GEARING FOR MPG
Axle ratios have a direct affect on vehicle performance, from acceleration to towing to fuel economy; choose wisely when you buy your next pickup
by Bruce W. Smith
Axle ratios are a common point of discussion among the performance-minded, yet everyday pickup buyers in the fleet and construction world rarely pay gearing any heed.
But they should: Axle ratios have a direct affect on vehicle performance, from towing to fuel economy. More so on gas V-8s than diesels.
“Lower (higher numerically) axle ratios such as the 3.73s and 4.10s, provide the best acceleration and towing performance in pickups,” says Dawn Piechocki, Ford’s Vehicle Engineering Manager for the new F-150s, Expeditions and Navigators.
“But in real-world driving, unladen, those lower ratios cost about 2mpg in highway fuel economy compared to 3.15s, which is the ratio [Ford] uses in pickups when trying to maximize mpg,” says Piechocki. “But in city driving the lower axle ratios have little effect of fuel mileage.”
In years gone by, pickup manufacturers used to offer a wide range of axle ratios depending on the truck and engine/transmission package. In the gas V-8s it was common to have the option of 3.42, 3.55, 3.73 or 4.10 axle ratios.
Today, with EFI engines being much more efficient and powerful, and transmissions getting more splits, axle ratio options are limited.
For example, the 2014 Silverado/Sierra 4x4s with the 5.3L/6.2L V-8s are only offered with 3.42 or 3.73 ratios. But the latter is only available with the “Max Trailering” package. The 2015 Colorado V6 is only available with 3.42s.
Keep that in mind, especially if you plan on changing tire sizes after you make your truck purchase.
Tire diameter directly affects the final-drive ratio just as if one changed the gears in the differential.
For instance, switching from LT265/R17s to LT285/R17s, or roughly a 1-inch taller tire, is equivalent to changing the axle ratio from 3.42 to 3.30. So acceleration performance will fall off and the truck will probably hunt for a lower gear when pulling grades at cruise.
Fuel economy will also take a hit—but not from the gearing change alone.
Taller, wider, heavier-lug tires weigh more than the typical OE street/all-terrain/all-season tires.
So even a one-inch change in rolling diameter comes with an increase in rolling mass and rolling resistance, which can can easily chop 2-to 3mpg at highway cruising speeds.
Bottom line, choose your gas pickup’s axle ratio wisely.
Here’s what I suggest:
- If you are going to add taller tires, get the lowest axle ratio (3.73 or 4.10) offered.
- If you tow or carry a big load in the bed, do the same as above.
- If you just want the best mpg and never tow or haul a load, get the tallest (3.23-3.42) ratio offered.– Bruce W. Smith