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1967, Ford, Mustang Coupe
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Discussion Starter · #1 ·
Have a mainly stock 1967 coupe sports sprint. 289 motor, nothing real special.

I have recently learned quicker acceleration at the cost of gas milage and top power when dealing with the 3rd member

I plan on switching mije out for a ratio of 3.25:1 because milage is a priority and I'd rather not replace tiers that often. But as far as open differentials and locking or posi, track lock, and all the other options, im not so sure what to do.

I want her street able for daily use, thats priority one. You smart folks know any suggestions?

Down the line ill be replacing the heads for some aluminum ones, tri y headers, a new oil pump, a mild camshaft, a 4 barrel carb, ect.
 

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'68 Diamond Blue Fastback
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It all depends on what you want / expect from the car. I'm running a 5.0HO/GT40P engine & .68 T5 with the stock 2.79 rear. Probably won't see 5th gear much but it will be there when I want it for economy cruising. Unless it's a drag car I don't understand why people run an overdrive transmission then gear it down. To me that defeats the purpose of having the overdrive in the first place.
 

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If your car is mostly stock, you probably have a 2.79 open rear. Going to a 3.25 will not get you better gas mileage.

If you're worried about replacing tires, just keep you foot off the floor board.
 
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So here is the gist of it. A numerically higher gear ratio, as well as taller tires will let the engine rev easier, it will be easier for the engine to move the car, as well as reach the power band sooner. However, you will be cruising at a higher RPM, and be decreasing MPG. A locking device in the rear will also decrease MPG.
A numerically lower gear and shorter tires will result in slower acceleration, lower cruise RPM, and better MPG.
Do you know what gear ratio is in the car now? If you plan on certain performance mods, especially with a hot cam, you may need a "racing" high stall torque converter. Good luck
 

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67 Fastback
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A 3.25 gear will give you a quicker acceleration but will cost you a little in gas mileage. I agree with awhtx, if you're not getting on it there is no need for the posi. I'm going with a 3.55 gear but I'm also installing an AOD so it shouldn't much of an effect on my mileage.
 

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So here is the gist of it. A numerically higher gear ratio, as well as taller tires will let the engine rev easier, it will be easier for the engine to move the car, as well as reach the power band sooner. However, you will be cruising at a higher RPM, and be decreasing MPG. A locking device in the rear will also decrease MPG.
A numerically lower gear and shorter tires will result in slower acceleration, lower cruise RPM, and better MPG.
Do you know what gear ratio is in the car now? If you plan on certain performance mods, especially with a hot cam, you may need a "racing" high stall torque converter. Good luck
You have it backwards on tire height. Increasing tire height is like making the gear ratio numerically lower.

To the OP, mileage cannot be your main priority with the mods you have in mind. Making the exhaust less restrictive is the only mod that may help with efficiency.

For what it's worth, I ran 3.55's with a C4 and 225/60r-14 tires for years. It was fine, and fun for daily driving. It did only get 13 mpg with a heavy foot.
 

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My 289 has a dual exhaust, Summit 4V manifold and Summit 500cfm carb, otherwise it is a bone stock C-Code

Stock 2.79 gears.

If I manually shift it will chip the tires from 1-2 and 2-3.

Cruise on the interstate and get decent mileage.

Its going to cost $1000+ to buy the gears and have a shop install them. The axles are coming out so you may as well replace bearings, seals, fluids and gaskets. The rear brake line probably needs replacing as well. The shop will also tell you to replace the u-joints since the driveshaft is out.

The same $1000 will get you a intake, carb and exhaust if you provide the labor.
 
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Find somebody's calculator online and there are several. Figure out what rear tire you plan to run and what your cruise rpm will be. Plug those in and it will tell you what gear you need to run which miles per hour based on your rear tire size and rpm. With an automatic you should allow a bit of fudge(5 percent or so) for the converter slip assuming it isn't a lock up and the C4 never was. The calculator at wallaceracing.com does a pretty good job.
 

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Being you have manual, you more then likely have 3.00 gears. 3.25 isn't a big jump. Personally I think 3.00 are ok. Maybe put a limited slip. Before you start switching out gears, have your distributor rebuilt and recurved. Full mechanical typically doesn't come in until around 4000 rpm. A rpm range where you don't drive too often. A shop will probably have it all in around 2500 and probably change the amount, to allow more initial timing. All this does wonderful stuff to low end torque which makes a noticable difference in both performance and improved fuel economy.
 

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About 12 years ago I finally changed from 2.80 open differential to 3.25 Currie Traction lock. (8” rear end)

Wish I had done it years ago, family and work were priorities. Life happens.


That was only change I made, everything else was the same.
My right foot is primary cause of mpg change.
Driving normally, I may have lost 1-2 mpg.

I had this for driveline:

289 (.030 over)/TCI C4/Edelbrock Performer 1406 carb, intake, cam/351w C9 heads/Duraspark ignition/Tires 235-60-14 (needed replacement) changed to 235-40-18

Only time I look back is if a cop is following me😂
 

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About 12 years ago I finally changed from 2.80 open differential to 3.25 Currie Traction lock. (8” rear end)

Wish I had done it years ago, family and work were priorities. Life happens.


That was only change I made, everything else was the same.
My right foot is primary cause of mpg change.
Driving normally, I may have lost 1-2 mpg.

I had this for driveline:

289 (.030 over)/TCI C4/Edelbrock Performer 1406 carb, intake, cam/351w C9 heads/Duraspark ignition/Tires 235-60-14 (needed replacement) changed to 235-40-18

Only time I look back is if a cop is following me😂
how do you like the 235-40-18 tires i just put some on but the car is being worked on so I havent had a chance to drive yet. i was nervous about putting 18s on a 67
 

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I like them very much on my 65 coupe.

I have Pirelli p zero Nero all seasons due to weather in my area.
Summer high performance tires tend to get hard around 40F...summer mornings can be that cold.
The Pirelli’s run slight narrow at 9” section width.

235mm = 9.25” section width


They are on some used 18x7.5 with 5.75 back space ( with 1” wheel spacers front and rear).
I had no clearance problems with fenders.

The 235-60-14’s had minor issue on passenger rear only when loaded or big potholes, but that was due to backspace being too shallow.

Backspacing is your friend.

Last winter, I made front suspension modifications to lower the front and decided to roll front fenders to allow additional clearance with slightly lower stance.

Rear leafs are 5 leaf mid-eye from Mustangs Plus since mid 90’s.
I sold the rear sway bar due to excessive oversteer.
Front sway bar is 1-1/8” also from Mustangs Plus.
I now have Viking coil overs in front and still have KYB gas-a-just in rear.
The KYB’s are too stiff, very noticeable on washboard roads.
Better JB Weld your fillings in.
I purchased the suspension as a kit front/rear over 25 years ago.
I have been quite happy that setup, especially since it was affordable to me back then.
 
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