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Discussion Starter #1
My wife and I have a 2019 SLR with factory tires. The rear is starting to wear and I'd like to have a new one on hand to put on soon. The factory tire is a 305=30-20.I want to go to a 295-50-20. I've even seen a 295-45-20 I liked. Any comments or concerns about my plans???
 

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Might want to compare sizes with this chart. I went with 315/35/20, but have read that narrower tires offer better traction on wet roads. I;m sure more knowledgeable folks will chime in shortly.

 

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Discussion Starter #3
Thanks! I want to go narrower for sure and, I want a taller sidewall. Once the front tires wear down I would buy proportionately similar tires there too.
 

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Two great sites for tire info are - https://www.discounttire.com/learn/tire-size-calculator and https://www.willtheyfit.com/. The DT Tire Size Calculator provides a more accurate speed impact, but WillTheyFit provides a visualization of the change between tire sizes AND also allows you to specify the wheel diameter AND changes in wheel offset. Unfortunately, the speedo calculations on WillTheyFit show high when they should be lower and vice versa, but the DT Calculator is spot-on.
Looking at the tire sizes you mentioned, both the 295/50 and the 295/45 tires will be significantly oversized with a noticeable impact on speedometer accuracy. A 295/50 tire will actually be doing over 75 mph at a speedo-indicated 65 and the 295/45 will be doing almost 73 mph at an indicated 65. A 295/35 tire will be doing just over 67 mph at an indicated 65 and a 295/40 will be doing 70 mph at an indicated 65. Personally, I like to stay just under 5 mph over the speed limit so the 295/40 tire would place me at my favored limit at an indicated 65.
Good luck choosing your next tire.
 

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You may also have to go to a larger diameter tire on the front wheels to avoid ABS error codes (I think the computer threshold is around 10%). Also keep in mind that just increasing the rear tire diameter will raise the rear and tip your headlights down and slightly reduce your front airdam clearance. I changed to a larger aspect ratio tire in the front without doing the back (yet). I went with a tire that is 1.5 inches taller in total (same width) and had the opposite issue (headlights blinding oncoming traffic, but much better ground clearance. The ride is vastly improved and the handling really did not change much. I like the chunkier look as well. 1.5 inches taller is about the max you can do in front without clearance issues at the back of the outer lights at full lock.
 

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Discussion Starter #6
@techkraut... I’m not sure what you mean by 10% or get ABS error codes. If I get these rrror codes, cant I just ignore them until I go to a taller tire size in the front too?
 

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@techkraut... I’m not sure what you mean by 10% or get ABS error codes. If I get these rrror codes, cant I just ignore them until I go to a taller tire size in the front too?
I got this from another thread. I stayed within 10% and did not have a problem. It is possible that the ABS will be turned off, if you get the error. ABS works by detecting a difference in wheel speed that exceeds a threshold. If the wheel speed delta exceeds baseline, the computer doesn't know how to deal with it
 

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Discussion Starter #8
So, I shouldn't go any more than 10% larger in circumference than my stock tire? The circumference would determine the rotation of the tire right?
 

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I think there is already a small difference between the front and rear stock tire. Use one of the online calculators to check your front tires' circumference and then pick a size for the rear, that is no more than 10% larger. Or simply swap both front and rear at the same time.
 

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Discussion Starter #10
I hope that doesn't happen, but if having the ABS off is the worst thing, I can live with that....until I replace my front tires with a closer match to the new rear ones.
 

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I have a 2015 base Slingshot and have mounted replacement 18" tires ranging from the stock 265/35 size up to a 295/30 on the stock rear wheel. Generally speaking, as you increase the tire width, it also helps to drop the profile each time to help keep the speedometer reading as close to stock as possible. In my case, I was able to run up to a 285/35 18" rear tire while staying within about 2 mph of the indicated speed. A 295/35 tire would be running just over 65 mph at an indicated 65, while the 295/30 actually was turning closer to 64 mph at an indicated 65.
 

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Discussion Starter #14
I ended up ordering a 295/40R20 rear tire. Looking forward to getting it put on sometime this week.
Spring is here in the South.
 
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