Rear axle ratio choices

[metroplex]

Why kill performance? Sub 3.00s will yield a slug off the line, and in-town MPG will suck. A good CAI and tune with exhaust mods will get you to near 20 mpg town, near 30 mpg hiway. I am doing it with my '08, and performance is very good.

It won't hurt city driving MPG. The Crown Vic with 2.73s gets 20 MPG in the city while the same car with 3.55s gets 11-13 MPG in the city. Either way you slice it, the Modular engines get better gas mileage with 2.73s especially if you lug it around town.

 

[Five Oh Brian]

Metroplex, Ford used to offer 2.73's for the 8.8" rearend (they were the factory gears in my '89 Mustang 5.0L LX), so there must be some sets out there somewhere. However, like others have mentioned, you will really hurt in town economy in your Mustang as you'll lug the poor car so much to get going that you'll be harder on clutches, as well.

I appreciate your comparison to your Crown Vic, but the transmission gearing is very different in your CV's 4R70 trans vs your 5-speed manual in the Mustang. The CV transmission gearing is much taller (lower numerically) hence the reason you get much better mileage with taller rear axle gears.

Consider your Crown Vic's transmission with 2.73 gears...

1st gear = 2.84 x 2.73 rear gear = 7.75 overall ratio.
2nd gear = 1.56 x 2.73 rear gear = 4.26 overall ratio.
3rd gear = 1.00 x 2.73 rear gear = 2.73 overall ratio.
4th gear (overdrive) = 0.70 x 2.73 rear gear = 1.91 overall ratio.

Now, here's what rear gear you'd need in your Mustang to have the same overall ratio as your Crown Vic above...

1st gear (3.38) with 7.75 overall ratio would need a 2.29 rear gear.
2nd gear (2.00) with a 4.26 overall ratio would need a 2.13 rear gear.
3rd gear (1.32) with a 2.73 overall ratio would need a 2.07 rear gear.
5th gear (0.68 overdrive) with a 1.91 overall ratio would need a 2.80 rear gear.

So, to get the same in-town gear as your Crown Vic with 2.73's, your Mustang would need gearing that doesn't exist for 8.8" rearends. And the CV has taller tires than the Mustang (which I did not factor in above), so you'd need even taller gearing in the Mustang to approach the overall ratios of your Crown Vic (say, a rear end gear below 2.00:1).

So, you might actually want to consider leaving the rear gears alone and replace the transmission gearing (preferably just the overdrive gear to something like 0.50 which you would get in the Tremec 6 speed manual transmissions).

Oh, and lastly, my own rear end gear experiment (different car, same theory). My 1968 Pontiac GTO came with 3.23's, but I later swapped in 3.55's, 3.90's, and then 2.56's for highway mileage. With the 3.90's, car went 13.60's in the 1/4 mile and got 10 mpg highway. With 2.56's, car went mid 15's in the 1/4 mile and got 16 mpg highway. So, the 2.56's netted my a 6 mpg improvement on the highway, but cost me 2 full seconds in the 1/4 mile - yuck!

 

[BurntRubber]

solution, Ford's new 2010 low emissions vehicle. in fact ∞ highway and city miles to the gallon! beat that with 2.00 gears.

 

[o0Dan0o]

I, for one, would like to see the mileage he'd get with 2.73. Hell of an experiment if you ask me.
Dan

 

[Sparty92]

Well I always shift at or below 2k RPM with my GT, and I frequently lug the engine around the city. I never understood why people NEED to run the engine at 3000+ RPM with a manual.

No offense, but I think you bought the wrong car. Perhaps a Focus is what you really want?

When driving around town, I shift gears 1-3 at around 3-4K rpm. I easily get 20mpg. On the highway (strictly highway), I get 30-32 mpg.

I have 3.55s and a 5-speed.

Get a Focus and a CD of V8 engine sounds....you'll be all set.

