Selecting an effective and balanced gearbox is essential in engine design. Automakers have to balance out a vehicle’s performance and fuel economy when determining a vehicle’s gear ratio.
With too much gear, car owners may complain of high fuel economy while others may be unsatisfied with a ‘gutless’ vehicle with too little gear. Therefore, would raising a car’s gear ratio (from 2.73 to 3.73) affect fuel?
What is gear ratio?
Gear ratio refers to the number of turns the output shaft makes when the input shaft makes one rotation. It is when two gears are rotating in opposite directions that the smaller gear spins twice as fast than the larger gear, causing the ratio between the gears, or the gear ratio.
As a result, the smaller gear may rotate several times by the time the larger gear makes one axis of rotation. Numerically expressed, a high number indicates a low gear ratio, while a lower one indicates a high gear ratio.
The lower the gear ratio, the more engine revolutions per minute (rpm) needed to maintain a given top speed with equivalent tire size. Gear ratios in transmissions ensure your engine maintains a safe and economical operating RPM range.
It gives your vehicle the ability to accelerate while still keeping a safe speed across the ground. Lack of transmission means that once you reach the maximum engine’s operating RPM; there would be no more acceleration.
How do gear ratios affect fuel mileage?
Gear ratios directly affect fuel consumption. At a given speed, high gear ratio positively affects your vehicle’s fuel economy and vice versa. By raising your car’s gear ratio from 2.73 to 3.73, typically you reduce your engine’s fuel consumption since you increase your car’s normal operating rpm during driving, translating to lower fuel economy.
Higher gear ratios (lower numerical values) mean the engine runs faster to achieve a given top speed; thus, better acceleration/torque and high fuel economy. On the other hand, lower gear ratios allow your vehicle’s engine to run more economically to maintain a given speed; hence, better fuel economy and higher top speeds.
Changing gear ratios can lead to a drastic shift in fuel savings or increased acceleration advantages. However, installing a new set of rear end gears is one of the most challenging automotive tasks to accomplish.
It requires precise measurements when lining up. If the mesh does not line up correctly, it may lead to excess friction in the driveline, whining sounds from the rear end while driving, and premature wearing of the gears.
Gear ratio fuel economy calculator
Using a gear ratio calculator helps in finding an effective gear ratio for your vehicle with new tire size. It aids in determining the best ring and pinion gear ratio for your car; thus, optimizing fuel economy, performance, and everyday driving.
Auto repair and tire shops provide information charts on gear ratios to tire size. Some other tables display the new gearing you may need to go back to your initial gear ratio when switching to either a bigger or smaller tire.
A gear ratio and a tire diameter chart give an approximation of ideal RPMs at 65mph (highway speed) for better fuel economy, increased towing power, and overall drivability. It represents a 1:1 gear ratio on a manual transmission, ensuring your car is as efficient as possible.
Auto transmission RPMs are higher than those in manual transmissions and overdrive transmissions tend to be a bit lower.Another way to easily calculate gear ratio is multiplying .12 by your new tire size, for example, .12*38”=4.56. You may also calculate rear axle gear ratio by dividing pinion gear tooth count by ring gear tooth count.
Tire specifications have a three number format (275/75R16). In this case, 275 is the tire tread width in millimeters, 75 refers to ASPECT (percentage of tread width to yield the sidewall height), and 16 is the diameter of the tire.
Truck and off-road tires come with specifications like 30*9.5R15, with 30 as the tire diameter, 9.50 as the tread width and 15 as the wheel diameter. In such scenarios, you do not need to calculate the tire diameter.
Furthermore, you need to determine your tire diameter when using an RPM calculator to find the gear ratio. The formula to calculate your vehicle’s tire diameter is (cross-sectional width*0.aspect ratio*2/25.4) + wheel diameter.
Which rear end gear ratio is best for fuel economy?
Generally, rear end gears, such as 2.79, 3.00, and 3.25 and so on, are excellent for freeway driving. However, they are not the best when it comes to accelerating from a dead stop or for 0-60mph.
Lower rear end gear ratios, those with high numerical values such as 3.55, 3.73, 4.11, etc. are best for acceleration. Lower gear ratios accelerate quickly from 0-60mph; thus, you will gain top-speed much faster.
Car owners need to choose a gear ratio that works best with their engines and driving habits. Small engines with high ‘peaking’ abilities work best with low gear ratios while big engines can easily ‘pull’ taller gears compared to small engines; thus they can work with both high and low rear end gear ratios, depending on your driving needs and skills.
Fuel economy is an essential consideration when it comes to gear ratios. Therefore, before raising your car’s gear ratio from 2.73 to 3.73, it is advisable to understand what effects it will have on your vehicle as well as your driving experience.
Steven Reilly is a qualified mechanic and his passion for cars goes beyond just the technical aspects. He is also an amateur racer and all round car enthusiast. When he is not driving them, he can often be found in his garage under the hood of a rare model. Steven Reilly has lost track of the number of hours he has spent setting up his fine collection of rebuilt models. He believes that cars can provide a constructive and fun opportunity to teach the youth important life skills. In line with this, he is developing a community outreach program, potentially dubbed ‘Cars for change’.