General Knowledge · Shifting Gears
What is the purpose of selecting a low range on a vehicle with an automatic transmission before going down a grade?
Explanation
Selecting a low range on a vehicle with an automatic transmission before a grade keeps the transmission from shifting up past the selected gear, which increases engine braking and helps control the vehicle's speed on the descent without relying on the brakes alone. Controls used for shifting are part of the key elements of shifting that CDL applicants must know under 49 CFR 383.111(a)(5)(i), but that paragraph only names controls as a required knowledge topic; it does not itself explain how a low range functions, which is standard driver-training and vehicle-operation knowledge rather than a codified federal rule. A low range has no connection to the parking brake and does not increase top speed, and no federal rule limits this control's use to hazardous materials transport specifically.
Source: 49 CFR 383.111(a)(5)(i)
Related Shifting Gears questions
- Why do most heavy vehicles with transmissions that lack synchronizers require double clutching when changing gears?
- What are the two basic ways a driver can know when to shift up in a vehicle with a manual transmission?
- Why should a driver shift down into a lower gear before a long or steep grade begins, rather than waiting until already partway down it?
- Why should a driver downshift to the correct gear before entering a curve, rather than while turning through it?