I'm looking for some help in getting solid advice on what tires people have run on wider wheels. I have looked over this chart (https://members.rennlist.com/flyingdog/928 Wheel Fitment.htm) many times and it doesn't quite have all the information I need.

Background: I bought some OZ Allegerita wheels from a fellow RLer for a '90 S4, to replace some Mille Miglia Cup 3 turbo twist lookalikes (very heavy wheels). The front wheels and tires are good and installed with no issues. I did not pay attention to the tires that are on the rear wheels and, at 315/30 R18, there's no way they're going on the car. The rear OZ wheels are ET63, 11". From the RL chart above it looks like 285 tires will work but there's no ratio given. If I look at this website (https://tiresize.com/calculator/) 285/30s will give an overall width of 11.2", wider than the 10.4" of the current setup. However, that website only recommends that tire be fitted on up to 10.5" wheels. The next size up for 285s is a 285/35, giving an overall width of 11.2". The recommended wheel width in that case goes up to 11".

So, is that the right choice for 285s? Any experience running those on here?

I really wanted the tires that came on the wheels to work, as they're cool looking Toyo Proxxes R888s but that's obviously not happening. It also looks like I'll have some used 265/35 and 315/30 tires for sale...

All help is appreciated.

Cheers

 

285's are best suited to 10 inch rims. The 315 section will work fine on a 11 inch rim the only question being whether there will be interference. Unfortunately I do not have the spacial measurements I once made to hand so cannot advise whether the 315 will fit in but given an offset of ET63. I would think there is a good chance it may work. On my S4 with a rear offset of ET60 and a 295 section fitted there was ample clearance. I understand a 315 can fit the S4 but the caveat is I am just not sure of the offset needed to achieve such. I have something in the back of my heat that ET70 may be needed to achieve this- doubtless one of our listers should be able to give you more definitive input.

Needless to say if have made a trial fit and they will not go -end of discussion on that point.

 

Did the previous person have them on a 928? Have you bolted them on just to see? I agree it's most likely too wide, but I'd be curious by how much it's off. Ever consider having the fender lip rolled?

I was down this road recently and you are correct, there is not 285/30-18 which specs a 11" whee. But that doesn't mean it cannot be done, "stretching" a tire on a smaller rim is very common, but you need a tire person that knows what they are doing.

Another thing to note, not all tires of the same "size" are actually the same measured size. You can like up 10 tires from different brands / models of the same ratio (in this case 285/30-18) and there could be as much as a 1/2" or more difference in real world width.

I have 11" et65 wheels on the back of my 87 and I really wanted to go 305 or 295 for the same reasons you are dealing with, that 285's are not "rated" for an 11" wheel and I was paranoid it wouldn't fit without rolling the fender.

I ordered 285/30-18 Sumitomo HTR Z iii's since I knew my local shop could make them fit (not kidding about this, make sure you have a tire shop you can trust. If you are not sure, figure out the place that does the custom low rider and extreme camber cars).

Bottom-line, I'm not happy with this setup. Stretching a tire makes them look smaller (which the low rider and extreme camber scene goes for).

Luckily I have 10" wheels going on my 79 that also needs tires so these 285's will go there and I'm going to order 295's for my 87. As you can see, I have plenty of room for a slightly wider tire.
I bought these tires knowing if I didn't like how they fit, they would go on my 79.

As for the "stretch" look, you can really see the effect on the inside of my rim below. That "look" is actually popular on the outside of the rim in some custom car scenes.

Your wheels are 2mm further inboard compared to mine. But again......all these calculations could be out the window if the tire you go with is wider than mine. Fun huh?

Every 928 fender is not 100% the same either. One of my cars I was running a very tight wheel / tire combo years ago. In order to make it work I literally pulled the fender out by hand. Sitting on the ground with my feet up against the rim, I grabbed the bottom of the fender (wearing thick gloves) and pulled. Gave me maybe 1/8" more clearance and you couldn't tell anything from looking at it.

 

Thanks to everyone for the responses so far. Because I am sometimes a dolt, I didn't even think to look at the tire size yesterday before I went to bolt on the new tires. I was so focused on getting the wheels to have the right offset that I just didn't pay attention to what was on there. With what's on there now the inside of the tire hits the wheel well enough so that I can't seat the wheel fully on the hub. It's too much wheel, for sure, but holy ice cream does it look meaty off the car. From trying to eyeball things as I was getting the wheel on the car it looked to be about 1/4" away from seating on the hub. The tire size calculator linked above indicates that the 315 tire/wheel combo is about an inch wider than what's on there now (265/35 R18, 10") so getting something that will end up narrower is needed.