 

[ski]

Get a tune and a CAI.
My manual 05 S197 GT with 3.55's, BamaChips 93 Race tune, and a C&L Street CAI gets 27 mpg at 70 mph on the hwy.
However, my wife's 09 auto Honda Fit Sport knocks my car's socks off with 45 mpg hwy.

 

[MustangGT08]

This is an interesting thread to me. I my my awakening to muscle cars due to a used car my dad bought when I was a new driver. A '65 Olds Cutlass Vista Cruiser V8 wagon that we thought was a slug. Until yours truly, the 16 year old budding enthusiast, discovered a huge Rochester Q-Jet under an air cleaner labeled "Ultra High Compression Premium Fuel Only'. After some investigation - Think about a wagon with 3 on the tree, 330CI/320hp 442 motor, and sub 3.00 gears and posi. Someone ordered it as a turnpike cruiser. Good gas mileage on the hiway, but was a slug off the line. Budding enthusiastic discovered that Sunoco 240, stabbing the clutch and throttle at about 25 mph in 1st would break the tires loose, and yehaw! Begged the dad to put better gears and a 4 speed in it. What a sleeper that would have been. Cops got on to my antics, called the dad. I had to chill - until I bought my first muscle car, a '68 390 AMX Go-Pak.

 

[SlideWRX]

However, like others have mentioned, you will really hurt in town economy in your Mustang as you'll lug the poor car so much to get going that you'll be harder on clutches, as well.

...No. Unless he manages to drive a speed where his usual gear won't have enough torque and he has to down shift to the previous gear, he will get better mileage. Considering 5th works fine down to 25 mph, he would be hard pressed to need a gear change to a lower gear. These cars can accelerate from idle in every gear.

I regularly shift at 1250 rpm or 1500 rpm, and there is no slipping of the trans. 1st gear is short enough with 3.55's that I regularly start in 2nd. Very little slipping going on. I usually launch around 1000-1500 rpm when launching in 2nd. the engine has plenty of torque. I do a mix of driving, and when I am paying attention I get 22-23 mpg. When I don't care I get ~19 mpg. I still run it hard on occasion, I just don't do it every time.

Considering that 4.10's apparently drop most people around 2 tenths in the quarter mile, I'll bet that 2.73's wouldn't fare much worse in the other direction; maybe 4 tenths slower.

 

[rugedraw]

Five Oh Brian hit the nail right on the head.

If anything, go with the 3.08 gear. Anything less than that, would be uncivilized.

 

[Five Oh Brian]

...No. Unless he manages to drive a speed where his usual gear won't have enough torque and he has to down shift to the previous gear, he will get better mileage. Considering 5th works fine down to 25 mph, he would be hard pressed to need a gear change to a lower gear. These cars can accelerate from idle in every gear.

I regularly shift at 1250 rpm or 1500 rpm, and there is no slipping of the trans. 1st gear is short enough with 3.55's that I regularly start in 2nd. Very little slipping going on. I usually launch around 1000-1500 rpm when launching in 2nd. the engine has plenty of torque. I do a mix of driving, and when I am paying attention I get 22-23 mpg. When I don't care I get ~19 mpg. I still run it hard on occasion, I just don't do it every time.

Considering that 4.10's apparently drop most people around 2 tenths in the quarter mile, I'll bet that 2.73's wouldn't fare much worse in the other direction; maybe 4 tenths slower.

You make some valid observations, but with 2.73 gears there would be a very noticable lug on the engine at launch and low speeds which will be hard on the clutch and require more throttle tip-in (and more fuel, accordingly) to get the car moving. Of course you don't have issues with your 3.55's as that's plenty of gear to launch from idle in any gear. Metroplex, on the other hand, is contemplating 2.73 (or taller gears) which are night and day different than your 3.55's.

Have you ever owned a musclecar with super tall freeway gears? If you have, you know that I am accurate in my observations (I've owned several with tall gears, both with and without overdrive transmissions). If you haven't, then you can only speculate without any empirical observations to base your argument on.