tire wall size question.
 

It is time to change the rear tires and was planning on going wider to the XJR spec.. Then realized that 295 and 275 has a different wall height. 275 / 40 / 20 vs 295 / 35 / 20...
Why is there a difference on the wall height? and if I change only the rear to 295 / 35/ 20, will the car throw an error mes? I was looking at 295 / 40 /20 tires, thinking wall height should stay the same and only change the width. Am I wrong to think this way?

Fronts still have some bite left... so wanted to change them later... So will it be a problem going for 295 / 40 / 20 for now then change the front tires to 265 / 35 /20 ? Or is it better just to match all 4 tires (same brand) and go for 295 / 35 /20 and 265 / 30 / 20

Not really good w tire specs so decided to come here and ask for help. (before putting on the wrong size and getting all sorts of error code and tire rubbing.. :icon_tinykitball: )

Thanks

EDIT:
After reading Thermos reply, I realized I made a mistake... I flipped the no between the front and back tires. Currently I'm running on 245 / 40R 20 on the front and 275 35R 20 on the back.. I was wondering if it was ok to just widen the tires and keeping the R number same, or I should go down one size to match the XJR tire profile. I was under the impression that if the R no stayed the same, the diameter of the tire stayed the same. But after reading Thermos comment, I learned that its either going up or down by 2%. So it really doesn't matter which way I go...

Then my next question is, if you guys had to choose between 265 /35R 20, 295 /30R 20 set up and 265 /40R 20, 295 /35R 20 set up.... which one would you go for? Would this size difference make any difference on the ride quality? performance?

gochurush, when you look at tire sizing, you have to realize what the numbers mean. The first number is as you state, the width of the tire. The last number is the size of rim that the tire fits on. This is where math comes in. The middle number is a ratio of how much sidewall you have to how wide the tire is. If you keep the same ratio, as you make the tire wider, it will get more sidewall. In your case, when you step from 275 to 295 tires, you need to drop the ratio slightly to keep the sidewall the same height. By keeping the same height, you will eliminate the chance of having troubles with our ABS system.

PUnching the numbers into a tire calculator, the 295/35R20 is actually a smaller diameter tire than the 275/40R20 tire by 0.6 inches (or 2.1%). If you were to go with the 295/40R20, you end up going the other way and having your new tire being 0.6" bigger (again, 2.1% difference).

The tire sizes that you suggest for the final setup (ie, 265/30R20 for the fronts, 295/35R20 for the rears) will not be a good combination. This will put too much difference between the front and rears, almost guaranteeing an ABS light to light and resulting in abnormal braking.

From what I see, XJR sizes are 265/35/20 and 295/30/20. The smaller sidewall ratio in the rear keeps things equal. The XJ I bought earlier this year came with incorrectly sized tires so I'm looking into fitting the XJR sizes as well.

Thermo, thank you so much for the detailed info.

After reading your reply, I realized I made a mistake... I flipped the no between the front and back tires. Currently I'm running on 245 / 40R 20 on the front and 275 35R 20 on the back.. I was wondering if it was ok to just widen the tires and keeping the R number same, or I should go down one size to match the XJR tire profile. I was under the impression that if the R no stayed the same, the diameter of the tire stayed the same. But after reading your comment, I learned that its either going up or down by 2%. So it really doesn't matter which way I go...

Then just out of curiosity, if you had to choose between 265 /35R 20, 295 /30R 20 set up and 265 /40R 20, 295 /35R 20 set up.... which one would you go for? Would this size difference make any difference on the ride quality? performance?

I run the XJR tire sizes on my SuperSport, as you can see in my signature, and the work great, no ABS issues and fit the rims perfectly, as stated by Thermo, there is a .6" difference in the diameter resulting in the speedometer being "optimistic" 2mph. The speedo can be corrected if you have access to an SDD and can get into the engineering mode, but I have not been able to find someone locally that can do it, so I live happily with the inaccurate speedo/odometer.

gochurush, when it comes to the amount of sidewall in your situation, the difference is going to be be minimal. So, are you going to lean more towards a better cornering car that might have a slight bit more acceleration at the cost of ride or go for the ride while "loosing" a bit in the cornering department? Either are going to work. The difference is going to be very minimal in the ability of the tires. You can probably offset it with a good tire that has good traction if you went with the larger diameter tire. Granted, are you really going to push your car to that limit? You shouldn't see any effects on how the car shifts or anything like that. Only when you get up into the 10+% error will you start seeing issues with shifting. But then, we are talking completely different set ups at that point.