These wheels and tires came off of some flavor of 911 but it's not clear to me either the series or the flavor (C2, C4/Turbo, GT?). I considered swapping these with the Carrera lightweights I have on my 996 C2 but it's equally unclear to me if that car will take the wheel/tire combination. I do think that the Carrera lightweights would look killer on the 928 but I'm trying to avoid expanding the scope of my insanity.

 

Get some washers, start stacking them on the wheel studs until they fit on the inside and see where you are with the outside fender lip.

Yes it's safe to tighten the lugs down and lower the car, I just wouldn't drive it like that (all though plenty of people do...)

Very common method to figure out exact wheel spacer width when doing custom work.

 

That's a good idea, thanks!

 

I would recommend going on to tire rack like you have done and looking up the actual tread width of the tires. The 285 can actually vary from company to company. Some 225s fit the same as 245s for example.

I always try to match tread width as close as I can to the wheel width to get the best fit and most optimal tire contact patch.

It also helps keep you from paying extra for tires that are wider than you need that will wear down funny.

I would go for the 35 ratio tires or even a 40 like I have on the set of 18s I have. They have a bit more give than a lower ratio tire.

 

Look up section widths by tire model at tire rack.

 

Then your speedometer is off, and the overall combo looks huge compared to stock, and fitment actually becomes an even more complicated issue.

 

I fitted 255 to the rear of my car so the ratio is different. So yeah, a 30 would be a better fit for that width or else you will have speedo accuracy issues as Hacker has stated.

It slipped my mind that the middle ratio number is the sidewall height relative to the tread width in a percentage.

 

From a technical perspective [ i.e. manufacturer's spec] as I am aware a 285x30x18 has no place on an 11 inch rim as they are limited to a 10.5 inch rim. That being said fitting such size on an 11 inch rim is a bit of racer's trick- presumably to gain a small advantage at the expense of something else whatever that may be. A 285x35x18 section can be mounted on an 11 inch rim.

I run a 285x35 section on my 10 inch rims and one is not aware of any physical/visual difference. I suspect a 285x40 would be excessively and look somewhat odd but each to his own of course. The 285 dimension is supposedly a given in that it represents the section width of the tire when it is loaded to maximum specified load at the maximum specified tyre pressure. Tread width can and does vary make to make. Typical tread width for a 285 tyre is about 10 inches but for DOT legal tyres you will find the width is more like 10.5 inches and racing slicks more like 11 inches of tread width..

A 285x35 will increase the actual speed relative to that indicated by about 4%. The interesting bit would be "which size is more accurate" given speedometers tend to be calibrated a bit optimistically to avoid litigation attempts.

Not sure if the OP has tried to fit the 315's he mentioned. As indicated earlier I suspect they will be a tad too wide but not by much with that offset and no spacers.

 

911 wheels have a front offset of 50-53 and is not a good match for the suspension geometry of the 928 where you really want an offset in the mid 60s or minimum high 50s.

I'd say start over with a correct set of wheels and tires.

 

Thanks everyone, for the great discussion.

FredR: I did try to mount the currently installed tires and there's a definite issue with clearance on the inside of the wheel well. I did not pay attention to clearance on the outside and so I have been mulling over the use of spacers, provided that there is room on the fender side of things. I like the idea of testing things out with washers as spacers.

Petza914: I think I can stick with the wheels based on the historical knowledge I've found on here but, of course, you may end up being right (which would cause some consternation in this here household). The fronts are not an issue at all and look great on the car. I think it's really just a matter of getting the tire choice right. I'll poke around further on TireRack and update things as I find out.

 

Not too surprising as I think about it. I had plenty of clearance with ET60 and a 295 section between the tyre and the fender - presumably the added width is just too much. If you can get the wheel in with spacer fitted you then have to pull the hub apart and fit new longer studs unless you can fit those modular spacers which I doubt would be possible without hitting the fender as those have a minimum thickness to accommodate the recessed bolting.

 

I had an 8" et50 Carrera Lightweights on my car when I bought it. Hit an average speed, mid-coener bump and the tire grabbed the inner fender lip. Luckily it only damage the lip and not the fender. After that event I changed to an et57 front wheel, wh ih wouldn't seem like much of a difference, but is.

If your going to use those wheels at least roll the front fender lips to get them tucked up and layed flat on the inner edge.