How do you find which tire sizes fits on your rim?

That has become obvious.

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That is why you need to go to a wheel and tire specialty shop after, I repeat after you put the coil overs on.

When you search for wheels on tire rack you can select stock size and +1", sometimes +2" options. They list 18" as stock for yours, and list 8 and 8.5 front and rear:
http://www.tirerack.com/wheels/results.jsp?&autoMake=BMW&autoModel=330Ci+Coupe&autoYear=2005&autoModClar=Performance+Package

Ok but Iā€™m looking for tires not wheels.

Well Iā€™m obviously not going to mount the tires at home now will I.

And I donā€™t see why is it imperative to get my tires mounted on the rim after the coilovers, with coilovers Iā€™ll be able to fine tune and adjust so it doesnā€™t rub. Donā€™t get crazy tires and be in the recommended tire size for the rim and it should be good to go.

Iā€™ll go off-topic . . .

I hope you like the way the car handles after everything is installed

I know a few people that have gone down this road, and many of them have complained how harsh the ride was, and that the car bottoms out very easily. Theyā€™ve also messed up their bumpers BADLY, because the car was lower, and the damage occured when going into driveways, underground parking garages, etc. Just about anywhere and everywhere, actually

And these were just moderately lowered vehicles with very tasteful rims and tires

Iā€™m not condemning anybody, just pointing out there may be some consequences. But I guess you already know that, otherwise you wouldnā€™t have started this conversation

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Et is offset which moves wheel in/out. The gap u refer to is the clearance between tire circumference and fender lip which is increased with lower profile tires which is why u have coil overs to adjust ride height. If coil overs are available it probably means there is a website that caters to your car.

Wrong, wrong, wrong! Youā€™ve changed the wheel size based on forum info and you just want a tire that fits the rim and assume it will fit. NO. Tires are specā€™ed out to fit a range of rim widths and there are lots of considerations for clearance.

Lets take a 225/40/18 BFG Sport Comp 2 for example. This tire can be used on a rim 7.5 to 9 inches wide. It fits BOTH of your rims. a 255/45/18 tire of the same model ALSO fits (8 to 9.5 width) both rims. 2 inches taller and over an inch wider. Will it fit the car? At front and rear? Depends.

And since you chose a staggered width for front and rear, donā€™t you want wider tires for the rear? But the same diameter, of course. And what about the handling change that occurs from a staggered tire selection? How are you dealing with that?

You need to do more research.

Sorry I think you misunderstood. In that certain tire range for the rim, there will be a optimal sizes, thatā€™s what I meant.

And I know my rear tires will be not the same size as the front.

Nowhere in any post did you say that. We are not mind readers. And even now the clarification you offer is not helpful.

Optimal size can vary. There is no ā€œoptimalā€ size for your rims without all the other info. Diameter is a biggie for clearance radially and tire width can vary quite a bit for a tire designed to go on the rimā€™s you specified. We are not making this hard for fun, it IS hard to change tire and wheel size to other than a factory size.

As an example, I put 18x10 rims on my Mustang with 285/35/18 tires. A size not offered from the factory. I measured a bunch and researched forums until I was satisfied the combo would fit. It did, perfectly because I understood what all the numbers meant and made a tire template to verify clearance.

Your best bet, at this point, is to go back to the forum and ask what size tires others have put on their similar year 330ā€™s that fit without rubbing.

With the greatest respect, Iā€™m worried that youā€™re attempting to do something that if done without the proper knowledge will result in a safety failure. There are times when all of us have to use the expertise of someone else. This might be such a case.

Another poster also commented on ā€œcoiloversā€. If your goal in using them is to lower your vehicle, that also will affect your carā€™s safety. Wheels donā€™t move straight up and down as they go through their suspension range. They tilt (camber). This is intentional to keep the ā€œtrackā€ stable. If you lower the suspension, you may make it impossible to properly align the car. You might also relocate the static position of the tires to where they lack the space to move up and down and rub against mechanical components.

And, not to be redundant, but if you change the ā€œoffsetā€ too much you may overload the ball joints and/or bearings. These have to accept huge shock loads without failure. Just as a pry bar enables you to pull nails you couldnā€™t with just a hammer, increased offset also puts much more force on the joints.

We mean well in recommending a professional. Safety is at stake here.

First, the right way to do this is to find out what original tire size came on the car. That information will be located on a vehicle tire placard which is usually on a doorpost or in the glove box - but occasionally will be found on the fuel filler door or on the trunk lid.

From that you need to determine what load rating the OEM said was right for the vehicle.

Then look up what fits under the fenders. The easy way to do that is to use Tire Rackā€™s alternative size for your vehicle. Does one of those sizes fit your wheel? If so, youā€™re good to go. If not, it might be time for a re-think.

Just be aware that it is load carrying capacity which is the most important thing about tire sizing.

Sorry I got sidetracked on wheels. Tire Rack shows these 18" for the ā€˜performance packā€™ option, so theyā€™ll fit:
225/40x18 front/255/35x18 rear. The rears are listed as suitable for 8.5"-10" rim widths.
Is this the info you need? If not, why not?

Since we donā€™t know the bolt pattern or offset of the OEM wheels or the wheels the OP wants to buy, I think we cannot assume the wheels will be correct. The tires might fit in the wheelwell and mount on the rims, but Iā€™m more concerned about the wheels interfacing properly with the hubs.

Bolt pattern is good. The wheels mount, another guys will have the same wheels as mine.

Bolt pattern and offset checks out.

My real only question is tire size, the rest is fine.

And that is why they have tire shops. Stop messing around and just get tires.

donā€™t fool yourself by rim size alone .
itā€™s not a matter of what size tire fits the rim . .you already said itā€™s 18".
NOW itā€™s a matter of what size tire will fit on the CAR !
Suspension travel and turning radius will dictate the overall size of the tire . .
like texases said the 225/40 and 255/35 might work inside your wheel well.

Will do. I was just curious about pricing.