What is the best rear end gear for my 1968 Mustang? - Ford Mustang Forum
 
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post #1 of 13 (permalink) Old 04-16-2006 Thread Starter
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What is the best rear end gear for my 1968 Mustang?

i got a '68 coupe and i am debating on what gear to put in the a##. i want to drive it on the "e-way" and the track. any ideas would be great!!!!!!!!!!!!!!!
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post #2 of 13 (permalink) Old 04-17-2006
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Somewhere on this site I think there is a whole tread devoted to what gears folks are runnin' you might check there to see what poeple are saying. You didn't say much about your car so it is pretty hard to say what gear you should be running. Let's assume you have a mildly reworked 289/302 (carb, intake, headers and maybe a conservative performance cam). Then I would say with an automatic go with a 3.25:1 if you do a lot of highway driving or if most of your driving is in town you can bump it down to a 3.50:1. If your running a four speed then a 3.50:1 or 3.70:1, If your transmission has been updated and you have an overdrive maybe go as low as a 3.90:1 depending on your driving habits. Stay conservative and you will be happier in the long run. To run a really low gear takes a lot more motor which also means suspension mods which means...it goes on. The gear ratios above strike a nice balance between drivability and performance. I've made the mistake of running low gears with a stock motor before(many moons ago). It doesn't work out. In this case it was a stock 283 Chevy with 4:56:1 gears. I was young and dumb at the time and gas was less that a buck a gallon. I ended up blowing the motor...it just wound up to quick and didn't have the RMP range to handle that much gear. If you are going to drive it regularly then stay conservative, your car will last longer and you'll be much happier with the results.

Mike
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post #3 of 13 (permalink) Old 04-19-2006
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Couldn't have said it better than mike. You do have to carefull, because if you listen to the newer car guys talk, they are all going with 3.73, or 4.11 etc. This is ok for them as they have the overdrive. If you have either the 3 spd auto, or 4sp manual, you have a 1:1 top gear, witch is equivalent to a 3rd on an AOD, or 4th in a five speed.
3.25s, or 3.5 is all you want.
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post #4 of 13 (permalink) Old 04-20-2006
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Hello.That's why I try to keep two cars.One of mine has a 3.00:1 and the other has a 4.11.One for getting groceries and one for getting whiplash.
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post #5 of 13 (permalink) Old 04-22-2006
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Cool rocket sled or shopping trolley?

Hi Andy,
Veronica makes a very good point. When Ford designed the car, it came up with a trade off between acceleration and mpg. You are about to change that trade off and move towards one or the other. Be honest when you make that decision, about what you want from the car or you will be very unhappy with the result. As Veronica succinctly put it, she has 2 cars so there is no trade off. Also, since you may have slipped a 351 in there, a taller rear would not be that bad, say a 3.0:1 since the motor has a lot more torque, or a 3.25:1 if the 351 engine is a little bit worked.
Good luck and let us know how it goes.
cheers n beers
Mick:thumbsup

Time and Money, our two main weapons in the fight for the perfect stang. nb I dont have enough of either
Toy: 70 Coupe, 351Clevo, 11.7:1cr 2v closed chamber ported heads, twin GRA propane/LPG converters and 900cfm twin mixers, twin 2 1/2" exhaust system, RightHandDrive conversion, 15" wheels, stock power disks and steering. A work in progress.
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post #6 of 13 (permalink) Old 04-25-2006 Thread Starter
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sorry guys i should of let you know that right now it has a stock '68 302 but i put a edelbrock performer intake with a 725 cfm. demon barry grant carb. and you guys are right, alot of my friends are saying 3.73. i've even gone as far as got a quote from moser & strange. now thanks to you guys i am going to re-think my gear in my head as well the gear in my rear end. THANKS ALOT
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post #7 of 13 (permalink) Old 04-25-2006
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You may want to rethink that carb too, that's awful big for a stock 302. You may want to tone it down to a 600cfm Holley with vacuum secondaries or better still a 4100 Autolite. Believe it or not She'll run better. The Performer is a good conservative choice on intakes nothing wrong with that. Add headers and the correct cam and you'll have a nice combination that will run well and be easy to live with.

