Billinthedesert

Is there going to be a noticeable, real-world difference in gas mileage with a 3.15 vs. 3.55 vs. 3.73? I would expect the 3.15 to have some sort of an edge, but don't know how big.

ghunt81

My truck has 3.73's and still puts down some respectable mileage on the highway at 75 mph. Turns about 2100 RPM at that speed.

Billinthedesert

Thanks for that. I wonder what the RPMs would be for the 3.55 and 3.15 at 75 mph.

I looked at this:
https://www.grangermotors.com/blogs/...io-ford-truck/
and this:
https://www.hardworkingtrucks.com/ax...-fuel-economy/
and this:
https://www.edmunds.com/car-buying/h...kup-truck.html

Florida_F150

This calculator (link below) will tell you the exact RPM's for a given tire size, rear axle ratio, and gear your transmission is in. You can look up the ratio for each of your transmission gears easily online. Let me know if you need help.

http://www.crawlpedia.com/rpm_gear_calculator.htm

I'd think about what you want out of the truck, fuel economy vs performance. Most people would probably say your driving style has more impact on your MPG than your ratio, unless you're really cranking out the highway miles on a daily basis. On the other hand, the (higher number) ratio's will definitely benefit performance, and with today's transmissions with overdrive, the RPM's are still pretty low. I have a 4.88 gear ratio (with my tire size, probably equivalent to around 4.20 gear ratio) and I'm still under 2,000 RPM's at 65mph in 6th gear (2015 F-150). Everybody's needs are different but I'd much rather have the extra performance which is why I went with 4.88's.

marshallr

A lot less than you may think. A Tundra with a 5.7L V-8 and 4.30 gears is only about 2 mpg worse than a Ford with a 3.5L V-6 and 3.31 gears. I can't imagine any measurable difference between the same engine, same truck, with one having 3.31's and another having 3.55's.

And much more info is needed as to how the rest of the truck is equipped and how it is going to be used. Cutting rpm's doesn't necessarily reduce fuel mileage. Each engine has a sweet spot of rpm's where it is most efficient. Too slow, or too fast, and it isn't efficient.

If your axle ratio is too high, 3.15 for example then the transmission has to spend more time in lower gears in order to move the truck which reduces fuel mileage. If you're pulling any weight, driving on an incline, or just have a heavier truck higher 3.15 gears will really strain the engine and you'd likely get better fuel mileage with 3.55's or even 3.73's.

DK in NC

acdii

Yes, but with a caveat. RWD you will see better MPG than 4WD, that's a given based solely on weight alone. However, comparing an EB 6 speed with 3.15 gears to another EB with 3.55 gears I see at least a 3 MPG difference. My 14 weighed 5980 pounds, was RWD, and had a topper, my 16 weighs 6000 pounds is 4WD and has a topper, so all and all they are pretty much the same except for COD due to the different bodies. The 16 gets 17-187, and the 14 got 20-21 with a high of 24. The most I ever got out of the 16 is 20.4.

Now, the caveat, the 10 speed is a different animal in that it acts more like a CVT than a regular transmission so rear end gearing is not as big an issue as it used to be. To get the best MPG, on an EB, other engines maybe different, is to stay below 1500 RPM while cruising. With the 6 speed and 3.15 gears it turned 1450 RPM @ 60 MPH. The 3.55 turns at 1600 @ 60 MPH. Below 1500 the turbos are practically idle, but at 1600 there is some boost which impacts MPG. Having not driven the 10 speed other than in a test drive, I don't know where it cruises at, but I do know that if driven at 55 MPH with 3.15 gears, mild acceleration, and no passing, that it got 24 MPG easily. I "might" be able to squeak out 21 if I try hard enough with the 16, but it is such a dog that I don't bother anymore.

Bottom line is, I personally see a 3 MPG difference in nearly identical trucks as far as weight, engine and transmission goes.

Billinthedesert

Great replies, guys. One truck I am looking at is a regular cab 4X2 with 2.7, 10 speed and 3.73 E-locker rear axle. Could "tune" the MPG a bit with different tires. Stock tires are a 245 70R17.

sailorjerry

When comparing the gear ratios offered in these vehicles, the different offerings will not impact mileage in any way. Even identical vehicles off the dealer lot can can vary by several miles per gallon. Also that Tundra is 1000 pounds heavier than an F150.

My 5.0 F150 3.55 rear end often breaks 22mpg when interstate driving.

Steve83

No. Regardless of the RAR, you're still pushing the same weight, drag, & rolling resistance with the same engine. A few RPMs higher or lower doesn't change the engine's efficiency enough to notice. But the different ratios WILL affect how the driver feels the acceleration, which can subconsciously affect how he drives, which has a BIG impact on MPG.

But given the same actual acceleration, driving style, & other variables; RAR doesn't directly affect MPG.