Hi everyone, ive got a 1998 f150 v8 4.6L 4x4, 125,000 miles on the engine, and 3.55 gears in it. im running cooper discoverer atr tires p265/75/r17 at 40psi. The truck currenty gets 22 mpg on the highway @ 60-65 mph. it has a few mods, intake, raised throttle body, and underdrive pulley kit. it pulls about 1700 rpms going 60, and 1900 going 70. i do a lot of highway driving and what i was wondering is if i put 3.23 gears into it, would i see an increase in my mpg at those highway speeds? also would i have to put a set of the 3.23's into the front differential as well in order to keep the 4x4 system functional?

 

Would be a lot cheaper to just run larger(taller) tires. Becarefull though if you dont keep it in its power band the RPM will be to low and lug the engine and burn more fuel to maintain speed.

 

Honestly, those RPMs seem low for those speeds. I would actually try to run with the overdrive disabled for a tank and see how fuel mileage comapres. It might actually go up with a slightly higher rpm. Try it and let up know how it goes.

 

yea its definately a low rpm, the truck will cruise all day at that speed, accept for when it encounters a steep hill, then it will downshift. would the 3.23 gears give me better mileage around 70-75 than the 3.55s? and would i have to put them into both differentials

 

In all likelyhood a yes on the first question and naturally a yes on the second since it's a 4x4. I do tend to doubt that there will be a dramatic improvement in mileage so the benefit to cost comes into play now. Gotta remember/consider, wind drag is a square of the vehicle speed; higher speed requires more HP to make it happen. 22 on the highway isn't bad at all; I sure wish my Ranger could get that.

 

yea the mileage isnt bad at all, cant complain about it on a v8 4x4. i have a toneau cover so the drag is reduced alittle bit, other than that im going to be adding an ignition booster and an electronic radiator fan and between the two that should be rite around 20 horses total. just gotta pick up some better wires to go with the booster, i got the new plugs put in less than a month ago. im gonna be adding some new shorty race headers and a new exaust system sometime in the near future so that will definately help out a lot with power and probably alittle bit with the mileage too.

 

If you go from 3.55s to 3.23s, my expectation is you will lose fuel mileage. Your rpm should go from 1700rpm at 60mph to 1540rpm at 60mph. At that rpm at that load, I expect you will be under the torque curve and the engine will be lugging. This is my un-qualified opinion here. Take it for what it's worth.

Before you spend the money on a hardware change, try the experiment I suggested of turning off your overdrive for a week (or a long trip) and see what happens. If your mileage goes up at a higher rpm then lower numerical gears will probably hurt your mileage. The only thing this experiment will cost is a tank of gas. You might find you can change the rear gears for a higher numerical gear and gain more low end grunt.

 

i took the overdrive off on my way to school today (15 minute highway drive) and 60mph was at about 2300-2400 rpms. i noticed that i barely had to press the gas down at all to maintain the speed. I had to push the gas about twice as much with the over drive on. im goin from PA up to michigan this weekend so ill really be able to test it out. thanks for the help, i hadnt thought of it that way before. ill let you know how it goes.

 

Kool. Looking forward to hearing the results.

 

hey, i tried driving with the overdrive turned off and the mileage turned out to be about 18 on the highway so i turned the overdrive back on.

 

Ok. That worked in that we now know how a higher numerical gear will affect fuel consumption. Can't give you any other advice on the lower numerical gear.

 

Yea, thanks for the idea though, hadnt looked at it that way before. i think that once i get a few other mods installed on the engine ill go ahead and swap the 3.23's in and see how it goes. maybe with the lower gears in i could keep the overdrive off.

 

I don't think there's any advantage to swapping gears to stay out of overdrive. It was just an idea to test if a slightly higher rpm would give better mileage.

Good luck with the swap.

 

I doubt you'll ever recover the cost involved to do the gears, especially on a truck w/ over 125K miles already. I'll let you do the math but that ~$2K will buy a lot of gas. Figure 2 mpg increase, tops. Beware, this is an experiment, you mileage may even go down. 22 mpg is excellent for a 4x4, you may have already found the sweet spot. Lowering the RPM's by X% doesn't automatically mean the MPG will raise X%.

Have you recalibrated the speedo for the bigger tires? Your mileage can be off if not. What do you mean by "raised" TB?

 

I meant to say throttle body spacer, my bad. i think ill put the electronic radiatior fan on first before i do the gears though. thats said to give about an extra 1 mpg. but for rite now, in about 12 hours ill be goin back to michigan and im trying out this fuel additive called XTane, guarenteed minimum of 20% increase in mpg on any car or your money back so ill see how it goes. about the recalibration for the tires, the tires are the stock size that came with the truck and the off-road package, p265/75r17. theres definately still more potential for gas mile left in this motor and i intend to find it. i havent really begun to tap the weight reduction department yet but that wont happen for awhile and i havent cleaned my air filter since june so i gotta do that soon. thanks for your help and advice guys.