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Monday, October 20, 2014

Doug's Domain

Doug Vetter, ATP/CFI

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Sunday, October 31, 2010

Return to Snow Tires

(Image: Closeup of Pilot Sport A/S Plus wear pattern)It's that time of year again so in spite of a blustery fall day with temperatures in the mid 50's (cold considering it was 75 degrees here last weekend), I put the E36 in the garage and swapped the set of spent Pilot Sport A/S Plus for the set of Winter Sport M3 including the refurbished wheel.

This was the first time I'd been under the car since the rear suspension overhaul so I spent some time looking around. Aside from the fact that a light accumulation of dirt had managed to reduce the luster of the new parts, I found everything in good shape. I cleaned all the hubs and applied a new film of anti-sieze before I installed the wheels, torqued the wheel bolts, and pumped the tires up to 32 PSI.

A close inspection of the Pilot Sport A/S revealed significant wear to the inside edges of both rear tires. I've always had pretty even wear on the rear tires of this car in spite of an alignment involving two degrees of negative camber so I can only assume that this was due to scuffing as a result of the misalignment in toe caused by the failed trailing arm bushings. And surprise, surprise. The left side was worn to a greater degree than the right, which is consistent with the fact that the left side bushing failed first.

MPG Trend

(Image: Out for lunch one crisp fall day with the snow tires mounted)As regular readers may recall I record some basic data every time I fill the tank including the amount of fuel, cost, and CONSUM1 and CONSUM2 numbers from the OBC. I reset CONSUM1 every time I fill up and never reset CONSUM2, so I wind up with a short and long term average.

The long term average in this vehicle has been remarkably consistent at 24.3 MPG. That was, of course, until I installed the 18" CSL reps back in 2006. Mileage dropped exactly 1 MPG to 23.3 like clockwork, no doubt because the wheels are heavier and the weight is concentrated farther away from the hub. Thus, the engine has to work harder to accelerate the vehicle and some loss of efficiency is expected.

In fact, I was getting 23.3 consistently in April and May of this year. After the rear suspension overhaul, however, I noticed an interesting trend: the mileage kept getting better and returned to 24.3 a couple months after the overhaul. I have never seen it this high while equipped with the CSLs. Since the change in gas formulation occurs around here in April I think I can negate its effects and instead attribute the increase in MPG to a loss of friction in the rear end. Not surprising when you consider that I replaced every moving part back there.

Of course, this is only a theory, and I hope to use the mileage data of the next several months to confirm it. If my theory is correct I expect MPG to increase to roughly 25 MPG while equipped with the smaller 16" wheels.

Mileage: 199283