Stock tire size for 2011-2013 S60 T6: 235/40/18
2014+: 235/45/18

If your car uses 17" x 8" wheels you could use either 235/50/17 or 245/50/17. 1cm of width more won't rub or anything but it leaves you with more tire options.

Now, tires with bigger sidewall will give you a more comfortable ride with less responsiveness. Also, since your car came with 235/50/17, if you change them to 235/45/17 your speedo will read higher since the tires are smaller in diameter.

Edit: I thought the 2014+ T6s and T5s used tires with the same exterior diameter but it seems this is not the case. Now I see T6's with 18 in. wheels use taller tires than the T5's equipped with 17 in, my bad. I mean, what the hell Volvo?

As for your original question, I would use 235/45/17 since that's what the sticker on the door jam says.

 

2015 Volvo S60 - Wheel & Tire Sizes, PCD, Offset and Rims specs

Wheel size, PCD, offset, and other specifications such as bolt pattern, thread size (THD), center bore (CB), trim levels for 2015 Volvo S60. Wheel and tire fitment data. Original equipment and alternative options.

www.wheel-size.com

 

235/45 is indeed correct. 50 will be taller and throw gearing off (and speedometer and odometer). Up to you to decide whether it's bothersome to you or not.

 

I'm running 245/45 R17 on my 2014 T5. Recommended size from TireRack for my 17" wheels. Ride quality improved greatly over 235/40 R18 tires on stock 18" Titania wheels.

 

245 from 235 isn't a big deal, as you're dealing with only a 10 mm difference = ~1% of rolling difference.

Going from 45 to 50 series though will increase diameter by 3.7% (a difference of 3 MPH at 70 MPH).

OP, here's a useful tool for you to use to try out a few sizes to see how different the sizes impact you.

235/45-R17 vs 235/50-R17 Tire Comparison - Tire Size Calculator | Tacoma World

235/50-R17 tires are 0.93 inches (23.5 mm) larger in diameter than 235/45-R17 tires and the speedometer difference is 3.5%

www.tacomaworld.com

 

Or here: Tire Size Calculator

 

My 2016 came with 235/40/18's but those tires wore out fast so I downsized to the 16 inch 215/55/16. Much cheaper tires and overall more comfortable ride. The "16" rim is called Oden.

 

Compared to my V70's 16-inch wheels, I am weary of the ride and noise of my 2017 V60's 18-inch wheels and tires, so I am resuming my search for wheels. The tire size websites above are a real help, but is there a comprehensive website for Volvo wheels listing all its OEM wheel models and sizes?

Edited to add - I found it: OEM Factory Rims - Genuine Volvo Wheels