A 305/30-18 tire will be too tall and cause your speedometer to read way off. A 305/35-18 tire also requires at least a 10.5" wide wheel and can not be safely mounted on the 9.5" wide rear wheel. I have tried 2 options to get a wider rear tire on my 2015 Base Slingshot that didn't impact the speedometer or nanny systems.
The first is to use an American Muscle 17x10.5" 10th Anniversary COBRA style wheel which will allow mounting of a 30535 or 315/35 tire, but it has a 28mm Offset that will shift the center of the tire about 1.5" to the left. The wheel has similar styling compared to the stock 18" rear wheel, differing by having only 7 pairs of spokes compared to 8 pairs of spokes on the stock 18" wheel. The Black wheel isonly a little off-color compared to the grey of the stockwheel. I think that once I get the wheels powder-coated to match, the difference will be negligible. I've been running a Nitto 315/35ZR17 555 G2 tire on that wheel for over 11,000 mils w/o problems with the nanny systems. I used the Black version of this wheel - Mustang 10th Anniversary Cobra Style Anthracite Wheel - 17x10.5 - Rear Only (94-98 All), but they don't show it on their website at the current time. I paid around $150 for it and have seen it as low as $100.
The second option I tried was a CP32 wheel 18x10.5", 35mm Offset in Black, but they don't show the 35mm offset version on their website right now. I had an Achilles ATR Sport 2 305/30ZR18 tire on it, but my tire guys had a very hard time getting that tire to seat on the wheel. One tire guy damaged the tire trying to get it on and the second guy had to use a large strap to force the tire to seat on the wheel. I also didn't care for the ride from the 30 profile tire.
When considering a change in tire size and/or wheel size or offset, always check the tire manufacturer's website to see what size wheel they recommend for that specific tire size. Failing to follow the manufacturer's recommendations can relieve them of liability if something goes wrong. You should also check fitment using the following sites. This site - www.willtheyfit.com provides a visual image of the differences different tires & wheel size/offset make on tire placement, but the site's speedometer impact numbers are off. This site provides a more accurate estimatation of speedomter impact - https://www.discounttire.com/learn/tire-size-calculator.
Thanks for the info. Picking up my 2020 SL from the dealer this morning, Can't wait to drive it!
I think the tire size was mis-typed as the SL has a 20" rear tire and a 275/40/20 tire would make sense in this situation.I ran a couple of 275/40/18s on my previous '16 SL original wheel. Speedo was only 1 mile off. Back then, there was a rep from Discount Tire on here and he said 275 was the max for that wheel. Worked for me.
I have a set of SL wheels / Tires the rear is a 20I want to put the 305mm rear tire on my new SL without changing the fronts. Living in Jersey I can't use the really low pro tires. The stock tire is 265/35 R18. Can i go with a 305/30 R18 and not mess with the traction control?