18 in tire size 2001 996.1

I have read a lot of threads on this. Wanted to ask about tire size on 18 in cup 3 wheels, front 18x8/ ET 57, rear 18x10/ ET 58. Plan to get front tires 235/40 x18 and rear 285/30 x18. Street driving. Any concerns or other suggestions? thanks Dave

Dave, you may have a pretty big differential between front and rear diameters at your proposed sizes. If you are going 285x30x18 in the rear, I would stick with 225x40x18 in the front which is already a 1/2 inch bigger in diameter front over rear. 235x40 may be as much as 1 inch taller than the 285x30 rears. So check your measurements at tire rack. However, I have a couple of sets of wheels and I have run 225x40x18 and 285x30x18 track and street tires for years, but just decided to put on a set of Michelin all season 4s for year round street. But there are no 285x30s all season tires available only 285x35. Because of the rear diameters being 1 inch larger in the rear than front 225x40, I went with 235x40 in the front to eliminate a taller rear with too much rake. Point is that you have lots of configurations you can go with, but be careful with matching tire height with front and rears. Especially if you have PSM. On 8/10 inch wheels (18) consider the following. 225x40x18 w/ 285x30x18 OEM 235x40x18 w/ 295x30x18 or 235x40x18 w/ 285x35x18 Sure you can probably also do 225x40x18 and 295x30x18 as well for relatively even tire heights. I don't has psm on my 996.2. Just a LSD, so other will have to pipe in on configurations as to its effects on PSM. Also, keep in the back of your mind that the wider and taller you go with tires, the more it may influence what camber you may need to run as well as wheel spacer size limitations. As an example, as mentioned earlier, I have always run 225x40 and 285x30 on my 996.2 with 15mm spacers front and back. By going to 235x40 and 285x35, I had to get a set of 12mm spacers. I will be trying them out tomorrow to make sure it's a fit without rubbing. Good luck.

Thanks for the information I charted tire sizes and diameters here, attached Excel file The 235/40-18 are 25.4 in diameter vs the 225/40-18 are 25.1 in diameter The 285/30-18 are 24.7 in diameter So there would be a 0.7 in difference in front vs rear tire diameter with the 235/40 tire vs a 0.4 in difference with the 225/40 front tire That 0.7 in difference is still only a 2.8% difference between front and rear tire diameters, which is likley ok for the ABS system But if the 996 GT3 used the 225/40 front tire with the 285/30 rear, I will probably go with that

I'm fighting the same dilemma but my car is a C4. I was told by tire rack the following: OEM Want too go with: 225/40 18" (et 50) = 245/35 19" (et 57) which the tire would be 25.7" od 265/35 18" (et 65) = 295/30 19" (et 51) which the tire would be 26" od So the ratio would be (25.7/26) .0988 or less then .01 difference. They also didn't say offset was a concern or running spacers.

Something else to think about that may or may not bother you is the visual look of a 235x40 in the front with a 285x30 in the rear. The diameter difference is visually evident with much more rubber in the front.

In the past, I found the "Tire Comparison" calculator at https://tiresize.com/calculator/ to be helpful to visually see the diameter difference. When I bought my MY99 a year ago, I put OEM 225/40 and 265/35 on 8" and 10" turbo twists. The difference in the sidewall bothered me. The 265 was too "stretched" on a 10" wheel. I recently switched to a 285/30 in the rear (still 225/40 in the front) and am much happier with the look. The diameters are closer together, and the sidewall profile is more similar. A 295/30 in the rear would be an almost perfect diameter/sidewall match to 225/40 in the front.

I do DD street driving had 225/40/18 front and rear 285/30/18 on 5 spoke rims. I am currently on Continental extreme tires. I have to say the 285/ 30 /18 were terrible on uneven and jacked up roads, could feel everything. I ended up getting a good deal on 275/ 35/ 18s Continental extremes and they feel so much better now. The Gap also looks better in my opinion. In the future when tires are needed again, I will back to the 285 but with 35/18.

Are you using any spacers on your 5 spokes?

The OP doesn't state whether this is a C2 or a C4. If it's a C2, diameter doesn't matter all that much and you can pretty much pick what size tires you want to run. F/R ratio's are only relevant on the AWD cars.

C2, thanks

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I have been driving around for 10 days on a set of 235x40x18 and 285x35x18 Michelin AS4s on my second set of Carrera 5 wheels. Always ran 225x40 and 285x30 on the street and track. Side note, I had to remove my 15mm spacers and replace them with 12mm with the new tires sizes. My car is set to GT3 specs so my car is lowered and runs a moderate -1.5 camber in the front and -2 in the rear. Wow, if you want a softer ride on the street that absorbs bumps, this is the set up. I run GT3 seats and my suspension is set up for the track so it's pretty stiff. This tire configuration softens everything on the street. But for the track, I would stick with 225x40 and 285x30 or possibly go to 225x40 and 295x30 on 8 and 10 inch wheels.

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great info. Do you have any pics? Does 225/40 & 295/30 bridge the gap on the sidewalls closer than a 285/30 on the rear? I�m OK on tires as of right now (unless this weeks road force balancing says otherwise, but miles/age is OK) and on the next set I wanted to accomplish 2 things: Slightly wider in rear with a little more sidewall to better match front sidewall reveal. one thing that always bothered me was the difference between F&R sidewall height.

