Not a stupid question.
Your transmission is a 4 speed, the 4th gear is called overdrive, so you have R-1-2-3-OD
A transmission in which the highest gear ratio is less than a one-to-one ratio. This means the drive shaft turns faster than the engine crankshaft. The overdrive feature saves fuel and, because the engine runs slower, engine wear and noise are reduced.
On stop and go traffic at low speed (under 50mph) it is better to have OD OFF (to reduce shifts and internal clutch and overdrive band wear)
If you cruise at a constant speed leave OD ON
Rob Hernandez, AFM co-Founder.