Maybe I’m missing something here @Shatneyman and @Slingrazor but why would the width of the tire affect the speedometer?
well, the width is .8" wider,...agree, no need to worry,... diameter is .6" bigger, side wall is .3" taller, circumference is almost 2" bigger. Changing the rotation in the rear but not the front. Hope it made sense.
The tire height is changed. A 255 35 20 is 27", if I went with a 305 35 20 the height is 28.4". I'm only going to 275 35 20 which is 27.6"
The tire sidewall is actually a ratio of the width. For example in a 255 X 35 X 20 The rim height is 20 inches, and the sidewall is roughly 35% of 255. Now if you go to a 275 X 35 X20 the sidewall height is about 35% of 275, or slightly more. the actual tire diameter is the rim size plus 2 times the sidewall height.
I was just about to post the same thing @McClendons! Although the sidewall height is not exact from tire to tire. They can be more than 1/4" difference in diameter from style to style or brand to brand.
well, the width is .8" wider,...agree, no need to worry,... diameter is .6" bigger, side wall is .3" taller, circumference is almost 2" bigger. Changing the rotation in the rear but not the front. Hope it made sense.
The tire height is changed. A 255 35 20 is 27", if I went with a 305 35 20 the height is 28.4". I'm only going to 275 35 20 which is 27.6"
Minimal impact on speedometer with that one. Here ya’ go...
Just make sure to check the box for "mileage not accurate" when selling! 
Thanks all. I was thinking the number for the sidewall was the actual sidewall height. Thanks again. Guess I better check mine out since I replaced the Kenda 305 with a Nitto 315.
Edit: Just looked it up. Looks like my 315/35/20 is running 3.53 mp faster than the 65 reading. Guess when I set my cruise at 80 going to OKC a few weeks ago I was going substantially faster than I thought. Looks like I was doing 84.3. I will need to keep that in mind when I try to see what my top end is. 
Thanks all. I was thinking the number for the sidewall was the actual sidewall height. Thanks again. Guess I better check mine out since I replaced the Kenda 305 with a Nitto 315.
A 10 up will have minimal impact. I would not worry at all about that. It's only when you start jumping major numbers, like 275 to 345, you need to worry. In those cases, you have to look at front to back so the nanny controls work right etc.
A 10 up will have minimal impact. I would not worry at all about that. It's only when you start jumping major numbers, like 275 to 345, you need to worry. In those cases, you have to look at front to back so the nanny controls work right etc.
Front to back has to be extreme to cause problems. I had almost 3 inches of difference and it all worked.
A 10 up will have minimal impact. I would not worry at all about that. It's only when you start jumping major numbers, like 275 to 345, you need to worry. In those cases, you have to look at front to back so the nanny controls work right etc.
@dangerdarrell, I made the same change, went from stock Kenda 255 to stock Kenda 305 to Nitto 315, and the delta from 305 to 315 is enough to keep an eye on. Round numbers, below 75 I just add 3, above 80 I add 5.
Wife has I Phone, downloaded "Speedometer" app to use to calculate difference between stock and after market tires/wheels. Worked out pretty close to Speedometer Calculator.
My general rule of thumb for changing tire sizes is as long as you stay with about 3% of the original circumference you shouldn't affect the nanny controls. As you go over 3%, expect to start seeing the nanny controls illuminate. Om my 2015 Base Slingshot, I've kept the stock fronts and gone as high as a 295/30ZR18 ( about .4" shorter than stock) rear. I'm getting ready to reinstall a NItto 555 G2 315/35R17 (about .3"taller) on an American Muscle 17 x 10.5" wheel.
I'm running a 345/25/20 rear tire with the stock fronts with zero issues at all. Speedo is off so slight, you can't really tell.(No more of a difference than running a new vs worn tire of the same size)
What I love about this group is we can be so techie sometimes. We love this shit!!
I know I have posted this previously.
I ran a 295 45 18 with a diameter of 28.45 on the rear and 265 35 18s with a diameter of 25.41 on the front.
There was over 3 inches of difference between the tires. I did not have any problems with the nany controls with it flashed for SL wheels.
Anything up to that measurement the sling will handle.
When changing wheel/tire sizes, the safest thing to do to avoid problems is to try and keep any variation between original and new combo sizes as close to the original as possible. Keeping the variation within 2% is best, as you go higher, you are more likely to encounter problems with the safety systems. These are my estimates based on what other folks have posted about their wheel/tire combos.
Discount Tire has a Tire Size Calculator to let you see the difference in sizes and impact on the speedometer - Discount Tire
This site will let you visulaize the impact of changes including wheel offset - Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit
Folks: Not being a tire guru but I am wanting to change my SL slingshot tires. Currently, I have the OEM tires of 225-45-18 in front and 255-35-20 in rear. Will the following tire sizes work on my SL slingshot without affecting any safety systems: 235-50-18 in front and 255-40-20- in the rear? Recommendations please if any.
Folks: Not being a tire guru but I am wanting to change my SL slingshot tires. Currently, I have the OEM tires of 225-45-18 in front and 255-35-20 in rear. Will the following tire sizes work on my SL slingshot without affecting any safety systems: 235-50-20 in front and 255-40-20- in the rear? Recommendations please if any.
I wouldn't put a 20 inch tire on the 18 inch front stock rims. You may have a safety issue there!
If what you wrote is a typo and you meant 235/50/18 fronts, then yes, you are just fine! The difference in rolling diameter from the stock combo is under 4mm.
My bad: The front in fact I meant to say 235-50-18 rather than 235-50-20. I fixed it on my original post. Thanks TravAZ for seeing the error and providing input.
Folks: I will be getting new tires soon and probably will be going with Michelin Pilot Sport 4S. I am torn between the 2 sizes below so you gurus let me know if either of these sizes will fit on my SL (OEM Front: 225-45-18, Rear 255-35-20) without having any ABS, traction, etc, ( nanny issues).
1. Front: 235-50-18 Rear 255-40-20
2. Front: 245-45-18 Rear: 255-40-20
If both will fit, which would you recommend and why? Thx guys.
New rear tire get's put on today. I am rolling the dice and going with a Nanking NS-20 275/35. Have heard a lot of good things from people that have had them on other performance vehicles. The major gripe seems to be that they wear fast.
