There usually isn’t much choice when it comes to frames or axles—but there is a little.
For frames, remember that hydroformed is stronger than a stamped frame. Hydroforming is a process in which water pressure is used to bend the frame rails into shape rather than stamping. The pressure bends the metal, but because the water helps maintain a stable temperature during the process, it preserves the strength of the steel.
From there, a boxed frame is stronger than a C-channel frame. If you are confused with those terms, a C-channel describes the cross section of a frame rail. If you cut a C-channel frame rail, it looks like an upper-case C. A boxed frame is essentially a fully enclosed C-channel.
If you don’t want to mess with all that, you can rely on a few numbers: the gross vehicle weight rating and the gross combined weight rating. The GVWR is the maximum allowable weight of the truck or SUV when it’s fully loaded with passengers, gear and fluids. The GCWR is the GVWR plus the maximum allowed trailer and tongue weight.
You also can rely on the towing capacity figure. As a rule, get more truck than you need. Let’s say, for example, you have a trailer that weighs 8,000 pounds. Get a truck that tows, say, 10,000 pounds or so. That way, you’re not asking for everything your truck has each time you tow. Makes sense, right?
For axles, you don’t usually get much choice other than axle ratio and whether the truck is two- or four-wheel drive, but those choices are important. In general, two-wheel drive trucks tow heavier loads, but that doesn’t mean you shouldn’t consider a four-wheel drivetrain for towing. If you live where it snows, or if your favorite campground gets muddy and messy, four-wheel drive might be worth the money. If you don’t really need four-wheel drive, don’t bother with it. Stick with rear-wheel drive, but be sure to get limited slip.
Another good rule of thumb applies to axle ratios: the heavier the trailer you’re towing, the shorter your axle ratio.
For example, if you have a boat that weighs, say, 5,000 pounds, a 3.73:1 rear axle ratio likely would be sufficient. If it weighs 10,000 pounds or more, something along the lines of a 4.10:1 or 4.30:1 ratio would be a better choice. For most towing applications, you should choose a ratio of at least 3.50:1. On older trucks, choosing such a gear ratio meant poor highway mileage and uncomfortably high rpm on the freeway, but the aforementioned overdrive transmissions have gone a long way toward boosting fuel economy and reducing highway rpm.