Who Runs 20" Rears?

First off, I should have done more research on the 19/20 combo....had I, I'd realized 19/19 would be a better route. But since I already went that route, I'm kinda stuck for now. RCR recommends 325/30/20 or 335/25/20, but as I've discovered there's not too many options, far less if you don't want run-flats and matching brand/model tires.

At any rate, I'm at the point I need to get RCR some tires for my SLC. I know the clearances are tight in general and even a slightly larger 345 won't fit, but I want to verify that I shouldn't have any issues if I go with the larger of the 2 recommended sizes.... I'm looking at the 335/25/20 in Continental ExtremeContact DW specifically.

Anyone run these? Or what tire do you run on the 20's? I assume these are fine, but definitely don't want some expensive paperweights. :) Thank you for any advice!!

- Del
 
THe problem is mostly not really width per se, but height. The recommendation of a 26.5" max diameter is based on experience with different cars and tire sizes. If you use the tire calculation tools available on the web, you can enter any size and see the diameter- that's the key parameter.

However, you should be aware that tires of the same nominal size from different manufacturers vary in size, sometimes significantly, making the tire size calculator tools an approximate aid at best, especially as you get to the edge, as you are trying to do.

That's why some people can mount a certain size tire from manufacturer M and have it not fit, but the same size tire from manufacturer H will fit.

Finally, "fit" is a relative term. If you are willing to do bodywork, wider and taller tires can be made to fit. And offset plays a role here as well- too much positive offset pushed the tire too close to the fender lip, sometimes causing a clearance issue. Alignment is key as well. If you have the car very low to the ground, you'll likely find that you need to clearance the fender lip a bit. It's not really a problem of defect- just raise the car to the specified height, and run the correct tire/wheel combo to regain the clearance.

As the experienced builders here know, even the "correct" size tire on the car often requires special considerations to get it on and off the car. The wiki even recommends a specific jack that can be used to jack up the control arms so the tire/wheel assembly can be easily slid out the relatively tight opening.

These are not ordinary cars, and the wheel openings are tight, as befits the race cars from which they are derived. It's not surprising that clearances are tight.

The stock-size wheels and tires fit well, given the constraints discussed above. But going off the reservation will always make the problems more complex and harder to resolve.

It's possibly expensive, but the only real way to be sure of a size at or past the nominal limit is to do a sample mount on your selected wheels, on your car, with your alignment.
 
I have a lot to learn about this car for sure, I appreciate you taking the time to inform me so I can make the right decision.

I've discovered in the past that the widths definitely can differ alot from manufacturers but didn't realize the height did as well. This whole time I assumed it was more inner fender clearance issues than outer fender, but what you said makes sense for this car.

I'm definitely not trying to do any kind of bodywork right off the bat to fit tires and want to go with what works. Honestly, I plan to keep the exterior as stock as possible and not divate from the manual for alot of the build to not create myself headaches. I can do that later if I chose, before it gets painted/wrapped.

It's crazy the 335/25 PZero's are 1.6" bigger OD compared to most of the others! I just went through every make/model of RCR's recommended tire sizes (335 & 325) for 20's, but can't find a tire that is shorter than 26.6". I'm unsure what to do from here since I want to keep it a maximum of 26.5" and not run into any issues.
 
This was MUCH harder than I anticipated, I think it was because of the small selection and big price differences... After much debating with myself ended up having to bite the bullet and went with Michelin Pilot Super Sports in 285/30R19 and 335/25R20 for a couple of reasons. #1 they're a great tire in both wet and dry weather. #2 They're run-flats, which I originally didn't want, but I'd absolutely hate to attempt to tow the SLC home. #3 I can track the car at some point if I wished without giving up driving in wet weather locally. #4 It's a SLC and deserves the best

While the Continental ExtremeContact's were the cheapest, they weren't run-flats and knowing how my luck is I'd get a flat and be far away from home. Mixed reviews about them. It's kinda dumb, but I didn't particularly like mix and matching different models. Just an OCD thing, I think.

Also considered the Michelin Pilot Sport Cup 2 and while they were run flats and kinda dug the unique tread pattern, I don't feel confident I'd be okay in the rain. I think once the car is on the road and I decide to take the car out and see a 30% chance of rain, I don't want to talk myself out of taking it because I'm scared to drive it on the wet roads. The SS was $100 more for the set, so chance it.
 

Howard Jones

Supporter
Here's something to think about now well ahead of time. Just how would you winch the car onto a flat bed tow? Where would you attach the tow cable to the car? And can the car be towed onto the flat bed with a front tire down.

You need a tow point on the front of the car. Not on the suspension, or the front radiator mount plates and the splitter brace is out also. So what to do?

Here's what I did for my track car. A much smaller mount point could be attached to the foot box with just a threaded bung protruding form the body work in front of the windshield. Then use a threaded tow ring when necessary. When not in use plug the access hole with a plastic plug on some sort.
 

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I also plan on getting 19 front and 20 rear. I plan to get the new Continental ExtremeContactSport in 285/30/19 front and 335/25/20 rear and a good price so if they wear out faster, as I expect the rears will, then not all that bad replacing just the rears. Also, as they just came out they should be produced for a period of time. Good to know they will fit fine. The Pilot Sport Cup does look nice also though, but same concerns about the wet.
 
Just called up there and Fran said 26.6" OD will be fine. Whew, I was getting worried!
In prior discussions it seemed the max recommended tire size was 26.77" based on experience and sizing for the Nitto Invo tire.

