Hey everyone, could use some advice in choosing a tire for my 70 challenger. I knew I should have done the mini tub while I was doing some of the rust repair work, but I couldn't take the stench of burning factory undercoating permiating my house anymore and now the rear is all powder coated LOL

The setup is a 2018 Hellcat pullout, MMX cam, ATI 10% lower, E85. Should make around 800whp now, but may possibly do an upper pulley in the future for a little more power. I'm using the 8HP90 trans
Strange S60 w/ 3.23 (lowest I could put in it)
1' offset hangers/b body width

2nd gear in the 8hp90 is 3.1, I'd be launching from 2nd.

the issue I have is speaking to Mark Williams, the critical speed of the slip section driveshaft will be somewhere in the neighborhood of 6500 RPM's.

Originally I wanted to run a 325/50/15 ET street R (I like to drive the car on the street as well as to and from the track). I could fit it just BARELY, but it should work. I'm a little concerned about the height though, I think the car will benefit from a taller tire and it'll keep me further away from critical speed

I don't think i'll be able to fit a 315/60/15 ET-R with the added height and width, so the only taller option I have is a 295/65/15 which gives up a bit of tread width.

DS speed of 6500 occurs at 168 mph with the 28" tire
DS speed of 6500 occurs at 180 mph with the 30" tire

Do you guys think a 295/65/15 is going to be enough tire for this car? I know the rollout will help along with reducing the effective gear ratio, but just worried about giving up the width. I think I really need to run a 30" tire, that brings me to about the same effective gear ratio as a factory Hellcat with a 28" tire and 3.09 rear so I know it should work decent. Just don't know if it's enough tire to plant the power with caltracs.

Car weight w/ driver?
Front and rear suspension?
Street car or race car with license plates?

People are laying down amazingly quick passes on 275/60R15 "pro style" DOT radials, but those cars are seriously scienced out. If the chassis doesn't work, it really doesn't matter how much meat you stuff under it.

Your guess is as good as mine for race weight. I never weighed it with the old setup, but it’s manual everything with no AC, RMS front end, all steel except for the hood and I welded in a mild steel 8.5 cert cage and some pretty hefty rectangle stock subframe connectors. .
Some pretty significant weight shaved off between the weld wheels, seats, hood, k member but then a lot added back with the cage and who knows how much the 6.2/8hp90 weighs.

I’m planning for it to be a street car that sees the track fairly often but can still be driven sort of comfortably there

I would imagine the weight would be around 3450 without me in it but I could be wrong,

Read is caltracs mono leaf with their low profile sliders.
First time using caltracs so really not sure what to expect.

Almost every fast leaf spring racer uses Caltracs, you should be good to go there, especially if you have stiffly valved adjustable shocks in the rear.

I wouldn’t sweat 20 mm of tire at all, guys are running mid 4’s in the 8th on a 235 drag radial, but it takes lots of work with the tune and suspension at that level.

I would expect yours is going to need at least some power management on the starting line though- between the trans, the explosive power delivery of a supercharger, and I assume a tighter converter than a normal street strip 727. On a radial prepped track it will be ok but on a normal t&t night it might be a struggle.

Tires are only one part of the equation. Not the only part. And has been pointed out here, some really impressive numbers are being laid down with those tires. But they have spent a lot of time to get the rest of the combo right.

With good shocks and a free moving suspension - no problem. I have seen more power on a 275 radial at no prep so you should be good to go with a prepped track.

Keep in mind that the 315/60/15 is slightly narrower and also taller than the 325/50/15. If I am reading your post correctly, if the 325/50/15 is just a tad too wide, and you want something slightly taller, the 315/60/15 may be just what you're looking for.

Thanks everyone, sounds like I might be able to make it work (with some work on the suspension tuning, which I figured would be the case). The car should be set up pretty good to hook with the coil over/double adjustable shocks up front, caltracs adjustable shocks out back and the split mono leaf/caltrac slider so it'll just me a matter of me getting it sorted out.

IROC78, I know plenty of guys have run the 325/50/15 with this setup and my measurements seem to confirm that. the 315/60/15 has a slightly wider section width though (despite the tread width being narrower). M/T says the section width is only wider by something like .3 IIRC but I think the height combined with the added section width would get me. I think the issue would come from where the inner wheel houising tapers into the 1/4 panel lip, the added height would put it much closer to that tapered section.

I could be wrong and it might fit, but from what i've heard the 325/50/15 is a very tight fit, so I would assume .3" wider and a whole 2" inches taller in diameter would be too much. I could be wrong, but it would be so close I think I'd need a tire for mockup to make a determination and I don't know if it's worth looking for one when i'm almost positive it won't fit without rubbing pretty bad.

If anyone has successfully done it though, please chime in as that might give me some motivation to find a mockup tire and see if it'll work on mine.

Dizusters 62 Turbo Savoy hooked fine with the same M/T 275/60-15 driven on the street Cal tracks, good shocks, trans brake, 9.55@142
Doug

My friend Douglas' '70 runs MT 295/65R15s. I like the taller tire myself, but I don't have the gears and/or cubes to pull that so I stick w/ 275/60R15s. That's his car below.

I run the 295-65 M/T ET Street R radial on my cuda. It weighs around 3800 lbs and has been 8.70s at 156+ mph. It will go just as fast on a 275-60.

315/60/15"s on mine. It is amazing the times being run these days on little 275's and 28x10's.

How about them guys on 235

Yes, them too.

I used to run MT Street Radial 315 60 15, but no longer available, also not allowed in our Street Eliminator Class, This season we all switched to MT Street Radial 295 60 15.

Initially, I was concerned in going to a smaller width tyre, but has been great & no issues. My PB o 295 to date is 7.40 @204.

Most of the cars in our class also run the 295, with 4 Bar, most are within the 1.15 to Low 1.20’s range for 60fts