Tow/haul mode changes how an automatic transmission shifts when you are towing or carrying a heavy load: it holds each gear longer, shifts more firmly, reduces gear hunting on grades and uses engine braking when you slow down. The axle ratio sets how much the drivetrain multiplies torque; a numerically higher ratio such as 4.10 pulls harder but turns more engine speed on the highway than a lower ratio such as 3.42.
What does tow/haul mode do?
- Holds gears longer so the engine stays in its power band under load.
- Firmer shifts, which reduce clutch slip and heat.
- Less gear hunting on hills, so the transmission is not constantly shifting up and down.
- Grade braking: many trucks downshift automatically on descents when you brake, using engine braking; diesels may combine this with an exhaust brake.
When to use it
Use tow/haul mode whenever you tow a substantial trailer or carry a heavy load, especially on hills. For empty driving it is unnecessary and can lower fuel economy because the engine runs at higher speed.
Axle ratio explained
The axle ratio is how many times the driveshaft turns for each turn of the wheels. A higher number means more torque multiplication: better acceleration and pulling power with a heavy trailer, at the cost of higher engine speed and fuel use at cruise. A lower number does the opposite. Bigger tires effectively lower the ratio, which is why owners who fit larger tires often re-gear.
Choosing a ratio for towing
- If you tow heavy loads often, a numerically higher ratio from the factory tow options suits the job.
- Modern transmissions with more gears narrow the difference, because deep first gears and tall overdrives cover both needs.
- Use our gear ratio calculator to see engine speed at cruise for any axle ratio, tire size and transmission, and compare transmissions in the gear ratios chart.
How to find your axle ratio
Check the build sheet or window sticker, the axle tag on the differential cover, or the RPO code label on GM trucks, which lists the axle ratio code. A dealer can also decode it from the VIN.
Protecting the drivetrain when towing
Watch transmission temperature, add a transmission cooler if you tow often, change fluid more often under heavy use (see how often to change transmission fluid), and choose a transmission built for towing if you replace yours; see our transmissions.
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