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Discussion Starter #1
Took the car out for a spin 2 days ago. It is so much fun to drive an old mustang.

However since I am actually driving it more now I am considering changing the rear gears. Would love some feedback and opinions.

Currently in the rear is 3.73. It is a 390 with a 4 spd toploader. I believe wide ratio but I would have to do some digging to confirm.

When I am cruising on the 55mph coastal highway we have at 60 or so, it is runing at some decent rpms. Higher than I would like. And I would like to take this a lottle further than the edge if town.

Here is a reference. And before I say all this, I understand that this car is not fuel efficient, never will be, and all that. I know what I signed up for and thats fine. But if I can get more than 6 miles to the gallon, like 8 or 10 it would help tremendously.

I took a drive to a gold course 12 miles away, turned around and came home. It took 1/4 tank of gas for that trip. Exactly. 16 gallon gas tank, thats 4 gallons. 6 mi. Per gal. And one big environmental footprint.

Also the change of gear is more so I can cruise at lower rpms and not put additional strain on engine.

Would going to 3.25 be a good ratio for my setup?

Does anyone have any real world feedback who also has big block and toploader snd give me their experience?

Also is it better to find a used 3.25 gear somewhere or buy a new one online?

Thank you
 

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With no overdrive, I think 3.25s are a good rear, especially if you’re not worried about ultimate performance.

I have 3.25s in mine right now. When my car had a C4 auto, it was very manageable for cruising. Now that I’m swapping in a T5, I’m strongly considering moving to a 3.50 gear.
 
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I have had all manner of gear and transmission in my car. This is my take and I'm not trying to start a wide/close ratio debate, which will probably come later. If you have the wide ratio, you will probably be quite happy with a set of 3.25 out back. If you have a close ratio then not so much. 3.25 with a wide ratio has about the same overall first gear as a close ratio with a set of 3.91 out back. An FE has the torque to cover the RPM drop. New is typically better.
 

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3.73 is into the performance/show off realm and the 390 should have plenty of torque for even one that is under 3.00. Just guessing that you have a 9" but same for an 8", they only take an hour to change the entire pumpkin so i would start shopping around for a used one and see what pops up first, even if it were a 2.80 open diff. That or a 3.08 should be cheaper and more plentiful from those that upgraded and see how you like it and how it matches with the engine. You can always re-sell and get a new one in the gear you are pleased with.
End up with 2, one for driving across the state and another for monster exhibition:)
 

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Discussion Starter #5
Good thoughts, appreciate them all.

Can someone tell me what a good used pumpkin would be with 3.25s?

I will look up new ones, but if I could find A decent used for less I could consider sooner.
 

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Good thoughts, appreciate them all.

Can someone tell me what a good used pumpkin would be with 3.25s?

I will look up new ones, but if I could find A decent used for less I could consider sooner.
When faced with the same question, I was actually able to find a guy here on VMF that sold me a complete 8" 3.55 Auburn posi setup. Maybe someone will jump in.

If not, try a search on your favorite auction site for "3.25 Ford 9 Center Section" (assuming you need a 9"). I think you'll find many choices new or used.

Phil
 

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Just as a reference, I have a T5Z transmission with 5th at 0.63 ratio and a 3.4 rearend on a stock 1968 302 J-code It seems to be a good fit.
 

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I have a 390 with a 3.25. It’s a great match up. I also have a gear vendors overdrive which puts me at about 2250 RPM at 70 mph very nice.

Even at 3.25 you’re going to be hitting 3000 RPM at 70 mph.
 

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I have a 3.00 open and all that torque is plenty to still skkkkrrrrrtttt all the way down a city block... So a swap would not hurt. Maybe add (if don't have it already) a posi (trac lok) too since you are in there ;)
 

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Bishop, how about a 3.00 axle ? the 390 FE will have no trouble with that. The wide ratio 4spd is better for almost everything. how tall are your tires ? The 9 inch axle I purchased came with a 2.75 gear, it was from a '69 390 car, with an auto. LSG
 

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I think you would have to go to a 3.00 or overdrive (t5, tko or Gear Vendor Unit) if you want to make any real noticeable difference in your mpg coming from a 3.73. I have a car with a T5 and another with a 4 speed so I understand the characteristics of wanting to keep the vintage tranny (shifter throw, weight, gear whine, etc.) I have a 66 GT, A code, 4 speed with a 3.00 posi and it does about 2500 rpm at 65 mph. I think the car gets around 14 mph.
 

