I have a 2004 Expy with the 5.4, bone stock. How much of a boost in mpgs would I see if the rear end gears were swapped to lets say a 3.08:1? And where could I get some?

Tony

 

Is the truck four wheel drive? If so, you will need to swap the front and rear gears. As far as gear changes, you might see some difference on the highway but it won't offset the cost of the gear swap. Your mileage in town would probably suffer due to the shallower gears.

 

I would be willing to bet that if you have 3.55s, and were to change to 3.73s, you would see gains as long as you stay under 70 mph.

My sons EB with 3.55s will not get near as good of mileage as mine with 3.73s, especially in town.

 

If you switch to a shorter gear(higher numerically) your RPM will drop at any given speed(all things being equal). This could be important especially if you do a lot of highway driving, but as was already mentioned you have to factor in the cost of the new ring and pinion gears plus labor to install them.

 

Randy's ring and pinion has a lot of useful info.

Randy's Ring & Pinion

 

That would be a numerically lower gear. A taller gear (numerically lower) will make the RPMs drop. When you go to a shorter gear you will be turning higher RPMs.

 

Mister CMK is right. I knew what I wanted to say, but must have been so tired my brain and fingers weren't working together. A 3.42 will have less torque and lower RPM than a pulling gear like a 4.10. Most manufacurers don't offer the buyer the option of choosing their gear ratio other than in pickup trucks. Most new truck buyers don't know about this and really don't care because they see something on the lot and if they like it they buy it, but it doesn't have to be that way. With a pickup you have a lot of flexibility with these kind of options. You can build the truck you want down to the gear ratio to either pull a horse trailer or to do very little towing, but run low RPM on the interstate to increase gas mileage. With gas prices as high as they are maybe knowledgeable people will start to demand different ring and pinion ratios to increase their gas mileage. Many foreign truck manufacturers put short gears in their trucks to make up for the lack of torque of a V6 or 4 cy engine. The trade of is higher RPM and lower gas mileage as a result even though you would think a small to mid size foreign truck would get great gas mileage. It ain't necessarily so because they have to make up for the lack of cubic inches in the differential.

 

You've got to know where the power band is for your setup. Just going to a taller tire will lower RPM a bit. numerically higher gears are benefical when going with a lot larger tire. Staying at 60 MPH or below helps alot as well.

 

Heh, I figured that is what you meant. I do have to comment on one thing though, some cars get better mileage at a higher RPM than lower. My roommate's Honda Accord (yes, not a truck, same idea though) gets better mileage at 80mph than at 60mph since the engine is getting higher in the powerband and doesn't have to work as hard.

 

This thread has been very informative. I've been thinking about how big a deal it would be to change the gearing in our 2001 Navigator from 3.73 to 3.55 or something lower. Most likely I will leave it as is since 95 percent of the driving is city and on 55 to 65 speed limit highways. I also wondered at what mph does the wind resistance starting have a big affect on mpg. Seemed like on ours the sweet spot is around 60 to 65 mph.

 

yep, that's very true. I had a Jeep wrangler that had one of those stupid idiot lights that told you when to shift. If I followed that, it'd make me shift about 2k rpm and it would lug in the next gear. Once I started ignoring that and shifting closer to 2500 - 3k rpm my gas milage actually went up by about 3 mpg. It's all about getting the engine closest to the range where it makes the most torque. That is where it's most efficient.

 

Your entire quote is very well said. I like walking on to a ford dealer's lot and seeing dozens of trucks equipped with 3.73 LS axles, it makes me feel like the guy doing the ordering knows his buisiness and or Ford knows what we want. On the other hand, when I see a 4x4 equipped with 3.55.1 regular axles, my immediate thought is, what a waste of a great truck.

Don't waste your hard earned money on gear swaps for the reason of better MPG's. By the time you off-set your cost of the gear set, you'll will have worn her out.

Tim