For all highway driving you would want higher gears. Lower gears help help off the line, but higher gears will make the truck faster at highway speeds & you'd get better gas mileage. Fords high gear ratios are 3.08 & 3.31. I think that the 3.08s are too slow of the line, but the 3.31s are a good combination.
I've got the 3.55 gears in my truck. And I too think they are a little sluggish at the top end. But they seem to give me more power than the 3.08's I had in my 83' F-150 off the line. In my opinion, 3.55's are just a good all around gear for getting the best gas milage out of your truck. If your just driving around town, or cruising at 70 mph on the interstate. And they help out if you tow anything too.
2001 F-150 XL REG. CAB 2WD 4.2L V6, 5-SPEED MANUAL TRANSMISSION. DEEP WEDGEWOOD BLUE/ARIZONA BEIGE, BEIGE INTERIOR. KENWOOD KDC-316S CD PLAYER, INFINITY REFERENCE 5"X7" DOOR SPEAKERS WITH ORIGINAL DYNAMAT. FLOWMASTER 40 SI/DO EXHAUST WITH 22" X 3-1/2" DUAL CHROME TIPS. BUGFLECTOR II BUG DEFLECTOR, VENTSHADE VENTVISORS, NYRACORD BED MAT. AND I USE VALVOLINE DURABLEND 5W-30 SEMI-SYNTHETIC ENGINE OIL.
I've had both the 3.73s and 3.55s and the 3.73s are the way to go. The truck is stronger at all speeds and RPMs, and the gas mileage difference is negligable. The truck with the 3.73 had a wicked 60-90 mph punch that was great for interstate passing, with or without a load. Loved it! Still trying to get used to the 3.55s...
Higher ratios (lower numbers) don't necessarily correlate with better gas mileage. My 2000 5.4L Expy 4x4 has factory 3.31's. Then I put on 33" tires so my effective rear ratio is now 3.01. I get absolutely no better mileage with the higher ratio. Hence, I'm going to install either 4.10's or 3.73's really soon. With no mileage increase and more sluggishness off the line, I see no reason not to install lower gears. And no, my truck is not constantly downshifting while driving along the interstate despite the high ratio.
The larger rolling resistance of the 33" tires offsets the change in"effective" gear ratio. All other factors held equal, including tire size, the 3:08's will generally give better fuel mileage than 3.55's etc...
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