5280/(31/12 x 3.1416)=650 tire revs/mile so 4.11 is 5000rpm x 60min/hr/(4.11 x 650)=112MPH 4.56 = 101MPH 5.14 = 90MPH
What kind of et are you looking for? You're gonna need a different rearend for 5.00s than you do for 8.00s. Yea, it's a pretty radical example, but you get the idea. Larry T
you might try using the calculator on Randysringandpinion.com http://www.ringpinion.com/Calc_RPM.aspx If you know your approximate MPH at the 1/8 mile, you can plug in your MPH, tire size and RPM...... and then play with the gear ratio until you have max RPM at that speed. For example if I key in 31" tire, with a 4.11 gear ratio, and 112 mph, it tells me the RPM is 5123 for an auto trans.
You may want to take a moment to re-think exactly what it is your asking for here... You've got an 1800 lb, 225 HP car, and you want to hit the traps in an 1/8th of a mile at 5000 rpm..... Hmmm... Ok.... Running your 1800 lb, 225 hp combo thru one of the online 1/4 mile calculators tells me your car is capable of 11.85 @ 117... But since we live in the real world, let's call it 12.0 @ 110. A 12.0 @ 110 translates from 1/4 mile to 1/8 mile as a 7.75, and probably 75 mph. So since your asking for a gear that's going to put you @ 5000 rpm thru the traps in a mere 1/8 of a mile,,, it's going to work out to something like a 6.0:1 rear ratio. Are we assuming you will be in top gear thru the traps? Can you shift thru all the gears in a mere 7.75 seconds? Are you willing to live with a car that has a top speed of 75 mph, and has to buzz to 5000 rpm every time you reach that speed???? Planning on doing any expressway driving with it? Perhapse I'm reading your question wrong... Perhapse you stated your question wrong... Perhapse you didn't take all the factors into account whan asking the question... Whatever the reason, I'm just saying.................... As per my personal expirence, I'v been down the 1/8 mile hundreds of times, and in a manual car, I'm lucky to just hit third as I enter the traps[4 or 5 speed]. In an auto, it's always just shifted to second. My favorite all time car for the 1/8 mile was a 340 powered plymouth duster with a 4 speed and the factory 3.90 gear. I could always get that car into 3rd well before the traps and build power as I hit the timing lights. The duster was right at 3000 lbs and darn close to 300 HP, so I think it translates pretty good to your 1800# 225 hp bucket. So if it's a street car that will see a little strip duty, go with 3.90's. A 50/50 street strip car, 4.11's. Strip car that will see limited street duty, 4.56's Strip only,[and 1/8 mile only], 5.13's
I agree with the 1/8 mile et of 7.75 for a 12.0 et 1/4 mile car but the trap speed will be closer to 90 not 75. My 64 runs the 1/4 at 12.8 at 108 with a 2.0 60 foot time. 1/8 mle trap speed is 87 with an et of 8.3 .
Since you are going to run 31's, I'd run 4.11's if you drive on the highway, but if it was my car I'd try some 4.56's. I've had 4.10's and 28's and was buzzing 4k to go the speed of traffic with a TH400 trans.
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