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6 mpg? Ouch. I've seen GT-500's running that for MPG.

3.25:1 is my suggestion.

With a 390, you would be ok with that ratio for both "performance" and some kind of relief when at the gas pump.
A Gear Vendors unit would drop your final drive effectively to 2.91 but they are a PITA to get the installation free
from harmonics. I know this because I have one installed in my '68.....
 

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Mine was built with a 3.25 but had a 3.50 when I got it. At 60 mph it was turning 3,000 rpm - completely unnecessary for a 390. I installed a TKO600 (0.64 OD) and it changed the car completely. Now it is quiet at 60 mph and turns about 2,000 rpms. Installing a TKO is a much bigger and more expensive undertaking than simply changing the rear. You can get a complete rear, w/ trac-loc, from Custom Built Ford 9 Inch - Custom Built Ford Rear Ends for <$1k.
 

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Your gas mileage estimation was completely unscientific. The fuel gauges are notoriously inaccurate, especially when you estimate that 1/4 of a tank on the fuel gauge is equivalent to 4 gallons. Why don't you do a scientific test using actual miles driven vs. actual gallons used?
Honestly, I wouldn't expect much of a gas mileage increase by changing from 3.73 to 3.25. You have an FE and nobody buys an FE to get good gas mileage. And I don't think anybody makes 3.73 gears for a 9". 3.70 is the closest ratio available in 9". I'd confirm which ratio you actually have before changing gears. Maybe you actually have 3.50 and going to 3.25 will be negligible.
 

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Well, if suggesting instead of changing out the center section in the rear axle I can instead suggest you swap over to an overdriven transmission, then I withdraw my earlier advice and agree with above.

When I bought my car it and the 2.79 ratio axle and a C4 and that was fine.

Now I am using a 4R70W and 3.55 ratio axle and that is way better.

While I'm spending your money...do that AND go EFI and you can get mid 20's MPG out of your car EASY. I've seen high 20's out of mine and that's running a 347 (EFI with overdrive).

Let's do the math. I did that 10 years ago and have driven 20,000 miles since (about 2k per year). I went from 15 to 25 MPG. That's 533 gallons of fuel saved. Figure $3 per gallon and I've saved $1600....so I'm almost half way to breaking even! Whoop!

Seriously though...if you can swing it, keeping the low rearend (high numerically) and going with overdrive would be THE BEST solution....but it is a bigger project than what you first asked about by a factor of about 5.

Phil
 

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I had no issues with a 5.0, WR Toploader with 2.80 gear in my 66. With a 390 and WR Toploader I think even 2.75's are doable. I'm sure you could pick up a good used open 2.75 or 3.00 cheap to try them out, then either buying a rebuild unit or just put a limited slip in.
 

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Just remember tire height makes a difference as well. I would suggest you figure where your engine makes peak torque and consider that with your final rpm at cruising speed and then along with tire height figure desired rear end gear. Got to love that Big Block torque! I have to run 4:56's to get the launch I want with a small block.
 

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Discussion Starter #19
Thanks for all the info. Sounds like i have some work to do.

For me it is less about gas mileage as it is not putting thr car thru excess stress by running it at high rpms on some of the coastal highways. Much of the driving i will be doing will be 60 or under. However, I would like to cruise at 60 or 70 and not have it be so strenuous on the car.

If I can just lower the rpms a bit I wouls be happy.

I had considered a t5 originally years ago, but went with the roploader for accessibility, cost and strength.

Changing the trana right now is not a project i want to undertake. I would rather change gearing to make it more cruise friendly.

I will have to get more details on the car and the rear gears it seems.

Here is the question tho. These cars came with a 4spd back in the day on the 390s. What was the standard gear for those cars that came with that setup?

Thanks
 

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Thanks for all the info. Sounds like i have some work to do.

For me it is less about gas mileage as it is not putting thr car thru excess stress by running it at high rpms on some of the coastal highways. Much of the driving i will be doing will be 60 or under. However, I would like to cruise at 60 or 70 and not have it be so strenuous on the car.

If I can just lower the rpms a bit I wouls be happy.

I had considered a t5 originally years ago, but went with the roploader for accessibility, cost and strength.

Changing the trana right now is not a project i want to undertake. I would rather change gearing to make it more cruise friendly.

I will have to get more details on the car and the rear gears it seems.

Here is the question tho. These cars came with a 4spd back in the day on the 390s. What was the standard gear for those cars that came with that setup?

Thanks
3.0 open. That’s what my GT 390 4 Speed came with.
 
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