so I’m getting some new tires. I currently have 245/40 pilot super sports on the stock MR wheels and I am planning on going with the same manufacturer/type.

question is the size. Yes I did research and yes I looked at old threads. The problem is that much isn’t mentioned in benefits of the stock tire size. So the only two I am considering is the stock 235/45 and going with the current 245/40s I have now. Here is what I have gathered so please correct me If I am wrong.

245/40 pros
longer gears, more grip, less bumpiness and road noise.

235/45s pros
cheaper, better acceleration, better fuel economy, accurate Speedo

please let let me know if you have anything to add and no I am not considering any other sizes. Also, this may be a dumb question, but if I was tuned on the 245/40s, will this effect my tune at all? Will I need to adjust it?

 

I believe your tune will be fine. I ran 245/40's on the stock wheel and pros are as you say. maybe it was just my tire choice but it felt squishy. i went back to the stock size and got better gas mileage and my car feels normal and great again.

 

No difference for the tune.

You have some pros/cons mixed up. 245/40 is a smaller diameter tire than 235/45, so it would have better acceleration but be more bumpy (from having less sidewall height). 235/45 will have "longer gears" and better fuel economy.

Another aspect is that because 245/40 is a smaller diameter than 235/45, it lowers the car ~7.75mm (0.3").

 

specifically for the super pilot sport tires, the 235/45s weigh less than the 245s. Wouldn’t less rotational mass increase acceleration? I saw a car and driver article that confirms that notion. Albeit, we re probably not talking about a big difference.

 

Hadn't noticed the weight difference. Yes, reducing rotational mass by X lbs has a larger effect on acceleration than reducing sprung mass by X lbs. That said, that 22 vs 23lbs may very well be something 22.4 vs 22.6 getting rounded.

All that said, IMO, choosing between 245/40 and 235/45 for weight savings is really splitting hairs. You're never going to notice it either way you end up going with.