I have reached the tire purchasing point of my build , and have a few questions for the weekend warriors who run thier cars at the track. I will be running the quarter at NewEngland Dragway at some point so I have decided on MT ET Street Drag Radials on 15" x 8.5" Centerline Auto Drag wheels in the back. Up front ,MT frontrunners on 15"x3.5" Centerline Auto Drag wheels. My question would be , can I move up to a 30" tall rear tire or will that have an adverse effect on my final gear ratio? I am on the fence between 28" and 30" . Some info on car: 71' 462 Demon 850 carb Mild Isky Cam Small valve heads ( Stage 1 irons in the fire)(pun intended) MT SS Headers 3" Torque Tech exhaust Stock trans convertor TH 400 ( upgrade to JW 9.5 planned) 8.5 posi w/ 3:46 gear (cone style carrier, upgrade to 3 series GM carrier planned) Curious as to what others are running in the rear, heightwise , that frequent the quarter mile? Thanks for any and all replies. ........Jim Tapatalk User
play around with this and some of the other Wallace Racing calculators....gets you in the wheel house most of the time... http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php
All well and good, and ty , but I have no information to input. Be helpful after the fact, but not outta the gate. Looking for people with similar setups to chime in on what height works for them. Jim Tapatalk User
Yes, tire height affects final drive ratio. The gear ratio will vary directly with tire height. Your gear ratio is based on a stock height tire, say 26.6" tall. With a 28" tire, old tire height/new tire height X rear gear = effective gear ratio, 26.6/28 X 3.42 = 3.25. With a 30" tire, you get 3.03. Ideally, you want to be going through the traps at the RPM where your motor makes peak HP. A taller tire will kill your RPM at the end of the 1/4 mile. What you want is a 26" tire. I recommend the MT P275/50R-15. https://www.amazon.com/Mickey-Thompson-Street-Racing-Radial/dp/B00540H88I Figuring out RPM at speed is easy. 336/tire height X rear gear X speed = RPM (without converter slip). Let's assume you will go through the traps at 105 MPH, 336/26 X 3.42 X 105 = 4641 RPM That is without torque converter slip, so add 250 RPM. You'll be at 4900 RPM. That is probably about 500 RPM short of your HP peak. If you go faster, the RPM will be more, slower, the RPM will be lower. The last thing you need is a 28 or 30" tire. Go with the 26" tire.
Larry, I value your opinions as I know they are based on real world experience. Thanks for the tip, I've already added the tires in link to my Summit wishlist. TY also for explaining why they are a good pick. Now to figure my front height/stance . I imagine that will be easier with rear tires mounted. Thanks again, Jim /Rott Tapatalk User
I've been told that 28's hook better than 26's. Is there any truth to that? If not then that would help me out on a street strip tire purchase.
So besides hooking better it changes your overall rear gearing as well. So I am assuming that is what helps it hook better too? And the fact that the walls wrinkle up? Lets say I'm running a 3.23 gear, going to a 28 inch tire brings that down to around a 3.08. Right? Which spreads your torque curve out more? Correct? Which would help out your mid range power? I'm running a 455 with a 268 cam that says peak power is rated 1500 to 5200 rpms which is degreed in 6 advanced so that brings power band down a little. And a tubo 350 with a 2000 stall.
No, it's just the taller sidewall that hooks better, maybe because there is more sidewall to deform, but I don't think Drag Radials wrinkle like a slick does. It has nothing to do with the torque curve or mid range power. When it comes to 1/4 mile, you want your engine to be at peak HP RPM as you are going through the traps. The taller tire reduces your gear ratio, and it will reduce your RPM at the end of the 1/4. Brings a 3.23 down to 3.07. I have 3.73 gears which come down to 3.54 with the 28" tire.
Thanks very much for your response larry. These are what I'm considering at the moment. Mickey Thompson ET Street S/S Tires 90000024554 28 tall https://www.summitracing.com/parts/mtt-3453/overview/ Mickey Thompson Sportsman S/R Tires 90000000231 26 tall https://www.summitracing.com/parts/mtt-6653/overview/
