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Omer 08-26-2011 04:42 PM

What size tires to use for best off the line acceleration 0-60 mph (1968 Firebird)
This is a 1968 Firebird, with no modifications to the body, a pure street driven car with all the accessories less a/c. It's got Hotchkis total vehicle system consisting of sub-frame connectors, rear leaf springs and front coil springs with Bilstein shocks and the front & rear sway bars. Also PST poly-graphite complete front end kit and body mount bushings. Overall the car is stiff and handles great around the corners.

The engine is a 350 SBC with stock 64cc #492 angle plug heads (2.02/1.6 valves), flat top pistons, around 9.5:1 compression, XE256H cam, weiand stealth manifold, 670 cfm Holley street avenger carb, 4-2-1 headers (Doug's Tri-Y) followed by a Magnaflow 2.5" dual exhaust. I happen to enjoy the wide torque band and the throttle response of this combination. It's never been on a real dyno, but desktop dyno shows about 400 torque @ 3500 rpm and 322 hp between 4500-5000 rpm. It doesn't see past 5500 rpm. I never race it. My kind of fun is the best acceleration off the line and 0-60 times. I think this setup has enough torque for me now, if I could ever manage to keep the tires planted all the time.

The driveline is kinda weak. I have a 700r-4 with no mods in it and the rear is GM 8.5" 3.23 ratio gears with a cone type limited slip. I like the 700r4 giving me a deep 1st, compensating for the low rear gear (I think going higher would give me even more wheelspin with a street tire). For now, I don't care that the 2nd gear is too far away from the 1st gear ratio since I have good low end torque.

Well, the driveline has been holding up for now, but I don't know for how much longer. I definetely need some improvements here, but first, I need to get rid of my sorry tires!

They're Kumho 712's 255/45/16 rear on 8" rims, and 225/45/16 front on a 7.5" rims. They're long overdue for a replacement and I'm trying to find out a sensible size for my setup. I'm not sure if the wide rear tires really suit my driving style. How about 225/50/16 Eagle F1's on all four corners? I know that wouldn't look anywhere near cool, but I think I will get better heat management with that size and much better adhesion over Kumho's. Also how about going down to 15" diameter wheels? Would more sidewall height give me better traction off the line? (and for the downside, hurt cornering stability?) I'd really like to see what you guys think about it.

There's huge information everywhere on how to get more horsepower, but when it comes to tires, there's a whole lot of options and even more misinformation. I think this is often overlooked but I feel this is where the biggest gain would come in, regarding 0-60 times. If I can manage to minimize wheelspin, I'll go after some cal-trac style bars, a better limited slip and a beefier 700r-4 in short term.

el pollo 08-27-2011 06:39 PM

I'll tackle this first: No, don't go to a 15" diameter for this car. The sizes that most commonly fit are the 235/60R15 or 245/60R15 (although the 245s, depending on the width of the wheel, could rub.

The best fitment, in my opinion for your car in a 16" diameter would be the 245/50R16. Now, there's not a huge variety, but you can get some well balanced tires for your car. Two tires right off that I would likely recommend if I were to sell them to you, would likely be the BFGoodrich G-Force Super Sport AS or the Michelin Pilot Sport AS Plus. Performance between both tires would compliment what you have already done to the car, without being so aggressive that you break something. And even if you don't plan on driving in wet weather, the siping will allow better cooling efficiency.

What makes the Michelin better than the BFGoodrich, though, is the banded Aramid (also known as Kevlar) strips that are laid over the steel belts. Although more expensive, they don't react to the cold (or hold memory-producing flat spots) like a nylon overlay.

Funny, I haven't seen the Kumho 712 in a long time. I didn't think anyone would still have those on their cars!

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