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I'll post some pics later today. Yes, the 295x30 should increase the diameter depending on the tire used by 0.4 to 0.5 which pretty much gets it even to the diameter of 225x40. From a visual standpoint it will look better. I would much prefer to run a 295x30 rear vs 285x35. But we are limited with all season tires and even more so with winter tires. Guys in the warm weather town's don't have to worry about cold weather sizes and don't have this problem. But as i mentioned before, i probably wouldn't run 235x40 and 285x35 on the track because it most likely will be too soft for my preferences on 8/10 in Carrera 5s. So I'll stick with 225x40 and 285x30 or go to 295x30 in the rear and try it out when I order a new set of Bridgestone RE71RS tires. APEX wheels has done a great job at identify tire sizes for wheel sizes for track driving that works. The concepts would translate over to the street. Obviously they don't concern themselves with all season nor winter tire sizes.

Folks, I'm in the same boat. My C4S came with the following; 225/40 R19 295/30 R19 I immediately switched to winter wheels and tires because i took delivery in the winter. The tires that were on the 19" in rims were over 6 years old and in need of change. I know that the 996 did not come with 19" wheels from the factory, but understand that many have gone with them anyway. I'm planning on swapping the tires on the 19" summer rims for the same 224/40 and 295/30. Does anyone see any potential issues with this? Again, I have never driven the car with the 19" rims. I bought the car from a dealership so I'm unaware if the previous owner had any complaints about the set up.

Need to make sure on the C4 & C4S that the wheels are within 3% of each other while rotating otherwise you risk over heating the transfer case. On my C4 I'm running 245/35 & 295/30 and they are almost 1v1....I have to run a 15mm spacer on the front (using 19" 997 wheels) otherwise they'll rub the strut tower. You just can't put whatever tires you want on the 4 wheel drive cars.....rear wheel cars it doesn't matter but 4 wheel it does.

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Thanks @Viper1000 how long have you been running that setup? The front of the vehicles is the same from the C4 to the C4S correct? What was the offset on your 997 Wheels? Is the spacer in the front necessary for all variations? Not sure how long the previous owner was running my current 295/30 + 225/40. Is this the correct formula to determine tire height? Tire Height=(Tire Width�(Aspect Ratio/100)�2)+(Wheel Diameter�25.4) With this formula I get a 3mm difference in height on my existing tires and a 5.5mm height difference on the 245/35 & 295/30.

I just started running this set up as I had to cancel my tire order and reorder the correct size. The C4 is a narrow body, C4S is a wide body so the offsets will be slightly different. The rotating ratio for 4 wheel drive cars needed to be within 3% each other. You can use the Tire Rack web page to look at ratios as each tire has the rotating rev's for each tire size. My (997) rim offset (F) ET57 (R) ET51 while the stock 18" rims are (F) ET65 (R) 50. The 19" fit fine but I needed to add a 15mm spacer in the front as the tire (245/35/19) would slightly rub the strut tower. Your current set up is stressing your transfer case as the 225/40 is rotating less (in one mile) than the 295/30 by a lot.

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@Viper1000 , lets compare notes. I'm getting different numbers than you, so maybe I'm missing something. I'm on Tirerack.com. Checked 2 different configurations. Using Michelin Pilot 4S as my comparison. 295/30 = 799 Rev/Mile 225/40 = 797 Rev/Mile 245/35 = 807 Rev/Mile Unless I'm missing a detail here (which is very possible), the 295/30 + 225/40 is closer in Rev/Mile than the 295/30+245/35.

On my 2003 C2 - I run APEX 18x8.5 ET42 and 18x11 ET60 - with Michelin PS2 235/40-18 front and PS2 295/30-18 rear. Very nice fitment. No complaints from the PSM.

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Nice set-up. I have been eyeballing a set of APEX wheels for awhile. Haven't settled on which one yet. They all look good.

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Hard to go wrong with the VS-5RS. They look great, are very light, and fit perfectly.

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They do look great! My 2 sets of Carrera 5 spokes have been strong and durable on street and track. But I am getting close to selling one set and getting a set of APEX wheels. Got a few more big ticket items to finish up first and then APEX wheels.

Here are pics of mine:

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Let me see what I came up with and how....but are you using a 19" rim? For some reason I thought the 225/40 was not within the 3% range.....

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Looking at it you are correct. Still both within 3% so either set up would work for your car.

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Glad it is working out for both of us. before today I didn't even know this was a thing, i learned something new today. I'm so glad I'm on this forum!

Since we are talking tires sizes I wanted to provide some feedback on the Michelin Pilot Sport All Season 4 tires using 235x40x18 and 285x35x18 on Carrera 5 spoke 8/10 wheels. I just put them on my 2002 C2 two weeks ago and spent yesterday afternoon/evening driving from Chicago to Pittsburgh on my way to Phila for the weekend. First if all let me say that they are great tires. Glad I purchased them for street driving "year round". The "bigger" tire sizes work well on the 8/10 wheels and street. More rubber and diameter make for a much softer and smoother ride over the bumps. Grip was excellent in 35 degree temps thru low 50s. No experience yet with warmer temps. No problem wipping thru the Pittsburgh turnpike curves. Quite frankly, I never questioned grip. The key to running these sizes on 8/10 wheels is finding the proper air pressures. Too much air pressure and the wheels float and track every line, nook and cranny. Too low and they feel too squishy. Started off at 34 front and 38 rear and it was tracking all over the highway following the lines. Too much air. Dropped the pressures to 32 front and 36 rear and they felt great and no longer followed the lines. Then tried 30 F and 34 R. Too soft. Ran 32/36 most of the way and specifically thru the Pittsburgh curves on the turnpike and they handled extremely well. 32/36 seems to be the tire pressure sweet spot on my 996 at the air temps I experienced. Bottom line: These tires are very good and the 235x40x18 and 285x35x18 sizes work very well on the 8/10 wheels for the street. Proper tires pressures are key to characteristics. 32/36 tire pressures worked best with my set-up. Lastly, with my car set up to GT3 specs for height and camber, i had to go from 15mm spacers to 12mm to fit. More camber or higher ride heights will allow you to use 15mm spacers on a 996.2 without a problem.