would think that type of tire spec sheet has to be somewhere, and it would sure make this simpler and more accurate, especially if only 5-10 lbs has a significant effect at the wheel (I have no idea if it does or doesn't).

[skyblue2002]

Originally Posted by Max Well

Using the website sor provided in his 20 Nov 2017 1014AM post in thread http://x3.xbimmers.com/forums/showth...1433123&page=5 , found at https://www.etkbmw.com/bmw/EN/search...3+M40iX/USA/36 , I was able to compile almost all of the wheel (not tire) weights. There were only five missing (in yellow) so hopefully someone can find those to allow chart completion. I also added a column to now allow wheel option to be shown with the individual data sets.

I spent a fair amount of time trying to do the same for tires but it's just too difficult to find exact weights since some of the tires seem custom to BMW and there are so many of them. If some can find definite weights for specific tires just post them here and I can compile as well.

So as an example, the 698M 19" total 107.6 lbs and the 699M 20" are 130.4 lbs total G01. Once the tire weights are known then maybe someone with knowledge (? skyblue2002) can share the rotational inertia figures and explain whether it would translate into a significant performance difference.

“Dammit Jim, I’m a Doctor not a physicist...”. In all seriousness, I think the physics are pretty involved and we would probably have to know the wheels’ weight distribution to do any calculations. Anyone with more knowledge feel free to correct me, but I believe it all revolves (pun) around the moment of inertia. More mass further out from the axis of rotation requires more energy to change the velocity. The classic example is ice skaters doing a spin - when they pull their arms and legs in they spin faster - the energy already there goes into the velocity as the mass becomes centralized. Ideally, a wheel would have all of its mass at the axis but this would obviously be a structural challenge...

[Max Well]

Originally Posted by skyblue2002

“Dammit Jim, I’m a Doctor not a physicist...”. In all seriousness, I think the physics involved are pretty involved and we would probably have to know the wheels’ weight distribution to do any calculations. Anyone with more knowledge feel free to correct me, but I believe it all revolves (pun) around the moment of inertia. More mass further out from the axis of rotation requires more energy to change the velocity. ...

- I've always wanted to use that and this seemed appropriate, well-written!

And I don't want to lose the forest for the trees on this, but I do think there is a need to understand some basics of what happens to the G01's performance curves when we choose certain wheel/tire sets. And I'm not referencing high-end competition track performance as one would have on the Ring, but just for understanding daily 'high' (not city) speed drives on Interstates/Autobahns.

This shouldn't be a difficult question to answer but unfortunately the data sets aren't available for us to make rational decisions. Do I go for the great look of the 'x' wheel/tire or do I get the most aerodynamic profile with 'y' set? And I would suggest to those wondering why someone buys a precision performance vehicle and is discussing this - it isn't always about the fuel economy but rather most aerodynamic profile on a straight-away. End result is fuel economy, but aerodynamic efficiency is my interest.

[ando]

30i, P, 692, 19x7.5, 245/50, All season RFT, Pirelli P7, 500 highway miles, 75 mph, 26.3 mpg, Eco Pro Mode

[Max Well]

Originally Posted by ando

30i, P, 692, 19x7.5, 245/50, All season RFT, Pirelli P7, 500 highway miles, 75 mph, 26.3 mpg, Eco Pro Mode

Excellent, ando - thanks for sharing! The more independent and varied data we record the more effective and reliable the information will be. I have expanded the spreadsheet to now include contributor, tire pressure (which has an effect on rolling resistance), and total rim/tire weight to aid in comparing. If you happen to recall your tire pressures (and estimated rpm at that speed), ando, that would be great but if not that's OK.

And if anyone can find the unknown data in the yellow cells it would be appreciated. I can't find it anywhere, but it would be best to have a complete data set if possible.

And although we only have a(n) N=2, already some interesting comparisons. Fascinating that the M40i has better Hwy efficiency, but only with a lot more data sets will any potential conclusions be able to be considered.

Thanks again for contributing to this, ando!

[Max Well]

Here are some links about the new Bridgestone Alenza 001 tire, which was designed exclusively to meet the demands of the growing premium SUV/SAV market. Wish I could find the weight of the 21" tires...

http://www.sgcarmart.com/news/events...s.php?AID=3395
https://www.bridgestone.com.sg/ALENZA_001.html
https://www.carid.com/bridgestone-ti...749825175.html
https://www.redbook.com.au/news/2017...?csn_tnet=true

[ando]

Originally Posted by Max Well

Excellent, ando - thanks for sharing! The more independent and varied data we record the more effective and reliable the information will be. I have expanded the spreadsheet to now include contributor, tire pressure (which has an effect on rolling resistance), and total rim/tire weight to aid in comparing. If you happen to recall your tire pressures (and estimated rpm at that speed), ando, that would be great but if not that's OK.

And if anyone can find the unknown data in the yellow cells it would be appreciated. I can't find it anywhere, but it would be best to have a complete data set if possible.

And although we only have a(n) N=2, already some interesting comparisons. Fascinating that the M40i has better Hwy efficiency, but only with a lot more data sets will any potential conclusions be able to be considered.

Thanks again for contributing to this, ando!

Thanks for organizing this data. Tire pressures were 37 all around. RPM was about 2,500. I noticed that too regarding the fact that the six might have better overall highway fuel economy where as the four cylinder would regain this advantage in the city. Still have a small sample size.

[Max Well]

Originally Posted by ando

Thanks for organizing this data. Tire pressures were 37 all around. RPM was about 2,500. I noticed that too regarding the fact that the six might have better overall highway fuel economy where as the four cylinder would regain this advantage in the city. Still have a small sample size.

Excellent, thanks again for contributing to this project, ando. As we get more figures from others with different wheel configurations, and some of us run in a different mode (Sport etc), hopefully some trends will begin emerging.

[Max Well]

After seeing ando's data I went back to the spreadsheet I set up in 2014 which looked at HP needed to maintain a certain speed. Since the G01 models differ in their Cd (30i has Cd 0.29, M40i Cd 0.33), I was curious to see how they stacked up against each other and the 2nd Gen F25 X3 35i with its Cd of 0.36. At higher speeds in this sim the aerodynamic differences become really apparent - so that the G01 30i at only 248 hp has a nearly identical top speed unlimited as the F25 35i at 300 hp, around 145 mph. Kudos to BMW's Engineering and Design team for their aerodynamic accomplishments with the G01!