Is this possible? Would there be any negative side effects?

 

Answered my own question...yes, I can!

 

Yes you can.

Damn, too late.

 

Generally if you have a 255 mm wide rear tire you can put on a 275 mm one but, you will increase the ride hight, something you probably do not want. The car is already riding fairly high.

Generally, if you want to go to a wider tire on the same rim, you have to calculate the sidewall height and probably step the aspect ratio down to keep the same approximate ride height. I forget what aspect ratios are available on our cars. I know that I have the 275, 19" rears but, not sure if it is 35 or 40 aspect ratio. The aspect ratio tells you the height of the sidewall as a percentage of the width of the tire. Thus, if you had a tire with a 275 mm thread-width and has an aspect ratio of 35, the height of the sidewall would be, 275 x .35 = 96.25mm. You want that sidewall to be roughly the same size in the replacement wider tire, which means you have to go with a lower aspect ratio tire, likely a 30 ratio.

 

I'm getting some 20" Sepangs, and will be putting some 265/35 on back and some 255/35 on the front. According to the tire professional I talked to I can go up to a 265mm tire on a 9" wheel. I would like a taller sidewall to get a better ride.

 

I don't blame you for wanting a taller sidewall. I think that the trend for lower sidewalls have resulted in far too many wheel damages by potholes and in increased ride harshness. They have to make the short sidewalls much stiffer thus, taking away rubber compliance that is part of the suspension.

If you are going 20" wheels, be careful because you likely will actually decrease the sidewall. If your present wheel is 18", you will loose a full inch of tire sidewall, if it is 19" you will loose 0.5" sidewall. Both will have significant effect on the ride quality; that is, you will get exactly what you do not want. That, assuming that you will want to maintain the stock tire diameter, something I would recommend. For my personal taste the car's stance is already a bit high, I would actually like to see it sit about 1" lower. But, I'm fighting the idea of installing lowering springs because they are always stiffer thus, ride suffers. I am actually happy with the ride on my 19" wheels. Seem like the electronic shock dampers works really well, giving me a much better ride quality than my '05 XK had with the 18" wheels and taller sidewalls and no electronic dampers.

If you wish, I can calculate for you the difference in tire diameter if you give me your present tire sizes, compared to the sizes that you intend to purchase. If you do, make sure it is the full number, like: 255/40/18, the first number being the tread width in millimeters, the second is the aspect ratio, third being the wheel diameter in inches.

Albert

 

Well, I've decided to go with the Continental Extreme Contact DWS

245/35/20 on the front, which will be 9.65 inch width and 3.38 inch sidewall
265/35/20 on the back, which will be 10.43 inch width and 3.65 inch sidewall (I may drop this down to 265/30/20...)

I currently have 19" Jupiter wheels with 245/35 on front and 275/30 on back. Aside from the atrociously loud Perellis that are on, the ride is decent. I will be replacing the Perellis on the 19's as well. I am thinking (based on others feedback) that the Continental tires will be a huge ride and noise improvement.

So I will have both 19" wheels/tires and 20" wheels/tires. I will eventually find out which is the better of the two...

 

The stock front tire full diameter is: 245/35/19 is 654.1 mm.
The planned front tire full diameter is: 245/35/20 is 679 mm.
Difference: 20" tire diameter is 24.9 mm taller, ride height of front will be about 0.5" higher. Sidewall tallness is the same for both tires.

The stock rear tire full diameter is: 275/30/19 is 647.6 mm.
The planned rear tire full diameter is: 265/35/20 is 693.5 mm.
Difference: 20" tire diameter is 45.9 mm taller, ride height of rear will be about 0.9" higher. Sidewall will be 0.4" taller for 20" tire.

Hope this helps,

Albert