Mike
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post #8 of 13 (permalink) Old 04-25-2006
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Hello here's a ballpark figure for how big your carb should be.
3456/(Max rpmX CID)=437cfm That's with 5 grand for realistic maximum rpm.The carb that you have will drown the car.Foul the plugs,puddle in the intake,poor vaporization,contaminate the oil, all kinds of bad stuff.That 4100 autolite suggestion is a good one.If you get a 480cfm 4100 the car will run much better than it does with XXXL Demon.For a small block that's a carb for a purpose-built ,roller rocker, roller cam with radical profile, solid lifter race motor that is completely, utterly unstreetable.If, for whatever reason, you think that my calculation is just for girly cars, go look at some new, still in the box Holleys.There's a chart on the side of the box to tell you what you need.Hope that helps.
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post #9 of 13 (permalink) Old 04-27-2006 Thread Starter
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Re: What is the best rear end gear for my 1968 Mustang?

i do plan on building the motor up enough to handle the carb. for example i put smaller jets in it for the time being, and beleive it or not it improved the 1/4 mile time .3 of course it still runs a slow f@@##&g 16.40 but i think the rear end is a HUGE contributor to that. and the c-4 isnt in the best shape either. i do plan on putting a set of alum. trick flow heads & cam in it. also putting a B&M trans. in it. so i guess i am back to the gear which, thanks to you guys will probably be a 3.55.p.s. Veronica yes your suggestions did help and by the way, i named her MISTY . thanks again!!!!!!!!!!!
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Re: What is the best rear end gear for my 1968 Mustang?

It's still to big. The point was a demon carb is not a street carb and part of the reason the car is so sluggish is because you're dumping to much gas into it re-jetted or not. Carrol Shelby in a race prepped motor was , if I remember right, only running a 715 Holley. Nothing that Ford built as far as small blocks go came with anything near that size. Tri-powers on the 289 only flowed about 600cfm. The dual quads flowed about 1000cfm but again that was on a full race motor designed to run at full throttle. Resist the bigger is better symdrome-its not. Size does matter but in this case smaller is better not bigger. Even with the heads and all, you will have to over cam your motor to the point that it isn't streetable to use the Demon. Now it is your car you can run what you want in it but believe it or not if you want to go fast in street trim, you need to lose the carb and go with something smaller. If you don't think you can get enough cool point with your frieds running the 4100 or Holley 600 the pop for an edelbrock carb, it's different enough to get attention. A tri-power set-up will get you a lot of attention too if you can afford it. The point is there are a lot of better options than the Demon.

Mike
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Re: What is the best rear end gear for my 1968 Mustang?

I found this very helpful when deciding on a gear ratio. It is the formula for determining the RPM, MPH, with different gear ratios and tire sizes. It is on an MS Excel spreadsheet. I can email it to you if you would like. I tryed to attach it this but it wouldnt take. The way it is set up you can figure out RPM, MPH, Gear Ratios, by pluging in the different numbers and it does all the work.
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I agree that the 750 Demon is too large of a Carb. 600 cfm is the way to go.

sgomustang,

If you could e-mail that excel file to me I would appreciate it ..... I will leave my email address in a PM to you.

Dennis
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BTW, with regards to that carby. You said you put smaller jets in to compensate for it being too large. This is a very common missconception. The fuel is drawn through the jets by the vacuum created through the venturis. When you have a carby that is too large, the engine isn't able to draw the required volume of velocity of air through that carb, meaning also not drawing enough fuel. So what happens is they tend to actually run lean. If you must use that carby, have the car dyno tuned, and map the air/fuel ratio. This way you can get the best jet size possible, (with a carb thats too big)
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