Which Rear Gear ?

I have two differnt rear gears for my car and im unsure of what ratio i should use.
My car is a 89 Mustang Lx with a 429 thunderjet power is approximately 400hp and 500ftlbs of toque. Powerband is idle to 5500rpm. Im running a c6 with a towing converter, slightly higher then stock dont know exactly what its stall speed is. My choices are 3.27 and 2.73 I dont have the overdrive so im thinking 2.73 would be nice but what would be the acceleration difference between the two ? The main purpose for the car is just to cruise around in maybe go to the track once, but its not a race car by any means.

thanks

I would go with the 3.27, you'll still be able to cruise, rpm depends on tire height, but you won't like the 2.73's for acceleration.

Im using the stock tire size 225/55r/16 on the 91 "pony rims"

With that tire, you're not going to get very good traction with either gear.
The car will be more fun with the 3.27's but how much rpm at cruise speed is acceptable to you is your opinion only. Neither gear is considered a very low gear or even close to a race gear, they both are street ratios. Ford put 3.25's in mustangs with similar height tires. What's more important to you, low rpm cruising or acceleration? I'd go with the 3.27's and if I thought they were too low, I'd put a taller tire on back. You're going to need something bigger on back anyway or you're going to spin like crazy. I run a 275/60/15 BFG drag radial on a 9" rim on my 67, it does pretty well, but the rubber comes off like butter.

Here's a link to a calculator that might be of help:
http://2.3liter.com/Calc1.htm#ETFromMPH
scroll down to the rpm for speed in gear calculator

The widest tire Ive heard that I can fit on the rim without hitting the quad shock is 245/50r/16.

Forgive me if this sounds like a newb question. But If Im spinning like crazy with 2.73's why would i need to get more acceleration out of a 3.27 that would spin even more. Or would the 3.27 rear really improve the pull of my 2nd gear?

Whats important to me is an overall street car that can really pull hard. I have a big block so i know fuel consumption will be high itd be cool If I could get 15-20mpg with the car. But it is not my daily driver.

My goal with my big block mustang was to have it look close to stock and be in the high 12's low 13's for quarter mile time.

For some further detailed info on my build I have a 429 thunderjet with a stock rebuild long tube headers into x pipe with a 2.5inch flowmaster exhaust. accel ready to run ignition ,and a 750 holley with vacuume secondaries. No powersteering electric fan, and a drive belt pulley system.

If you want to run high 12's then go with the 3.27 gears for sure IMO.

Later Mark.

From a dead stop, you'll spin with either gear with that tire and stock suspension. With the 3.27's it will pull harder once you get traction. It will also pull harder from a rolling start. 15-20 mpg is probably out of the question. :( I wouldn't even consider the 2.73's but that's just me. Pulling really hard and 15-20 mpg are two different things, you need to decide which you want. I doubt you'd see much mileage difference with the 2.73's but if it made the difference between getting 10 mpg and getting 11 mpg, maybe that's important to you. It's not your daily driver, don't worry about mileage, have fun, run the 3.27's. :D

What would this gear be like for my combo ?

Big Improvement in acceleration ?
How much loss in MPG with this gear and the c6 ?

If this car is for cruising, which is what you said...then you want the 2.73. That will put you around 2kRPM @ 60MPH... You don't have overdrive, so you need taller gears. If you want a street/strip rig, then you need an OD ratio much lower than 1 so you can raise the diff ratio to offer you the launch you need at the track since you won't be using the OD, just the lower gears, which combine to keep the motor at the target RPM range...which is of course dependent on the way the motor is set up... Lots of details to work out, sure, but the easy answer to your question is 2.73 since that is your option.

i'm running welds on my 86gt and i got 265-50's on it by flipping the quad shicks over.

Right now the car has the 2.73 ratio

Its shifting at full throttle from 1st to 2nd at 70 mph and I would like it to shift
at around 45 mph.

What gear size would you recommend ?

All else being equal to achieve your 45 mph shift point you would need roughly 4.20 gears....

With the weight of your car and the stated intended use I bet you would be happy with a 3.5 or maybe as low as a 3.73 gearset. Anyhthing lower will be annoying and expensive to drive anywhere on the highway.

Seeing as you already have them I would try the 3.27's first and see how you like them....

With street tires and a 400 hp big block it isnt going to matter much what gear you have in it you will have a traction issue if you try to nail the throttle it's just simple physics. How much you smoke the tires will depend on the weight of your right foot. :D

dkp

The widest tire Ive heard that I can fit on the rim without hitting the quad shock is 245/50r/16.

Forgive me if this sounds like a newb question. But If Im spinning like crazy with 2.73's why would i need to get more acceleration out of a 3.27 that would spin even more. Or would the 3.27 rear really improve the pull of my 2nd gear?

Whats important to me is an overall street car that can really pull hard. I have a big block so i know fuel consumption will be high itd be cool If I could get 15-20mpg with the car. But it is not my daily driver.

My goal with my big block mustang was to have it look close to stock and be in the high 12's low 13's for quarter mile time.

For some further detailed info on my build I have a 429 thunderjet with a stock rebuild long tube headers into x pipe with a 2.5inch flowmaster exhaust. accel ready to run ignition ,and a 750 holley with vacuume secondaries. No powersteering electric fan, and a drive belt pulley system.

Who needs quad shocks anyway, they are not a big help. If you put a 15x8" with a 5.5" backspace weld on there you can fit a 28x11.5" tire and you won't have a traction problem. I have that rim with MT et drags 275/50 and it is good with 650 RWHP. Good Luck!