Gear Ratio Selection & Driving Pleasure

 

Gear ratios are a topic that is heavily dependent on how you use your car. I'm going to cover selecting the right gear ratios for a fun, primarily street driven vehicle that sees a little use on the track from time to time. 

Finding the right balance of ratios, starts with a careful study of the torque band. Let's use a peaky engine, with a very narrow torque band to understand this principle. 


Honda has designed its gear ratios such that as the driver shifts up, the rpm falls to the beginning of the power-zone. This creates the maximum area under of torque curve between minimum and maximum rpm in each acceleration segment.

 

For cars with a very broad spread of torque, the gears don't have to be spaced as close to achieve the same acceleration.  For all out racing, most teams look at gear ratios specific to a track to shave every hundredths of a second, while car manufactures look at variables like fuel economy and emissions. I'm going to focus on two areas:

1. Racing Variable: Corner Exit Speed Optimisation 

2. Street Variable: Fuel Economy Optimisation 

Racing

Let's start with the racing variable. You could go much deeper down the rabbit hole and start to use gear simulation software to optimise gearing for each track, but there is a balance between a fun hobby and an unhealthy obsession :) Think about the corners before the long straights. By maximising exit speed, your overall average speed should be faster. 

At corner entry and mid corner, the car should be perfectly balanced as there is no driveline shock from changing gears awkwardly or hitting the redline mid corner, and should allow you to control the car with the throttle and steering inputs gracefully. 

The average driver in a manual car shifts gear in 2 directions, vertically (1st to 2nd gear) and horizontally (2nd to 3rd gear).  The time between shifts is around 500 milliseconds and 1 second. This means if one driver shifts 10 times in a race, and another shifts 12 times, there could be a 1-2 second gap from shifting alone. 

This comes down to torque. If you have it, and you can get it to the ground without overwhelming the available grip, a taller gear ratio might leave you more options in the gear you are in. 

Fuel Economy

The red line on the graph below represents the torque curve of the engine. The rainbow colours represent fuel consumption, black being the most inefficient. This engine is operating most efficiently from 1,500 to 3,500 approximately. The final gear is very important to make the most of this range. 

In this case, the 6th gear selection should fall around the 2,500 rpm range and this will be ideal for economy if the car is driven on flat ground. The whole idea is to keep the engine RPM low to avoid torque losses from friction, but not so low that the engine throws fuel to keep the engine from bogging.