A fleet wants to convert from an overdrive top gear to a direct-drive transmission to maintain similar engine rpm per mile. To achieve this, what should be changed?

A fleet wants to convert from an overdrive top gear to a direct-drive transmission to maintain similar engine rpm per mile. To achieve this, what should be changed?

• A. Convert the gearing in its rear axles to a higher (numerical) ratio
• B. Convert the gearing in its rear axles to a lower (numerical) ratio
• C. It need not convert (the same rear axle ratio will produce the same engine revolutions per mile)
• D. Convert to lower-profile tires (which would turn more revolutions per mile)

Answer: (B)

Engine rpm per mile is set by the overall gearing: transmission gear ratio multiplied by the final-drive (rear axle) ratio, and then scaled by tire circumference. When you’re in an overdrive top gear, that gear ratio is less than 1, so at a given road speed the engine runs at a lower rpm than the driveshaft would in direct-drive. If you switch to direct-drive (1:1), the engine would tend to run faster for the same speed. To keep the same engine rpm per mile after moving from overdrive to direct-drive, you compensate with the rear axle. You need a lower numerical final-drive ratio (a taller axle) so that the product of gear ratio and final-drive ratio stays about the same as before. For example, if the old overall gearing was 0.75 × F and you go to 1.00 × F', you’d set F' to about 0.75 × F. That means using a rear axle with a lower numerical ratio. Lowering the rear-axle ratio is the right adjustment. Raising it would push engine rpm higher, and leaving the axle unchanged wouldn’t offset the change to direct-drive. Changing tire size could affect rpm per mile, but it isn’t the direct mechanism to maintain the same rpm per mile when changing top-gear from overdrive to direct-drive.

Engine rpm per mile is set by the overall gearing: transmission gear ratio multiplied by the final-drive (rear axle) ratio, and then scaled by tire circumference. When you’re in an overdrive top gear, that gear ratio is less than 1, so at a given road speed the engine runs at a lower rpm than the driveshaft would in direct-drive. If you switch to direct-drive (1:1), the engine would tend to run faster for the same speed.

To keep the same engine rpm per mile after moving from overdrive to direct-drive, you compensate with the rear axle. You need a lower numerical final-drive ratio (a taller axle) so that the product of gear ratio and final-drive ratio stays about the same as before. For example, if the old overall gearing was 0.75 × F and you go to 1.00 × F', you’d set F' to about 0.75 × F. That means using a rear axle with a lower numerical ratio.

Lowering the rear-axle ratio is the right adjustment. Raising it would push engine rpm higher, and leaving the axle unchanged wouldn’t offset the change to direct-drive. Changing tire size could affect rpm per mile, but it isn’t the direct mechanism to maintain the same rpm per mile when changing top-gear from overdrive to direct-drive.