I have a 94' F150.
It has 132K on the OD, 5.0 V8, 4R70W (4spd auto with overdrive), 3.55 axle, stock tires. I drive like an old man because I am one. I have zero interest in fast starts or drag racing.

85% my driving is country or highway. 15% City. Occasional small trailer towing.

My question for the group: If I changed to a 3.08 or 2.73 rear axle, would my MPG improve significantly and would the truck's performance be hurt in any other way?

 

With that engine your fuel milage would actually go down with a taller(numerically lower) axle ratio. EFI differs from carbs when it comes to engine lugging.. .fuel requirements are based on engine load so higher load(lugging) means the computer injects more fuel. I have real time engine data from my '90 5.0 3.55 truck that I had equipped with a tuner, on a rolling country road at a 50mph(80kph) the truck got much better fuel milage in Drive versus OD and I mean it wasn't even close, over 20mpg in drive at that speed versus almost single digits for OD.

So my advise for you is simple, disable OD every time you drive your truck unless you are travelling on major highways at 70mph or better.

 

I don't have real-world data to back this up, so take it for what it's worth (which is probably less than what Conanski's reply is worth). But Conanski is assuming that you will be lugging it with the taller gears. If you do find yourself deeper in the throttle to compensate for the lost revs, then yes, too tall gears will hurt your mileage. But I know that I often run my Bronco in 5th gear at ~35 mph (~1150 rpm) and I'm not into the throttle much at all. Obviously the performance is pretty dismal like that. but it'll hold a steady cruise down a suburban section road. So if that's what "driving like an old man" means to you, I'm not so sure you wouldn't see some benefit from taller gears.

But I do agree it'd go away pretty quick when you stood on it a little. And with an auto trans it might be hard to get the downshifts without standing on it (with a manual you can "regear" any time you want).

 

This is not my experience with my 2006 Ford Freestyle. I have a ScanGauge on it so I can monitor MPG real time. MPG always goes down considerably when the trans kicks down to a lower gear, such as on a hill. I let it lug against the taller gear as long as I can and try not to press down on the accelerator so it won't kick down for this reason. I am an old man too.

On my 1991 F150, before I yanked the 300 Six for a 460, I swapped rear-end gears from 3.55 to 3.08 in an effort to gain MPG. It made little difference to MPG, however. If it went up, it wasn't more than 1/2 MPG. However, the Six was happier with the higher ratio - this may not be true for a 302, however. But the main point is it did not help my MPG as I thought it would.

 

I'll need to try the idea of taking it off OD & check the mileage.

 

I'm no expert either. Consequently I will suggest listening to Conanski and Jass88. They are the only ones that have or had real time data. Having said that, I used to have a 94 F150 SC LB 4X4 351 W and 3.55 gearing. I went from the stock 15 inch tire size (29 inch diameter) to 16 inch 235/85 R 16's (31 inch diameter). Kind of like re gearing to taller (lower numerically) gears. Over the time I owned the truck and had 16 inch tires it averaged 15 mpg (about 13 US) overall. That's city, highway, dirt backroads and off road driving for 20 or 30,000 Kms. I was OK with the mileage but I sure noticed the difference in power. I drive pretty casual but I didn't like feeling that my truck was lacking in power.

It's up to you but with a 94 F150 and a 302 there's no way I would go with taller gears. I was actually going to install 3:73 or 4:10's.

 

Ddaybc gave me an idea. Know anyone with a jacked up Ford truck? See if you can borrow their wheels/tires for a tank or two of fuel. The taller tires will act like taller gears. Should be able to find a gear calculator online to figure what size tire would be equivalent to a 3.08 or 2.73 gear. Not a terribly practical test, but would prove the point with a little sweat equity.

 

A big part of the issue here is open loop vs closed loop operation. Lug the engine, high load, pcm goes to open loop and ignores the oxygen sensors and just dumps fuel into the motor. Easy to watch with a good scanner (or cheap one if obdii). Higher gears can do this. The 302 isn't known for its low end. Best thing for mileage is to slow down: force of drag increases with the square of velocity.

 

The gas mileage will definitely not improve with taller gears on the small block V8 especially since you have OD. All it will do is make the power band worse.

 

Your Freestyle has a CVT transmission...apples to oranges.

 

Also, it's completely designed to use that CVT with the differential it came with. 2006 to mid-1990's = apples to oranges.
The 302 has to run a higher engine speed to be in a good power range. The other truck engines make more low-end torque, so they don't suffer as much with lower-numerical gears.

 

https://www.ford-trucks.com/forums/1...10-gear-2.html

member here actually went from a 3.08 to a 3.73 I his 300 i6 truck and GAINED 3 mpg

 

I went from a 3.55 to a 4.10 with the 302, no other changes and gained a couple MPG. Also, the increase in performance was nice. I don' think you'll be happy with a higher gear than 3.55 with the 302 and not likely to see any improvement in MPG and certainly will lose performance. I'd stick with the 3.55's and turn OD off in town as others stated above.

 

It totally depends on what what kind of driving you do. Spend a lot of time on the highway 70 mph+, and taller gears are your best bet. Spend most of your time 55 or below, 4.10+ is your best bet.

 

What are you currently getting for mileage?
I have 4.9-E4OD-3.08 gears-2wd-235-75-15 tires, 85-90% highway.
On my last 2 tanks, I got 19 and 21 mpg.
I wouldn't go to 3.08 or 2.73. My truck doesn't like going over 65 mph, aerodynamic drag becomes a big factor at higher speeds.