Vendetta

There's a lot of posts about wheels and weight lately, cause and effect and so on. I just changed up the rims on my CL600 - I had the 18" AMG 5-spoke, and I upgraded to a 19" yet lighter split-spoke design. Tires were all-season Pilots and are now Goodyear Eagle F1 GS-D3s. After a week and nearly a 1,000 miles (NY-VA round trip), here's my experience.

1.) The ride is definitely sportier, i.e. rougher - but not significantly more than the 18s inflated to 40 PSI, which is about what I had anyway. When fall comes and I put the 18s back on with the all-season tires, I'll reduce the pressure to the 32 range. That way, the difference will be much easier to appreciate. (And by then, I'm sure I will appreciate it.)

2.) Road handling on the Goodyear F1's is obscene. If there were any questions about the car's demeanor around corners or long, tempting highway exit ramps, these tires put them to rest handily. Their threshold is seemingly far outside my experience or ability to test it.

3.) I find myself keeping a sharper eye out for road debris and potholes, but that's because they're brand spankin' new and will probably wear off with time.

4.) The car looks a lot sportier, obviously (see pics in the other thread). It turns a margin more heads, so the "sleeper" factor of the car is probably decreased by the same margin.

5.) I doubt anyone on here is losing sleep over fuel economy, but I was pretty surprised with what I saw on the trip computer today... The wheels weigh about 25lbs as opposed to the 30-something that the AMG's weigh. What surprised me is the new fuel economy. The same trip I routinely take from NY to VA on the old wheels got me 22.4-6 MPG, while the new wheels got me 24.5 MPG! These are true numbers, and include both the city and highway legs of the trip. Average speed according to the car was 63MPH, but cruising speed on the highway was between 70 & 80 MPH. Total distance was 328 miles.

So AMG2GO, damn - if you go with the 20-pounder RENNs, your gonna qualify for a freakin' Green Vehicle federal tax credit. :rollseyes:

-V

vraa

I need to get you a set of Miche PS2's.
They are considered to be a level above the Goodyears.

techbike

V, I think the jump in mileage is probably due to the change in wheel/tires. Although the +1, +2 concept says the overall diameter is the same, it reallyis a tad smaller. I experienced the same when I switched from 17s to 19s on my CLK55. But hey, it feels good don't it?

Vendetta

Right? Definitely gotta be the weight, Techbike. I'm sayin, if AMG2GO get's those RENN Monolites, damn..... 20lbs per wheel? HA! I can't imagine.

Hey, did you ever get to the dyno with the Mini group? Would love to know how it went.

Ciao neighbor,
-V

AMG2GO

If the diameter is smaller, then the affected gear ratio would cause the engine to actually rev more... and MPG would be reduced. The factory rear setup of 275/40/18 is 677.2mm in overall diameter, the new 19" setup of 275/35/19 is 675.1mm. Thats a loss of 2.1mm, which is insignificant. But if it is to have an affect, it would give slightly better acceleration, but lower MPG.

Differing MPG can be related to any number of things including outdoor temprature, slightly more or less stop and go, and a slightly more frisky right foot on one of the days. Not to mention maybe he ate more baklava one day and weighed the car down more... hehe But if these results are consistent, I would bet that what is actually making the difference is nothing other than wheel weight.

AMG2GO

And don't forget once you change tire sizes you can go into COMAND and tell it what your new tire sizes are. I am not sure what this affects exactly, probably more for the nav but it may actually signal other computers in the car. It is in the service menu of the COMAND system.

Vendetta

Oh, no doubt. The first thing I did was come on here and search for "calibration" to find out if Navi reads front or rear wheel rotation. (Front, by the way.) Love this forum!

techbike

Well, not necessarily. Because of the smaller diameter, although the engine revs a little bit more, it also works less at the same cruising speed; which could equate to more MPG. May also put the cruising rpm at a slightly more advantageous spot on the torque curve, further affecting MPG. Nonetheless, less rotational weight with a lighter wheel will definitely show some performance advantage, hopefully without sacrificing strength.
Very evident when at one autocross race recently, I switched wheels/tires for the afternoon runs from 18 lb. Minilites with street tires to 11 lb.Revolite magnesium alloys with Goodyear slicks and cut my run time by 2 full seconds. (slicks probably had something to do with it too)

techbike

Nope never got to do it yet. I'm a bit disillusioned with my Mini club at the moment. But that's another story. Thanks for asking.

AMG2GO

Techbike more revs = less MPG. It does not matter where the torque curve is... a lower diameter wheel always means more engine revolutions... and since weight, rolling resistance and aerodynamics remain the same, MPG is always reduced. Your acceleration will always improve, and your MPG will always go down.

techbike

We have no disagreement in theory; just my experience when I did the switch on the 55 from 17" AMGs to 19" SSR GT3s. My hiway MPG went from 21 to 23.5 so I'm offering some possible reasons; and I'm no engineer so I really don't know.

AMG2GO

Well that is inline with what we have been saying. You went with a lighter wheel... and MPG improved. Your overall diameter may have been the same, or more with the 19" which again would improve milage. Are you saying you dropped overall diameter (including tires)?

I think you may have misunderstood, I am saying if the overall diamater (including tires) is significantly smaller, that means more revs and that means lower MPG.

Vendetta

I'm no engineer either, but it seems that just having the lighter wheel was enough to make an impact on fuel economy. The overall diameter difference between 275/40/18 and 275/35/19 is statisticly negligable (less than 3mm?) for purposes of calculating RPM impact on fuel economy based on wheel size. I'd think one could drive over a wad of chewed gum and have more measureable results. Anyway, the MPG went up by literally 10%. That's not negligable, so I thought I'd share with the rest of the class.

(Oh, did I mention that I had my windows down, too? Let's see... how do I factor for adjusted drag coefficient to respect the decreased efficiency, respecting the increased drag induced by wind baffling while travelling at ~75 MPH?)

-V

vraa

Possibly lighter rotational mass = less work required for the engine to move X amount of distance.

Stop it damnit, you're making me actually think about Physics.

Vendetta

Hey, I'm just happy that I could theoretically get 40 more miles per tank. At the current fuel prices, these rims'll have paid for themselves by what, next Tuesday?

-V

vraa

Hahaha! Man you lucky SOB.
I'm getting 8 to 9mpg. Perhaps 10 on a good day.
I need to change my driving habits.

Vendetta

Brother, don't we all....