Tire size question/concern

BarryC · 09-14-2008, 06:56 PM

Hi all,

I'm in the process of adding 20" wheels to my car and was looking for some info regarding sizing and diameter. Based on the tirerack website the stock 19" tires have a diameter of 25.8" front and 26.3" rear. If I go with a 275/30/20 front, the diameter increases to 26.5" and a 295/25/20 rear the diameter goes to approx. 25.8" rear. Anyone know is this will create and issue?

I could go with a 255/30/20 front and 285/30/20 rear and the diameters are 26.1" f and 26.8" rear but I'm concerned about increasing the diameter on the rear as it will reduce acceleration a bit as it will increase the effective final drive ratio. Any suggestions?

TIA!
Barry

**ben@tirerack** · 09-15-2008, 05:15 PM

275/30R20 on the back would reduce the height and width a little bit, but the bigger issue here is the 20" wheels. Before you worry about the effect of a fraction of an inch diameter on acceleration, worry about the weight and rotational inertia of your 20's.

BarryC · 09-15-2008, 05:46 PM

Quote:

Originally Posted by ben@tirerack

275/30R20 on the back would reduce the height and width a little bit, but the bigger issue here is the 20" wheels. Before you worry about the effect of a fraction of an inch diameter on acceleration, worry about the weight and rotational inertia of your 20's.

Thanks Ben for the info! The wheels should weigh less than OEM and I'm looking into titanium fasteners to reduce weight at the rim (where it matters most).

Thx,

BC

swamp2 · 09-15-2008, 05:51 PM

Quote:

Originally Posted by BarryC

Thanks Ben for the info! The wheels should weigh less than OEM and I'm looking into titanium fasteners to reduce weight at the rim (where it matters most).

Thx,

BC

Titanium is substantially more dense than aluminum.
Even at the same weight a larger wheel has a larger moment of interia and hence behaves as if it weighs more.
Wheel put together with bolts are generally weaker than 1 piece wheels.
20" will ride poor and perform worse than 18" or 19". They are for looks only. 19" is a good compromise between looks and performance. The 18" probably slightly outperform the 19".

09-14-2008, 06:56 PM	#1
BarryC Major 121 Rep 1,198 Posts Drives: '15 M3 Join Date: May 2007 Location: Kernersville NC iTrader: (12)	Tire size question/concern Hi all, I'm in the process of adding 20" wheels to my car and was looking for some info regarding sizing and diameter. Based on the tirerack website the stock 19" tires have a diameter of 25.8" front and 26.3" rear. If I go with a 275/30/20 front, the diameter increases to 26.5" and a 295/25/20 rear the diameter goes to approx. 25.8" rear. Anyone know is this will create and issue? I could go with a 255/30/20 front and 285/30/20 rear and the diameters are 26.1" f and 26.8" rear but I'm concerned about increasing the diameter on the rear as it will reduce acceleration a bit as it will increase the effective final drive ratio. Any suggestions? TIA! Barry
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09-15-2008, 05:15 PM	#2
ben@tirerack M3Post Supporting Vendor 87 Rep 964 Posts Drives: 740i Join Date: Mar 2008 Location: South Bend, IN iTrader: (0)	275/30R20 on the back would reduce the height and width a little bit, but the bigger issue here is the 20" wheels. Before you worry about the effect of a fraction of an inch diameter on acceleration, worry about the weight and rotational inertia of your 20's. __________________ _______________________________________________ I have accepted a new position at Tire Rack and am no longer in the sales department. gary@tirerack.com is available to assist with M3 questions.
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