Gear Ratios

Why? A firm grasp of gear ratio math will allow you to fine tune your vehicle's performance—specifically, its acceleration, torque and top speed. Gear ratios determine the load on the engine, and that impacts both acceleration and top speed.

Transmissions Second gear is in the transmission. It is described by a ratio. If you have a second gear ratio of 1.8 to 1, that means the second gear in the transmission turns 1.8 times for every 1 turn of the engine.

Factors to Consider The higher the gear ratio, the quicker your car will accelerate, but lower mph The lower the gear ratio, the higher the miles per hour your car will be able to reach in that gear. (Keep in mind, your engine has to have enough power to push the car up to that speed.)

Some Typical Transmission Gear Ratios 1 st Gear: 3 : 1 2 nd Gear: 1.5 : 1 3 rd Gear: 1:1 4 th Gear:.85 : 1 5 th Gear and/or Overdrive:.70 : 1

How to Calculate Gear Ratio The number of teeth on the driven gear(output) divided by the number of teeth on the drive gear(input)

Driven(output) / Drive(Input) Find the Gear Ratio of the following of the following 1 st Gear 30 Driven / 10 Drive = Gear Ratio of 3:1

Driven(output) / Drive(input) Find the Gear Ratio of the following of the following 5 th Gear 10 Driven / 30 Drive = Gear Ratio of 1 : 3

Think about output speed difference 1st Gearvs 5th Gear 3 : 1vs 1 : 3

Rear Axle Gears For many performance cars, 3.73s and 4.10s are common rear axle gear choices. The rear end gear ratio refers to the relationship between the ring gear and the pinion gear.

Rear Axle Gears- Find the ratio- For example, if we divide a ring gear with 41 teeth by a pinion gear with 10 teeth we find that the gear ratio is 4.10:1 (41/10 = 4.10).

Final Drive Ratio Transmission Gear Ratio Multiplied by Drive Train Gear Ratio

Bonneville A TH200-4R overdrive automatic utilizes a First gear of 2.74, a Second of 1.57, a Third of 1.00, and a 0.67 Overdrive. It has a 3.73 Axle Ratio. What is it’s final drive ratio in 1 st Gear? With this transmission's First gear ratio of 2.74 combined with a 3.73 axle ratio, the final drive ratio >> yields a 10.22:1 (2.74 x 3.73 = 10.22).

Bonneville A TH200-4R overdrive automatic utilizes a First gear of 2.74, a Second of 1.57, a Third of 1.00, and a 0.67 Overdrive. It has a 3.73 Axle Ratio. What is it’s final drive ratio in overdrive? In overdrive, the final drive ratio equates to a Bonneville- ready 2.49:1 (.067x 3.73 = 2.49)

Bigger Picture? How does this determine the load on the engine and top speed?

Facts 1 HP = 33,000 ft-lbs/min A Horse can lift a lb weights 1 ft in 1 min HP = RPM X Torque / 5252 F = M x A Drawbar Horsepower : HP = lbs x mph / 375

Question What would the 1 st gear ratio need to be for at an ISX diesel engine to pull 100,000 lbs at 10 mph? It puts out about 2200 ft-lb of torque at 1500 rpm, and has a rear differential of 2.1? Ans: Start with drawbar horsepower… HP = lbs x mph / 375 HP = 100,000 x 10 / 375 = hp

Next Steps What would the 1 st gear ratio need to be for at an ISX diesel engine to pull 100,000 lbs at 10 mph? It puts out about 2200 ft-lb of torque at 1500 rpm, and has a rear differential of 2.1? Next HP = (RPM x Torque) / hp = (RPM X torque) / = (1500rpm x 2200 x tranny gear x rear diff)/5252

= (1500rpm x 2200ft-lb x tranny gear x 2.1) /5252 To Solve: Get everything on right side over to left side x 5252 = ft-lbs / 1500 = ft-lbs / 2.1 rear diff = ft-lbs / 2200 ft-lb = 2.11 AT LEAST!!!!!