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Discussion Starter · #1 ·
I currently have a 2.80:1 rear gears in my '66 coupe and would like to switch to something that is going to give me a little better street performance but will also allow me to do 70 mph on the freeway and not be running high rpms. I dont want to put 4.11's but I was thinking of 3.89's, would this be a good gear ratio for what I want?

BTW, the engine is a stock 2v which I switched to a factory 4v with a shelby intake. The tranny is a c4 auto.
 

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Go to prestage.com will give you all options for tire size ,rpm etc. Gary
 

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Well, depends. What trans do you have? Modified with and O/D? On a non-overdrive trans, top gear is 1:1. Most T5 are 0.67:1 IIRC. What size tires are you running? What is the desired RPM and speed? One you got these answered, it is just a math problem to solve. Try this link: prestage.com/carmath/calc_Gears.asp
 

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What is to high of an RPM? You may need to go with and overdrive trans to get the best of both worlds. Yu rpm's will be getting up there with a 3.89 gear. I don't have the chart handy at this time (at work) to tell you how high they will be.
 

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I dropped in a 3.50 rear in my 66 coupe with 289 and C4. I went from running around 2500 rpm at 65 mph to over 3000 rpm at 65. It was just too much for me, even though I was only on the freeway maybe once or twice a month.

Another strange thing was that my C4 would shift 1-2-3 before I would even get through an intersection at part throttle.

It all depends on your tire size, but in my opinion, even the 3.50's were too much gear for my 289/c4 combo. I didn't like working it so hard on the freeway.
 

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Discussion Starter · #6 ·
My c4 tranny is stock with no overdrive. I am currently running 225/60/14's all around for tires. I guess I want the best of both worlds to be able to get the street performance and be able to drive 70+ mph on the freeway at around 2500-2700 rpms so I can drive it to work/school when weather permits.
 

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I had the same setup as you with 3.55's for 12 years or so. I would spin 3600 rpms keeping up with traffic (70 mph or so). 3.55's certainly make your car get up and go from a start, but highway driving is more difficult (but possible). I decided to add a T-5 for highway driving and for the fun of a stick.

You might want to consider a T-5 in addition to, or instead of a rear end change. Given that your c4 has a 2.29 first gear and an OEM T-5 has a 3.35 first gear, that change would be like changing your rear end to a 4.19 behind your C-4. Plus you would have overdrive for the highway. I saw my highway mileage go from 13 to 19 mpg with the T-5.
 

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6T6mustang: I have 3.00:1 in mine (with c4). At 60 mph the engine is turning about 2600 rpm although it is still easy to turn the wheel (thats right,,, 1 wheel) at a stop light. And it's easy on the mileage. Mark
 

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My car has stock 3.50 open rear with toploader 4-speed. I think I need 2-3 more gears when I drive, b/c I regularly shift from 1st to 3rd, or 2nd to 4th and don't miss a beat with easy going sunday driving. I shift into 4th before hitting 30 mph most of the time. If I don't have my foot in the gas, I will slow down when shifting from 3rd to 4th. At 70 MPH, I get about 10 MPG, and I'm glad I don't have a tach b/c I know I'm burning some serious R's.

My Shelby also has a 3.50 rear with a C4 (shelbyized). This is much more tolerable for some reason around town (prob b/c I'm not doing the shifting), but still burns a bunch of Rs on the freeway. Going down hills, I need to brake more with the Shelby automatic than with my GT in 4th gear. Isn't that weird?
 
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