Wheel and Tires Options for 450 whp Dart and Autocrossing

[MuuMuu101]

Hey all! So, I don't post very often, but I figure I could use your expertise. I've got a '68 Dart that is currently being built and should hopefully be no more than 3 months away from being completed. The '68 Dart will have a 408 small block making 523 hp @ 6000 rpm, a T56 transmission, powering everything through an 8 3/4 with 3.55 gears and an Eaton TrueTrac. Suspension-wise, the Dart will be running a full Hotchkis TVS with 1.06" T-bars and Fox single-adjustable shocks. When I built this car, I wasn't necessarily expecting I'd throw 450 whp in it right away. The engine I bought was a Paxton supercharged 360 making 390 whp but when I took it to the engine builder to give it a once look over we found out that one of the pistons was cracked and unusable (I wasn't really shocked, I was expecting the worse even though it was autocrossing a week before I bought it). So, I told the engine builder to stroke it as I could sell/save the supercharger and the supercharger would have been too tall to clear my stock hood.

Now my potential problem is that the car is currently sitting on 18 x 9" wheels and 255/40/18 Michellin Super Sport tires on all 4 corners. The main purpose of the car is to autocross at least once a month, road race once or twice a year, and also serve its cruiser duties. The current tires are barely rubbing the quarter panels in the back so I'm having the shop who's working on it roll the fenders and possibly install an offset shackle kit that will move the leafs in 3/4". My question is, do you think this tire will be enough to handle the power? I was thinking about moving to a wider wheel and tire to be used for racing only once fender and shackle mods have been implemented but I don't think I'll fit much more than a 275/35/18 on all 4 corners, especially in the rear. I'd most likely upgrade to a Bridgestone RE-71R at that point.

If that's not enough tire, what are my other options of fitting a larger tire under there? I don't know how much a mini-tub will help. I don't want to fully tub it and move the springs in further as I may have exhaust, sway bar, and gas tank issues. I could go Tomwheel's Valiant route and flare the fenders a tad. What are your thoughts?

Picture just because...

[Rod]

a 255 is a decent size...the 275 all four would be better, you can have a VERY competitive car....driven properly and tuned... on the 275 at all four corners....wider is better...the shorter side wall 35 vs 40 will help the car react quicker to steering inputs and will have less roll

[Nine Ball]

Be careful about just trying to max out tire widths front and rear, without considering them as relative to a complete set of four. I see a lot of people try and stuff as much rear tire under a car as possible, and then wonder whey they have more push/understeer afterward. I'd suggest doing some auto-x or road course in the car with the current setup first. If the car is tail-happy and oversteers, then going wider in the rear will help balance that. If the car understeers and drives straight off of a curve, then you'll want wider front tires to balance the car. We've found that most front-engine, rwd cars are balanced in handling when the front and rear tire widths are the same. Wider is better, in terms of keeping things relative, too.

[MuuMuu101]

Be careful about just trying to max out tire widths front and rear, without considering them as relative to a complete set of four. I see a lot of people try and stuff as much rear tire under a car as possible, and then wonder whey they have more push/understeer afterward. I'd suggest doing some auto-x or road course in the car with the current setup first. If the car is tail-happy and oversteers, then going wider in the rear will help balance that. If the car understeers and drives straight off of a curve, then you'll want wider front tires to balance the car. We've found that most front-engine, rwd cars are balanced in handling when the front and rear tire widths are the same. Wider is better, in terms of keeping things relative, too.

Well, I'll have my sway-bars to help me out a little bit with regards to under/oversteer. I'm just trying to find out how much tire is going to be enough as I don't want to be doing burnouts in the straight or that sort of thing. As far as tire setup, I was always planning on keeping it square. Right now, I could probably fit a 275/35/18 in the front with minor modifications. The rear, with offset shackles should be able to fit that sized tire just fine.

[Gdub]

Keep it square and run 18x9 and 275s , thats what I have done for track. You can rotate them and get a little more life out of the tires and its even grip front and rear. I had BFG KDWs and I switch to a Kumho PS91s and because of size and softer tire it was a big improvement. I am keeping a separate set of wheels around for cruising because I like the staggered look better, 18x9 and 20x10 :) Plus I put a lot of road miles on them and dont want to wear out auto tires.

[Tomswheels]

Samy you need 18x10s in front for 275s and 18x11 or 19x11 in rear with 315 or 305 if you are ever going to put that power down effectively. Anything less is effectively pissing into the wind...