It's really all about how it's going to be driven. If it's going to be a highway car, then 2.73's. If it's more an around town type car, would likely find a bit better mpg with the 3.27's.

You want the gear that gives the lowest rpm at the normally used speed. Basically, drag racing in reverse. With all that being said, if you really learn to drive it, you should be able to knock down at least mid 20's. But that's only if you fix the loose nut (you) behind the wheel.

 

i do bracket race it in the local 1/8th mile and it clicks off some 9.4s and 5s and have even won my class a few times with it..

 

The only way to get an answer based on measurement rather than speculation and anecdote is by finding the BSFC map for the engine and then using that to determine which gear ratio lands you in the island during the bulk of your driving.

My expectation is that with an absolutely stock GT/manual the best highway/cruise mpg will be achievable with 3.08 or 3.27 gears but also in particular on a manual transmission driver behavior is the greatest determining factor in mpg. A thirty year old v8 powered car is just not going to be a fuel sipper - that doesn't mean you should just ignore efficiency but you gotta be real about it too.

If I was on a mission to maximize mpg on a GT I wouldn't worry about gearing.. 3.27 and 3.55 seem to be about goldilocks for a street Mustang that isn't being used for competition or smoke shows - I'd go for a less terrible intake manifold (to balance combustion quality across all 8 cylinders) and switch to a modern computer with widebands (pimpx, holley, ms3, etc)

 

ill add some info to this since it was brought up... i drive it daily to and from work on 4 lane straight road 60 miles round trip.. .and 2 to 3 times to local town to pick up parts and such on a 2 lane road... and once a month or so trip to jackson ms.. 200 mile round trip to go out to eat with my daughter.. involves interstate and in city driving...

 

In the 30+ 5.0L cars I’ve owned, the 3.08 cars would get 24, the 2.73 cars would get 26/27.
Jump to 3.55’s, 3.73 or even 4.10, they all got 20, stock or modified.

Granted, that’s running 70/75 on the j Interstate.
Now a buddy of mine with a A trim Vortec on a mild 306 with 3.08’s got 30 on a trip from Kansas City to Tulsa.

 

Supposedly Ford went with the 2.73 and 3.08 mostly because of the emissions requirements, the 3.73 and 3.55's netted better mileage.
My sons 94GT with ported explorer heads and intake with a small cam, 1 3/4 longtubes, and 3" exhaust gets 26 @ a steady 60MPH, its a manual trans car with 3.73. A tune is really helpful in these cars too if you've got a stock processor.
EDIT> I'd also do a foxbody upper intake conversion just because of how weird the stock throttle body set up is.

 

So, in that case, just run the 2.73s in 4th gear. That's pretty close to running 3.73s in 5th.

I've been trying to buy a 0.73 5th for a while. Every time one pops up for sale it's damn expensive.

You know, my T56 has a .74 5th... If that's where ya wanna be in 5th, you'd be good to go, plus, you'd have a whole 'nother OD with a big jump to .5 in 6th. I'm gonna convince you one of these days.

 

I would do 3.27's so it's not a total dog. Although 3.73's really aren't that bad. Just depends how much freeway you do. I really like the 3.55's in my car. With the .62 6th I cruise at around 2000 @ 70 with 275/40/17's

 

I would do 3.27's so it's not a total dog. Although 3.73's really aren't that bad. Just depends how much freeway you do.

a lot actually. so i think i'll do the 3.27s about to get to work on it so i can drive home from work at the end of the day.. lol theres a big footbreak race this weekend i think i'll enter with it.

 

A 302 has a 3" stroke and since 60mph is 1 mile per minute if you're sitting at 1450rpm @ 60mph (5th gear on T5, 2.73 gears, 26" tire diameter) that's 1450 revolutions per mile too.

That piston travels 6 inches per revolution (3" down, 3" up) so you're dragging each piston up about 725 feet of bore per mile.

Change to 3.73 gears and now 60mph is 1970 rpm which means you're dragging each piston 985 feet per mile. Unsurprisingly the relationship between gear ratios (around 35%) matches the relationship between cruise rpm and between linear friction factors - move one up by 35% and they all go up by 35% .

That of course is just one specific element of friction, there's also the energy lost into accelerating and decelerating the piston (which increases at half the square of the increase in velocity so it's much greater than merely linear) and of course the energy lost into increasing the total acceleration of the full rotating assembly, transmission and driveline (same algorithm) and the roughly linear increases in friction from dragging lifters across valve stems and pushing valves against springs and guides and rotating camshafts in bearings and rotating crankshafts in bearings and rotating power steering pumps and water pumps and rotating transmission shafts and bearings... the frictional losses in all of those increase by approximately the same % as the static RPM increases.

So at face value lower rpm means lower net friction per unit of time any way you frame it. But combustion engines also have brake-specific-fuel-consumption characteristics which is a nerdy way of describing the conditions under which they most effectively turn explosions into motion and that is measured in both load and rpm. If you can technically move the car 60mph at 500 rpm but you have to be at WOT to accomplish that you likely won't be operating at peak volumetric efficiency on a spark ignition engine. There is a combination of both load and revs which extracts the most work from the least fuel and it differs wildly among different engines and different configurations of the same engine.

