Hello guys! Im new to the forums I own a 2014 xj SC and im wondering if I can fit 255/35/20 in front with out any issues? I know these cars can be picky with what you do to them. Thanks in advance.

 

fredom, based on looking at the overall dimension of the tire, as long as you get the same offset, you should be good. The new tire is going to be 0.7" smaller in overall diameter, but 0.4" wider. BUt, because you will only see half of that because the tire will center on the rim, you will only see 0.2" of additional tire sticking out at the body edge.

On a side note, hopefully you are planning on running 295/30R20 on the rear. That is the size you need to make the wheels spin at the same rate. You can go a little narrower, but you are going start getting a wheel speed difference. This is not a big problem if you have a RWD car. If you have an AWD car, then it can be a problem. I can speak from experience that you can have a 3.3% error between the tires and an AWD car will absorb it for the most part. It was a non-Jaguar dealership that installed the wrong size front tires (so my front tires were turning more slowly than the rears). I knew something was wrong as I could feel the ABS kicking in every once in awhile, especially on a turn. So, I would say that is about as far as you can push the wheel speed difference. So, 285/30R20's are going to have a 1.1% error and 275's are going to have a 1.9% error.(ie, both are going to have a slower spinning rear wheel (assuming an AWD car), so, the front is going to be attempting to drag the rear around some).

 

Danm! Thats an excellent explanation, thank you. I will add 295s in the rear but they still have some life left. I wanted to add 265 up front but got a good deal on 255s I am rwd, do you think I im going to run into problems with 255/35 front and 275 in the rear for the mean time?
Thank you again for that explanation.

 

fredom, you should be good. Just anticipate the car possibly jerking or just not feeling right if you are making a higher speed turn that is fairly sharp. This would be the ABS kicking in because you have exceeded the limits inside the computer for the ABS to now activate.

 

What about 305/30/20 in the rear? Do they fit? Ive found a brand new pair for really cheap.

 

fredom, you may get a little bit of rubbing. But ,this could be easily fixed by talking with a body shop and having them roll the fenders a bit. That would give you the clearance you need. The rolling would not be that expensive either. The bigger (and unknown) thing is the long term effects of running different diameter tires on the car. I ran a 3.3% error tire on my car for a month with what appears to be no detrimental effects. But then, I also took it easy on the car and didn't do any hard pulls or drive it where traction was mediocre at best. The 305's are gonig to have a 0.7% error with the 255's up front. So, this different is minimal, but how this will affect the car long term, hard to say. Car should drive find for day to day use. I am just worried about what is going to happen to the transfer case from having to always make up the difference in speed between the front and rear tires.

Before I say "yeah, go for it", I would want to know more about how the transfer case is designed. If it is an open style differential, then having this offset is not that big of a deal. But, if it is a viscous coupling setup, that could be bad long term.

 

Ah ok. You’re very knowledgable and thank you for your response.
I dont have a transfer case, Im two wheel drive. Do you think I would run into such problems?

So what do you think would be the best tire set up running the widest tire possible front and back?

thank you again.

 

fredom, if you don't have a transfer case, then keeping the overall diameters the same and you will be good. When I had a 3% error in tire size, I could feel the ABS kick in once in awhile. If you keep the difference down under 1%, you should be good under most conditions.

As for widest, I think in the rear you are going to be limited to about a 305/35R19. This is where the tire is going to start getting wide enough to touch the outside fender area. Then, when it comes to the fronts, this is always a hard one to judge with all the different angles that you can possibly have. I would think sticking with a matching say 255, but there is no size 255 that really comes close (ie, within 1%).

I would say a good combo to try is say a 255/40R19 in the front with a 295/35R19 in the rear. That will give you a bit more tire under the car and will keep things almost spot on (off by 0.4% between the tires). This will result in a -2.5% error in the speedo. So, at an actual 70 mph, you speedo is going to indicate 72mph. Not a big deal. but, something I think you should be aware of.

If you really want to see, you have 2 options. You can find a used tire place and see what they have in tires sitting on the rack and have them mount up some tires to simply bolt to the car (no need to balance them). See what they look like and you should be able to get a general feel for whether they would rub or not (this is where the shop will be able to assist you some). Then you can get an idea of the largest I can run front and back is ......

The second way is to simply go out with some paper or wood blocks and build up spots on the tires to simulate the wire tires. You can then slowly move the car and see if you have any contact. The masking tape holding the blocks in place will give and let the blocks fall off of the tire beofre damage is done. You can then measure the blocks. Or the reverse method is to simply twist up the car some by slowly driving up on to a curb (this will push the wheel into the wheel well). You can then use a ruler to measure the gap between the tire and the fender. From there, you can double the measured gap (the new tire, half the new width will be going out, the other half goes to the inside). You can then play wiht a tire calculator program (see Tire Size Calculator) and find a tire with the same overall diameter, your new width and it will tell you the aspect ratio (the second number in a tire size, the 40 of a 255/40R19). Then you can even play with changing rim sizes and whatnot. Granted, you have to be careful once you start playing with rim sizes as you are changing more variables and you can easily change one that the same diameter tire works on a 19" rim, but not on a 20" rim. You may alos hear this called "offset" or "backspacing" (2 different ways of measuring a common reading).

 

Hi,

For visualisation this are exelent tool:
https://www.wheel-size.com/calc/
It uses the rim ET value, suspension, fender etc clearance as we well to find out if your new tires will fit or not.
Idea are that you set your current tire size on box left and measure clearances from you vehicle with current tire. Then you add the new tyre size on right side box and it will calculate the chances.
I used it when i was seeking an 18" rims for wintertyres.

 

My current setup is 255 40 20 in front and 295 35 20 in the rear. No issues at all. I also use to run 305 in the rear at one point. I have the AWD model if that makes a difference.