classy37

Guys, I am at my wits end here trying to match up front and rear tires. Any help is appreciated. I've searched and searched and put in hours of spreadsheet action and I don't know whether I've gotten any closer to the answer...

I just bought some OEM 19" S wheels and will be putting them on my AWD sedan. That said, I have 2 goals-
1) Keep front and rear total diameters AS CLOSE AS POSSIBLE. I put this in caps because I still have not seen a definitive answer as to what is an acceptable difference that will not harm the AWD. It seems reasonable that normal treadwear differences should not be an issue, but again, I have not seen a concrete threshold stated anywhere.
2) Keep total diameters as close to stock as possible. Two reasons for this-
(a) I don't want to under any circumstance introduce more wheel gap.
(b) I'd like the speedometer to be accurate. This is not nearly as important as (a) and I'd live with a small difference, but this point is moot because both (a) and (b) require the same thing- same diameters as stock; solving one will take care of both.

So... I FINALLY settled on scenario 2 below until I realized that the ~1.25" difference from stock diameter will introduce a HUGE wheel gap. This is unfortunate because this scenario brings front/rear diameters very close, to within 0.02".

I've highlighted what appear to be my only remaining options but I'm concerned that the diameter differences front/rear at too high at 0.138" and 0.177"? Again, its very difficult to get a clear answer because its not known what the threshold is. I just came across a thread where Neal from Tire Rack states that differences as low as 0.5" have led to both short and long term problems! Add a little uneven tread wear to 0.138" over the course of time, and 0.5" does not seem so unlikely. So I don't know what the do. I would like to get nice and beefy tires in rear and I don't want to run the same spec tire on all 4.

I think that if this becomes a be all - end all, official thread, to finally reach a conclusion it would be super helpful to us AWD folks.

Any advice is appreciated!!!!

Mike

iTrader: (1)

I may not be able to answer your questions, but I can definitely give you some more info.

OEM tires on the sport wheels are 26.9" diameter, front and rear. Theoretically, if you were to wear them down to the wear bar, they would be 25.33" diameter. That's about a 5.5% discrepancy.

Realistically, most owners go through the rears about twice as fast as the fronts. So, halfway worn, the diameter of the front tires would be 26.115". With fresh rears, that would give a difference of 3%. AFAIK, VDC will accept this.


However, the issue here is AWD, and the maintenance schedule for the AWD specifies tire rotation, and changing all 4 tires at the same time. It is unlikely that you'd ever get more than a 1.5% difference between the front and rear tire diameters, and I've been told (hearsay from an Infiniti rep) that the system will be affected by a 1% difference.


Also, remember that the tire diameter changes depending on the width of the rim you mount the tire on. E.g. a 225/40/19 tire mounted on a 8.5" rim will have a smaller diameter than a 225/40/19 tire mounted on a 7.5 rim.

classy37

Thanks Mike.

You're absolutely right and I didn't take the quoted part above into account. Arghhh back to the spreadsheet...

classy37

Another option is to buy 2 more rear rims and go 275/35/19 all around. This will keep the total diameter just 0.17" smaller than stock.

Does anyone see a problem with running 275s in the front?

Mike

iTrader: (1)

Funny you should bring that up... I asked that very question today. My concern is any sort of rubbing. A lot of members run lower offset wheels with 245 front, so clearance on the outside should be fine. I'm more worried about rubbing on the inside.

I'll let you know if I ever get a firm answer.

Vince

Frustrating I know. I just went through this. Dealer says wheel width and tire diameter has to be the same.

Tire Rack said .1" delta max. Who knows. I actual wound up buying the OEM 8.5" wheels new for all four and stopped on the tire selection. I have the 18" wheels and tires, car's new. I'll use these for winter.
Good luck.

classy37

So what's the maximum that can fit in front without rubbing? Is it 265?

Also, just so I know I'm understanding this correctly- (difference between diameters of factory and upgraded wheels)/2 = increase/decrease in wheel gap

ie 265/35/19 have a wheel diameter of 26.303" compared with stock diameter of 26.744". This means that I should expect 0.22" more wheel gap, keeping the suspension that same.

Mike

iTrader: (1)

It would be (the difference)/3.14/2. (diameter converted into circumfrance, divided into the radius; wheel gap would be the distance from the tire to the fender, so the change in radius would be the factor in this)

The change in ride height would be the same.

I have no idea what the max in the front would be... yet

Neal@tirerack

iTrader: (2)

What, if anything, does the owners manual say about the tolerance? My Audi owners manual gives me the acceptable range of tire diameters I can use front/rear. If it's not there then I'd contact Infiniti. I've talked with techs in CA from Infiniti before when I had my 6MT about other things and they were very helpful. Their customer service line got me in touch with them at the time.

There are several suggestions that have been offered to drivers who are replacing tires on their four-wheel drive and all-wheel drive vehicles. Some vehicle manufacturers recommend that all tires maintain the same rolling radius and circumference, while others suggest that all tire circumferences remain within 1/4- to 1/2-inch of each other. Other vehicle manufacturers recommend that all four tires remain within 2/32-, 3/32- or 4/32-inch of each other, or within 30% of each other in relative remaining tread depth.

Regardless of the vehicle manufacturer's recommendations, the least stressful application is when all four tires are the same tire brand, tread design and equivalent tread depth.

classy37

C = pi * D; D = C / pi

Your formula is equivalent, but you're recalculating the diameter. It's already given.

C / (pi * 2) = D / 2

Mike

iTrader: (1)

Oops. Didn't know diameter was given...

I was delerious from lack of sleep

I did have a discussion yesterday about fitting 275 front tires. It was purely theoretical, but this was the result.

1. Coupes are 100% confirmed to be able to fit 245 front tires on the stock rim. This demonstrates at least 10mm clearance on the inner side of the front.
2. Coupes are 100% confirmed to be able to fit stock tires on the stock rim with 20mm spacers. This demonstrates at least 20mm clearance on the outer edge of the tire.

Given that information, a +38 offset rim of appropriate width should be able to fit a 265 tire with no issue. The guy I was discussing this with was very sure that with a +45 or +47 9 or 9.5 inch rim, a 275 should be okay in the front. The issue here is that the inner edge of the tire may rub something, as nobody has tried this; everyone likes low offsets.

It seems either you or I will be the guinea pigs

He also suggested the possibility of mounting a 275 on a 10 or 10.5 inch rim to give yourself a little extra clearance from an ever so slight stretch; the offset is conservative enough that the outer side is never a problem.

Loki123

Tire size calculator

classy37

Did you not read any of this thread?

classy37

I wouldn't mind being the guinea pig as long as tire rack would take back the tire if it did not fit. I don't believe they would, but maybe I'll give them a call and find out.

Also, 265/35/19 is not a popular size for whatever reason, and is not available for many of the brands/models I am shopping...