Calculating MPG on 4WD

Why is it that a 4WD vehicle has an advertised MPG so much lower than 2WD. Example: Toyota Matrix 4WD has roughly the same 20/26 MPG of a Subaru AWD. But with a 4WD shouldn’t the low mileage be only when you’re (infrequently) using the 4WD and the rest of the time you’d get the mileage of a 2WD Matrix?

First, with any AWD or 4WD system, you’re always carrying around the extra weight of the components needed to power the other set of wheels. I’m pretty sure this is not insignificant.

Second, with some non-full-time AWD systems, power is sent to the other set of wheels pretty frequently, such as during moderate to hard acceleration. You’d have the extra drivetrain losses each time this happens. I don’t know if the Matrix (which is AWD, not 4WD) falls into this category. I know that my car does.

Third, I’m not sure about this one, but whatever coupling system is used to send power to the other set of wheels may cause some extra frictional losses just by being part of the overall drivetrain system, even if it’s not sending power to the other set of wheels.

To add: average weight of 4WD components depending on vehicle (physical size) ranges from 250 lbs to approx. 400 lbs.

Transfer case, front differential, cv-joints/driveaxles and front driveshaft.

FYI, there is almost always a difference of 2mpg between 2wd and 4wd.

One other factor to consider when comparing mpg between vehicles is the rear end gear ratios.

You cannot expect the fuel economy of one car to match that of a different make, with its different engine, transmission, and weight. Matrix 4WD matches Subaru AWD drive? Coincidence.

You cannot even safely compare the fuel economy of the Matrix 4WD to other Matrix models. The 4WD version uses a 2.4L engine and a 4-speed automatic. All other Matrix models use either a 1.5L engine or a 2.4L engine, both with a 5-speed transmission (manual or auto). These other models would be expected to show improved fuel economy over the 4WD version, just from the transmission alone. That statement is true even if you operated entirely in 2WD mode.

Thanks, just to clarify, my question is based on the fact that every time I look at a 4WD small SUV or equivalent it has about the same MPG as the Subaru AWD. Not trying to compare models, just still wondering why a 4WD doesn’t get better mileage than an AWD. Weight seems like a good partial answer–but the fact that a 4WD isn’t sending extra power to the wheels as the AWD does with some frequency still seems to me should make a difference.

It’s not just the weight, the gearing is important, too. The rear end of a 4WD truck is usually lower (higher numerically) than a 2WD or AWD vehicle. This is because it is supposed to pull you out of the lousy situation you put yourself into on those dirt roads you like to travel. And even if you don’t like to travel them, there are a number of stump jumpers who do, and that’s who the trucks were designed for.

It works like this: An AWD or 2WD vehicle has a 3.40:1 rear while a 4WD truck has a 4.11:1 rear. This means that in an AWD truck the camshaft turns 3.4 times every time the wheels turn once, but in a 4WD truck the camshaft turns 4.11 times. OK, OK, the transmission has gears, too, but you get the idea.

You’re not comparing apples to apples here. I know the Pathfinder and 4runner both use to come in 2wd versions. The 4wd versions were pretty much identical. The gas mileage difference was about 2mpg.

The Toyota Highlander comes in 2wd and AWD and the difference is about 2mpg. Try comparing the SAME vehicle in 2wd to the same vehicle in 4wd.

Some vehicles w/4WD have AWD as the default drive setting with no RWD only capability. Some Explorers are that way.