Does getting wider tires mean less turn radius?
I know it reduces fuel efficiency but what other downsides are there?

Im going from stock 8.5s to 10.5

It does not effect turning radius

You'll loose some straight line acceleration due to the added weight. I definitely could notice the difference between stock and 275/35 rubber, even though that rubber was on a lighter wheel than stock.

The difference is minute going from 245 to 275 in acceleration. If anything, my wheels spin less on launch. You're generating a **** ton more grip with 275's than a 245. The trade off is beyond worth it.

Also fuel efficiency is a hit or miss. Not always worse. Sometimes the same.

Get a 18x10.5 or 18x11 wheel and slap a 275 and call it a day. You'll never go back.

Yeah I got 18 x 10.5 +15 with 275s on it now, thanks guys

You won't regret it

245 to 275 is a big jump and you will probably experience increased bump steer. Ruts and waves in the road surface will become much more noticeable.

It does not affect turn radius

Wider tires have a bit harder time going through debris like water/snow/ice etc since they collect more of it vs a narrower tire. However, they do plant the car in the corners better and offer more steering response.

Why are you running 10.5 or 11 for 275? 10 is enough. You don't really need to firm up your sidewalls that much with stretch.

I mean, want and need are two different things. My yoko 295's sit pretty square on 10.5 inch rims. My next tires will be 285 because i'm a cheap bastard. They'd be 275 if i was doing mostly DD. I've seen 275's on 10.5 inch rims and the stretch is about perfect for sidewall preloading.

The way you want to set up the car differs on your objectives.

wider wheels with a slight stretch of the tire is what you want for cornering and handling. for straight line speed you want a tire that you can get into its optimum heat range and deflate the pressure to grow the length of the tire tread on the ground for straight line grip.

the worst thing to do is run a wide tire on a narrow rim for its width, it gives horrid feedback, delayed turn in response, and the tire contact patch isn't being used well as the tread isn't being stretched flat on the ground.

a skinnier tire tread to wider tire tread have the same exact contact patch size on the ground, different shapes, but the contact patch is a function of tire pressure and weight. you don't gain more contact patch from wider tires, you just change the contact patch for better handling grip when going wider, but if you have say a 10" wide wheel, you will be better off running a tire with a tread width of 9.5" (255/35/18) than a 285/30/18 with a contact patch of 10.7".

For an engineer, you're relying on visuals vs. calculations a bit too much no?

wider contact patch, regardless of area, keep tires cooler.

So yes, the area doesn't change, but your tires are happier.

Don't forget to account for lip width, rather than wheel width, in your calculations.

Yup!!! Because tire data is impossible to come by!!! And manufacturers won't publish **** because its all "trade secret" or whatev. That's why we're forced to guess which cambers will work for us. Currently up to -3.3 degrees up front and i'm finding more grip. I might have to keep going up.