Del - it looks like you already pulled the trigger, not sure there's anything to do at this point but the feedback I've gotten from everyone I know who has run flats is they are tossing them as soon as they can. Handling and ride suffer due to the very stiff sidewalls is the common complaint. A quick google search shows run flats aren't favored by owners though you do have to take it with a pinch of salt as those with bad experiences will be more vocal. My friends haven't had blow outs but my friend with a gt3 running sport cup 2s is ready to toss them in favor of non run flats.

If you're able to change your order I went with a slightly smaller pilot super sport. I have a slightly higher risk to sidewall blowout from hitting potholes due to my 325/25/20 rear but I figured this was an acceptable risk to get that tire and in a non run flat configuration.
 
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In prior discussions it seemed the max recommended tire size was 26.77" based on experience and sizing for the Nitto Invo tire.

Del - it looks like you already pulled the trigger, not sure there's anything to do at this point but the feedback I've gotten from everyone I know who has run flats is they are tossing them as soon as they can. Handling and ride suffer due to the very stiff sidewalls is the common complaint. A quick google search shows run flats aren't favored by owners though you do have to take it with a pinch of salt as those with bad experiences will be more vocal. My friends haven't had blow outs but my friend with a gt3 running sport cup 2s is ready to toss them in favor of non run flats.

If you're able to change your order I went with a slightly smaller pilot super sport. I have a slightly higher risk to sidewall blowout from hitting potholes due to my 325/25/20 rear but I figured this was an acceptable risk to get that tire and in a non run flat configuration.
Thanks for the feedback! Makes me feel a little better about going with a 26.6" tall tire. I've always heard about the ride with run-flats as well, but figured with my luck it'd be good move... but now I have to go back and see if the ones I got are actually run-flats. What's you input on the Pilot Super Sports? Sounds like we have the same tire, but wonder why you went with a 325/25 over a 335/25? Fixing to go look at your build thread, seems 20" rears aren't the common option, lol.

Cam - could you please post a link to your website for your log thus far? I thought I had it bookmarked, but don't and can't find it on here or online.
 
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Stephan E.

Supporter
Hello Del,

First I would like to congratulate you to your purchase. Mine was ready February 17th,2017. I have the combo of 335/25/20 in the rear and 285/35/19 in the front on Michelin Pilot Sport Cup2. I simply love the way they look and I am certain they will stick like chewing gum. Price is high but considered we do not have a spare, the run-flats are a good compromise. Fit is tight and required the wheel arched to be cut away to prevent contact. May I ask how long you waited for your kit, and was it complete when you took delivery?

Cheers,

Stephan
 
The post in which I discuss tire selection is here:

superlitesite.wordpress.com/2016/12/19/8-wheels-up

You can bump back to the main menu and bookmark that though my progress has been exceedingly slow :(

I went with 325/25/20 due to availability, the PSS is not available in 335/25/20. I was willing to lose the 10mm of width and slightly lower diameter to get the tire I wanted to run.

I haven't driven on the PSS yet however, everyone I know who's run them swears by them. A google search will show they're one of the most popular sport enthusiast tires on just about every platform. So based on feedback I was set on going with PSS and not using run flats which led me down the path to where I ended up.

Yes - the 20" rear seems to be quite rare, posssibly due to the very limited tire selection. Did you go with the CF10 or one of the other wheel designs?
 

Stephan E.

Supporter
Hello Del,

I conquer with Cam. I sacrificed width for performance and I was concerned about not having a spare. The price was solala but I do not see a problem with availability. Its a standard equipment Corvette tire. The run flats were a must for me. I chose the Cups 2's since we are having plenty of race tracks here in NJ and PA.

Cheers,

Stephan
 
The post in which I discuss tire selection is here:

superlitesite.wordpress.com/2016/12/19/8-wheels-up

You can bump back to the main menu and bookmark that though my progress has been exceedingly slow :(

I went with 325/25/20 due to availability, the PSS is not available in 335/25/20. I was willing to lose the 10mm of width and slightly lower diameter to get the tire I wanted to run.

I haven't driven on the PSS yet however, everyone I know who's run them swears by them. A google search will show they're one of the most popular sport enthusiast tires on just about every platform. So based on feedback I was set on going with PSS and not using run flats which led me down the path to where I ended up.

Yes - the 20" rear seems to be quite rare, posssibly due to the very limited tire selection. Did you go with the CF10 or one of the other wheel designs?
That's pretty much same thing I came up with about the PSS's too. I definitely needed something for the rain here in Louisiana since even with a 30% chance I'll probably run into a shower and don't want to have that make me not to want to drive it. PSS's seem to be the happy medium between rain and dry.

I went with F14's on mine. I just assumed all PSS were run flats, I had to go back and make sure my 335's were, lol. I definitely could have gotten away with cheaper tires, but the OCD in me made me buy matching sets, lol.

Hello Del,

I conquer with Cam. I sacrificed width for performance and I was concerned about not having a spare. The price was solala but I do not see a problem with availability. Its a standard equipment Corvette tire. The run flats were a must for me. I chose the Cups 2's since we are having plenty of race tracks here in NJ and PA.

Cheers,

Stephan
We have some tracks, but nothing really close by so it wouldn't be a normal thing for me to go run at one. Definitely agree with run-flats, least makes me feel better if anything.
 
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