: New tires, what size can I use?

Benjamin Simon
10-14-07, 12:06 AM
I have the 1SG in 2005. Currently, I have the 235/50-18 in the front and 255/45-18 in the rear. Are the rims different sizes? I would like to 255 on all four if possible. Is this a bad idea if it is possible?
Also, will having a different aspect ratio cause any problems? 50 vs 45? BTW, what exactly is aspect ratio?

10-14-07, 12:39 AM
If you go to Discounttire.com, you will find a tire calculator (after you plug in any car, yr, etc). That will give you several specs such as width, height, diameter, circumference.

In regard to rims, usually the rear rims are about .5 inch wider on high performance cars with wider rear tires. Disc Tire and Tire Rack can verify that.

Also, some Caddy shops have a tire dept. They can tell you what to use and what not to use. Usually, the larger Caddy shops have this dept.

10-14-07, 12:40 AM
I recently changed tires on my '06 STS from the OEM Michelin 235/50/18 front & 255/45/18 rear to Goodyear ResponsEdge 255/45/18 all the way around.
The rims are the same size, just staggered tire sizes.
I had the Cadillac dealership install & balance them and reset the TPMS.
No problems whatsoever.
I'm certain many others have done this as well without any problems.

Benjamin Simon
10-14-07, 12:51 AM
Going to 255/45/18 all the way around improve handling? That is my plan.

10-14-07, 01:04 AM
Going to 255/45/18 all the way around improve handling? That is my plan.

I found that handling & braking did improve.
I'm happy with running 255/45/18's all the way around.
Just be sure to give the new tires some "break-in" time (at least 500-1000 miles) before judging them as being to stiff/harsh riding.
Initially the new tires were a bit hard & rough but after 1000 miles they're pretty smooth now.
However, they are a wide aspect high performance tire so therefore they're not going to give a real soft & supple ride.

10-14-07, 10:35 AM
I have a 2005 1SG AWD and changed to 255/45/18 Michelin Pilot Sport/AS for all four wheels after 30 k miles (I now have 45 k miles). I also rotated them front to back when I changed the breaks.

The wheels are all the same size on the 2005 1SG and there was an option in 2005 for the 1SG called the "performance handling package." The performance handling package upgraded the tires to 255/45/18 Pilot Sport on all four wheels using the the same wheels as the standard 1SG. This package was not available for the AWD for some reason but I went ahead anyway and changed my 1SG AWD to the same size tire on all four wheels. I went with the All Season version (255/45/18 Michelin Pilot Sport/AS) for the better rated light and heavy rain performance.

My experience has been that the performance in the rain is very much better. The car also feels more solid when turning even on dry roads, but I don't know if this is due to the better performance of the Pilot Sport/AS or due to the increased size in the front. I don't know about the snow yet but I will let you know after this years ski season. The car did fine in the snow last year with the OEM tires due the stability/traction control and the AWD.

My only regret is that I drove the car for 30 k miles with those OEM tires slipping and sliding in the rain. So my advice is go ahead and upgrade your tires. :yup: