Hi i was thinking thinking of doing a gear swap, my current setup is 2002 f250 extended cab 4x4 v10 with 3:73 stock factory gears no engine mods so far my truck has a 4'' procomp suspension lift and im running 325x60x20 duratracs on 20'' american eagle wheels, I would like to improve the overall fuel economy of my vehicle in the city and hwy what would be the right swap 4:56 or 4:88 or higher ???.......also would be pulling a 3000 pound utility trailer to the campsite once in a while any input greatly appreciated

 

I would go with the 4:56's

 

I would agree with dual and go with the 4.56 unless you anticipate towing heavy with it on a regular basis in which case you might want to go with the 4.88.

 

Exactly.........

 

The more RPMs the engine turns, the more fuel it needs.

3:73 gears will have a lower RPM, and use less fuel than 4:56 or 4:88 gears.

4:56 or 4:88 will give faster acceleration than 3:73.

 

actually no, hes turning much lager tires than stock, right now the truck is working ten times harder to power the bigger tires. And on the highway most likely the truck is downshifting on ever ant hill. And another thing the proper gears will save his transmission from blowing apart. When i installed 4.30 gears i gained MPG on my 5.4 and im running almost OEM size tires (255/85r16)
To the OP think very hard about this, do 4.56 gears if you tow light and not often. The V10 is more powerful then the V8 but id go 4.88 gears. The 4.88 will put you back to stock tire/gear combo, give you the grunt needed to turn the 35's properly and keep the V10 in the correct rpm for driving/towing

 

Going from a 32 inch tire to a 35 inch tire reduces your effective gearing from the 3.73 ratio to a 3.41 ratio. You need a 4.08 ratio to return to the effective stock gears, or a 4.10 is close enough. You can calculate all of this using the automotive calculators available on this site. Automotive Calculators offer to you by Ford Trucks Enthusiasts | www.ford-trucks.com

If you are worried about fuel mileage, my advice is to NOT install 35 inch tires. You increase rolling resistance and wind resistance when you lift a truck and install larger tires, none of which will help at the gas pump.

 

If he took the tires and rims off the truck and ran just on the brake rotors (taking tires & wheels out of the equation) 3:73 turns lower engine RPM as a final drive number than running the exact same thing but with a gear ratio changed to 4:56 or 4:88.

Yes, lower gear numbers 3:73 will not do as good a job of acceleration when larger diameter tires are used.

The OP asked if going to a lower ratio gear would get better fuel milage.

I considered only the gear ratio when making my statement. Different tires can raise or lower the engine RPM to go the same speed.

 

Surprisingly it works the other way around. Sure you burn more at higher RPM but the distance covered makes up for it.

Acceleration and deceleration (braking)
Fuel efficiency varies with the vehicle. Fuel efficiency during acceleration generally improves as RPM increases until a point somewhere near peak torque (brake specific fuel consumption.[18]) However, accelerating too quickly without paying attention to what is ahead may require braking and then after that, additional acceleration. Experts recommend accelerating quickly, but smoothly.[20]
ref: Energy-efficient driving - Wikipedia, the free encyclopedia

 

here, wiki this;

http://en.wikipedia.org/wiki/Gear_ratio
A 2004 Chevrolet Corvette C5 Z06 with a six-speed manual transmission has the following gear ratios in the transmission:
Gear Ratio
1st gear 2.97:1
2nd gear 2.07:1
3rd gear 1.43:1
4th gear 1.00:1
5th gear 0.84:1
6th gear 0.56:1
reverse 3.38:1
In 1st gear, the engine makes 2.97 revolutions for every revolution of the transmission’s output. In 4th gear, the gear ratio of 1:1 means that the engine and the transmission's output rotate at the same speed. 5th and 6th gears are known as overdrive gears, in which the output of the transmission is revolving faster than the engine's output.
The Corvette above has a differential ratio of 3.42:1, meaning that for every 3.42 revolutions of the transmission’s output, the wheels make one revolution. The differential ratio multiplies with the transmission ratio, so in 1st gear, the engine makes 10.16 revolutions for every revolution of the wheels.
The car’s tires can almost be thought of as a third type of gearing. This car is equipped with 295/35-18 tires, which have a circumference of 82.1 inches. This means that for every complete revolution of the wheel, the car travels 82.1 inches (209 cm). If the Corvette had larger tires, it would travel farther with each revolution of the wheel, which would be like a higher gear. If the car had smaller tires, it would be like a lower gear.
With the gear ratios of the transmission and differential, and the size of the tires, it becomes possible to calculate the speed of the car for a particular gear at a particular engine RPM.
For example, it is possible to determine the distance the car will travel for one revolution of the engine by dividing the circumference of the tire by the combined gear ratio of the transmission and differential.

It is also possible to determine a car's speed from the engine speed by multiplying the circumference of the tire by the engine speed and dividing by the combined gear ratio.


To get to your higher plane of thought, one must pass through the reason why I said what I did. You are tossing in variables, which do make a difference, but are not part of the point I was making.

 

Do you two need to be separated?

You are both correct and incorrect, to a degree.

Your engine will make the best fuel mileage at the RPM where it is the most efficient. The idea is to make the effective gearing match the point where your engine is happy. Too low of a gear (numerically) and you fall below the efficiency (lugging the engine). Too high of a gear (again numerically) and you lose efficiency and increase internal engine friction.

There are many, many variables to the equation and no one correct answer.

 

If you're worried about gas mileage and you only tow 3000 lbs, sell the Super Duty and buy a Ranger.

Sorry, but that's just the way it is. You can't take a 7000 pound truck, jack it up in the air, put big tires on it, and expect to improve your gas mileage. It's just going to go from bad to worse.

 

Wow lots of info being posted...it makes sense not to lug the engine and once in a while im guilty of doing it, thinking im saving fuel but actually i may be doing more harm than good!!!... most of my out of town trips and camping involve steep mountain passes so the overdrive comes off, now if i was to leave the stock gears in the truck and just keep driving it, there is a good chance i can do some serious damage right???....as for gas mileage this is not my daily driver but more of a grocery getter or pick the kids up from school a few miles down the road so im not to concerned about that, so what gear ratio then does everyone agree upon that would be the closest to stock or factory specs? I have read many posts where people recommend 4:30 with factory tires over 3:73 I have never driven a truck with 4:30..that ratio would requireme to go 4:88 is that right?

 

Yes, with 35" tires 4.88's will return the effective ratio back to 4.30 or so (Actually when I did my calcs for my soon to be 305/70-18's and my 4.88's it came out to 4.39). Many folks here swear that the 4.30 is the dream gear for these V-10s if you tow much. I have read where some claim to get the same highway MPG after the 3.73 to 4.30 swap and actually do a bit better in town.
I know that I get better towing mileage with my 4.88s (with 32" tires) than I did with my stock 3.73s, but running solo it's a little worse, hopefully it will be a happy medium when I step up to the 35's.

 

We just changed our 5.4 automatic from 3.73's to 4.30's last summer. We lost about 1 to 1.5 mpg empty but gained about 1 mpg while towing.

That's with 31.5" tall tires. According to the tachometer it's it's turning 2000 rpm at 60 mph.