I need to replace my rear tires BAD. I am currently running a 265/35 all the way around. OEM size is 225/45. If I cut back the width on the rear, 245/35, how bad would the ride be and would my speedo be off by much? I would think my speedo would not be effected since I am going to run the same size side wall.

 

1) the sidewall number is the percentage of the width. So 245/35 has a sidewall of 85.75mm, and 265/35 has a sidewall of 92.75mm. Your total diameter would drop by 14mm, or about 2.2%

2) Making that change means that your ABS/TCS/DSC will see the rear tires as continually spinning 2.2% faster than the fronts (814 revs per mile vs 797 revs per mile), and this variance is enough to throw a fault in the ABS/TCS/DSC system.

3) ignoring rolling circumference differences, you will have about 8% more grip on the front than on the rear, leading toward a risking balance problem. Any time you will start to exceed the grip available (driving hard or rain, whatever), it is the rear that will lose traction first. With your DSC system compromised from the sidewall, you can't rely on it to act correctly, or even act at all.

I'd highly recommend getting the same size tires as the front.