What are Car Gear Rear End Ratios All About?

What are Car Gear Rear End Ratios All About?

When it comes to gear ratios in automobiles, these are normally used in the transmission and the drive axle.

Its main function is to multiply power, as when all ratios are multiplied the final drive ratio is calculated.

Regardless of the engine type the car has, proper gearing ratio proportions draw the bottom line between having a car that accelerates and runs smoothly and lightly and one that doesn’t.

Rear End Ratio

The rear end gear ratio refers to the ring gear and the pinion gear measurements. This can be calculated as follows.

Rear end gear ratio = Ring gear tooth/pinion gear tooth count

Tire diameter also adds to the equation when mentioning final drive ratio. The reason being because of the tire diameter changing, same as the engine’s RPM at a given speed.

This also can be calculated with a simple formula:

rpm = (mph x final gear ratio x 336*) / tire diameter

If the tire diameter gets reduced, the engine speed increases. On the contrary if we increase the tire diameter by using larger tires, the vehicle will increase its acceleration because it has a higher numerical gear or low gear without the need for changing gears.

The reason why this is so important is because transmissions normally have several gear choices, hence the transmission allowing the vehicle to have a more efficient acceleration with lower gears, and to maintain an RPM peak performance using higher gears. This as a result saves fuel.

Today’s transmissions utilize overdrive high gears in the range of 0.70:1, which allows for reduced engine speeds. These overdrive transmissions in combination with a 4.10 axle ratio allows a fuel-friendly final drive ratio of 2.87:1 (4.10 x 0.70 = 2.87) in high gears as a result.

Why Is This So Important?rear end gear

By knowing the basics of gear ratios and power leverage, the car’s acceleration can be easily improved without needing to invest large amounts of money in engine enhancement.

Note that smaller engines need lower rear end in order to accelerate properly in contrast with engines of higher capacity where a higher rear end is required to provide for smooth cruising, as well as a higher top speed. Meaning that if you install a bigger, more powerful engine, you probably want to change to a higher gear ratio.

This is because shorter wheels require higher gearing for them to be capable of running the same distance as larger wheels. The smaller the size engine, the more power they produce as the gear ratios increase. That explains why an MGB with 14″ wheels is best suited with a 3.90 gear ratio and the earlier MGA with 15″ wheels came up with a lower gear ratio than the MGB.

Therefore, picking a proper gear that is compatible with the engine and driving requirements is key because smaller, high winding engines that make good “peak” power need lower gears, or they will fall flat on performance. Big, torquey engines can be chosen for taller gears and not need short (low) gears as much as a smaller engine does. The main aspect to understand this is that it doesn’t matter how enhanced and powerful the engine is, choosing the wrong kind of gearing will never allow for it to reach peak performance.



If you happen to have questions please visit our website where we have all the basic information about gear end ratios and how to learn to use the proper ratio to fully unlock your car’s full blown speed potential.


Bryce Newell is an automotive enthusiast who loves to write about the latest news, products, and DIY projects. While Bryce is an amateur in the field, he is passionate about cars and has been since he spent weekends in the garage helping his dad rebuild a 68 Dodge Charger as a kid.

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