Engineers consider all of these things there isn't a single factor they can change without balancing it elsewhere; intake runner length and diameter was not chosen randomly it would have been cheaper to just feed the air into the shortest possible path using the least possible aluminum. Cam profile was not chosen randomly it doesn't cost any different money to grind a different cam profile from a billet.

We can make broad assumptions and tell anecdotes but without objective data and rigorous measurement it's all speculation and should be regarded as such.

 

My concern would be if you go too high a gear ( 2.73) you will have to be in the throttle more to make the same speed as a 3.08 or 3.27....
The 83 in my pic has been in my family since 84-85 .. Factory 5 liter, carburetor car with a SROD transmission ( .070 OD in 4th) and the 7.5" rear had 3.08 gears....
This car has not run since the 90's but I do not remember it ever having any problems getting down the road having to step into the throttle to keep it going at speed.....
I cannot remember the fuel mileage, but over all this was a decent set up ..
This car made the trip from Eugene OR, to San Fran, CA multiple times and flat hauled ass over the mountain passes getting there.....
Car is getting a rebuilt explorer engine w/ the GT40P heads, a Summit 500CFM 4 barrel , TKX and 350 gears ( 9 inch).... Wish I could tell you how it runs now, but its not quite done yet.....
I hope this helps some....

PS. I am NOT an engineer........
Well, it seems as if today I cannot upload a picture..... FUK computers....

 

just took a lunch break.. 3.27s are in.. about to install axles and driveshaft and abs sensors... ill hopefully have it off the lift by 300 and will report back tonight

 

Dude, at least wait long enough for 2-3 members to get in a pissing match over an irrelevant point in a post that was off topic from the original post!

 

In purely stock form the Fox cars with 2.73s always got better mileage than the ones with 3.08s. It is not a major difference, like 1 or 2 mpg at most and could be a rounding error or weight differences.

 

done... did an around the block test... everything cool.. i'll fill up on my way home from work today and we'll see how it does in the next few weeks..

 

Damn that was fast. Last time I did gears it took me two days of swearing and a wife that threatened to leave me if I ever opened up a differential again from the smell.

 

When I get on something, I don't dilly around too much!

 

To far to drive. I'm in up MA.

I'll get to it at some point. I'm just not as quick as you are for obvious reasons.

 

Oh dang yeah a drive fo sho... if the fox it's a little easier.. no disk breaks on rear to disassemble and no abs sensor to take out. start with a 28 to 30 shim under pinion bearing.. set backlash to around 8 or so.. then check pattern.. usually the factoy carrier shims are 270 tho or so... so just remember if you add 10 thou to one side.. take 10 out of the other.

 

 

 

That's my shop at work. Shop at home a little bigger. Then 4 more small shops for special stuff

 

alright...here is my thought and why:

• 3.27 rear
• .73 5th gear (from 94-98 v6 T5)

Here is my why. One of the bigger issues w/ hwy mileage is the huge gap between 1.00 4th and .068 5th. By closing that gap some with the .73 + numerically lower rear, you don't have the big gap between the top gears (going uphill for instance) and the final ratio is slightly taller than a 3.55 + .068 combo, but it makes 1st/2nd better for daily driving.

 

i'm so happy with the 3.27 gears. very decent for the type of driving i do. i've put about 250 miles on it and will fill her up today and check the milage.

you mentioned the .73 5th gear.. i'm not sure which one i have in the car. as the T5 is from a Fox that i had and kept as a spare.

 

I've been trying to buy a 0.73 5th for a while. Every time one pops up for sale it's damn expensive.

 

What's a .73 worth? I just put one in my car, I didn't think it would be noticable. It is I lost about 5 mph / 200 RPMs on the highway with 4.10s. I don't like it.

is there a taller 5th than the .68?

 

I enjoy 3.55s on a blown '91 GT with the hair dryer option. On the open road it is a blast to be able to do 120mph+ in 4th gear!

 

I enjoy 3.55s on a blown '91 GT with the hair dryer option. On the open road it is a blast to be able to do 120mph+ in 4th gear!

PSH......2.73's get me close to 130 3rd, so much better...........I kid of course.
Well, I kid that it's so much better, but not that I'm still running 2.73's.

As for the OP, I only have experience with 2.73's and knocked down upper 20mpg when I had a stock engine.
With current HCI 5.0, still hit low-mid 20's with same 2.73s in same car.
75mph runs right around 2k rpm, just loping along.

 

I remember knocking out 29.9 MPG with a stock 5.0L LX with a 5 speed and 2.73s. Of course that was back when Sammy Hagar couldn't drive 55. So basically best case, long trip, all highway, cruise set at most likely 60.

 

Also had 100% gasoline. E10 hurt mpg quite a bit.

 

yep trash gas!

 

uh... e10 has at most 3% fewer btu's and contains oxygen so you get slightly less pumping loss (because you're not moving quite as much inert nitrogen through a bunch of tubes per unit of energy released) .. you'd need lab equipment to reliably identify a difference in mpg.

 

Once the switch hit mainstream my wife's commute that was very consistent yielded 2mpg less with her 01' Cobra.