In general, a numerically lower transmission gear ratio improves a vehicle's fuel economy. Some factors that can affect the relationship between gear ratios and fuel economy are vehicle weight and payload, average driving speed and the grades of the roads the vehicle is frequently driven on.
Mechanics and automotive enthusiasts often refer to numerically low gear ratios as high and numerically high gear ratios as low, since numerically lower gear ratios perform better at higher speeds and vice versa. The improved fuel economy gained from a numerically lower gear ratio comes at the cost of acceleration and low end torque. If a transmission's gear ratios are too numerically low, the engine and drivetrain will struggle, and fuel economy and performance will both decrease. This is especially true when the vehicle is hauling, towing or driving uphill.
For that reason, automotive engineers carefully select factory transmission gear ratios based on a vehicle's weight, payload capacity, engine horsepower, torque capabilities and intended driver use. Thus, a full-sized pickup truck is likely to have a numerically higher gear ratio than an economy car or luxury sedan. Automotive enthusiasts, particularly racers and off-roaders who modify their vehicles' gear ratios, switch to numerically higher gear ratios more frequently than lower ones in order to increase performance, often at the cost of highway fuel economy.