Good and bad idea? (archive)

The taller tire will reduce wheel thrust, which will reduce acceleration force to some degree. However, in this case, going to the 3.46 not only overcomes the difference, it will actually net a positive result.

Since wheel thrust is directly proportional to tire diameter and gear ratio, you can easily compare the two systems:

Tire rolling radius increase in % = +3.82
Gear ratio increase in % = 7.1

Essentially you end up with a gain of 3.3%, which will reduce top speed slightly, and increase RPM at all speeds by that amount. So at 60mph actual, your speedo will read 62mph. Not too bad.

Now, the only other losses associated with this are the increase in sidewal flex, and tire weight. But, with a 3% gain from the gearing improvement, in terms of in-line acceleration, you will still be better than stock by the tiniest of measures, at the very worst, you will be equal to it. This assumes you select a very high quality tire in order to gain from structural advantages to overcome a slight increase in sidewall height.

The only real problems I see with this is:

A. Tire clearance. The 275 is a very tight fit to the outside wheel arch, specifically on the drivers side in most cars. May require the bead be rolled under to provide clearance. The M cars have quite a lot of room to the inside, but you can't utilize it with stock rims backspacing.

B. This is a very expensive solution in order to gain a little more tread width. The rebuilt rear diff, with the new gear set can run as much as $2,500. This also requires professional shop work to install the components.

A less expensive approach, to get the width, might be to go plus-one to 18" wheels as follows:

245/35 x 18 Front on 8" wide rims
285/30 x 18 Rear on 10-11" wide rims

This limits you to Pirelli P Zero Assymetricos (245/35 x 18 size availability is a bit slim), but that's fine, as they are excellent tires. The resulting change in gear ration is essentially nill at 0.6%. Judicious wheel selection should even net you a net reduction in wheel/tire weight to boot!

I would think this would be a less costly and certainly an easier way to get the job done, and will result in an improved aesthetic as well.

Only negative with this direction? The resulting ride will be noticably harsher and the steering a bit twitchier. Normally the shorter sidewall on the rear would result in it being looser, but the gain in tread width and tire quality should easily overcome that and net a positive result. Turn-in when cornering will be improved, much quicker, but so will feedback throught he steering of road surface changes.

So, that's the long and short of it, I think anyway.

OK so I was using the gear speed calculator program and I put in the M's stock 245/40 with the 3.23 diff...

Then I upped the tire size to 275/40 and looked at a 3.46 diff that someone (sorry forgot who) was talking about... looks like there isn't a whole lota change.

My question for the genious is how would the 275/40 3.46 setup compare to the stock setup in terms of acceleration all weight differences aside. It looks like it would only drop the top speed by 5mph or so.

Thanks,
Pat

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