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      11-06-2012, 05:30 PM   #17
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Drives: 2008 328i Coupe
Join Date: Sep 2009
Location: Houston, TX

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2008 E92 328  [2.50]
Originally Posted by just4kickz View Post
Originally Posted by J-Spec Dan View Post
that. you've got more rotating mass per corner with 19's + tires VS your 17s + tires. and you can't just look at one corner. you have to add up the weights in all four corners. also, your car is lower with 17s VS 19s so it could reduce drag.

and although the mpg calculator in our cars are nice, just keep track of how many miles you run on a full tank (or whenever you fill up). that'll give you a more accurate calculation.
Guys, I stated that I calculated these fillups. The trip computer was not used, even though it was only 0.1 mpg off.

Originally Posted by TroyDeJ View Post
It's not the wheel weight as much as the rolling resistance from having a wider tire, this is why economy cars have very narrow tires
This is the conclusion i've come to as well. It sucks because wide is way, way, way better looking.

Originally Posted by MikeR1994 View Post
Exactly. The width of the tire alone creates more resistance since the contact patch is so much larger.
Another major factor is the chemical compounds in the rubber. "Eco" tires may sound like their production might have a smaller carbon footprint or something, but they're called that because their rolling resistance is reduced because of the chemicals in the tire. Porsche and BFG engineered a sort of eco tire that still grips like hell for that new hybrid 918 or whatever it's called. I'm curious to see what a set would run for.
I love new tire technology and can't wait for some new stuff to come out, especially the Z2 Star Specs

Originally Posted by CombatNinja View Post
FWIW, I recently switched out the heavy RFTs on my e91 328 for very light Conti DWs. I think the weight were around 27F, 29R for the RFTs and around 21F, 24R--getting close to 25 pounds of rotating weight as far from the hub as possible. My mpg went up by anywhere from 0.5mpg to 1.2mpg per tank. Nothing like what you are seeing. Of course mine were stock sport pack sizes...
I don't know why the difference is SO dramatic. I will have to see when I actually finish the tank since it's all speculation right now.