Close-ratio transmission

Definition

A close-ratio transmission is a transmission in which there is a relatively little difference between the gear ratios of the gears. Consequently, note that the word "close" here means "near", not "shut". The gear ratio numbers are in a smaller numeric range, hence closer together.

In the context of close-ratio transmissions, a transmission with large differences between gears is termed "wide-ratio". Close-ratio vs. wide-ratio are relative terms, with no standardization. Therefore, a transmission that a manufacturer terms close-ratio when paired with a V8 engine with a wide power band may be termed wide-ratio when paired with a high-revving straight-4.

Comparison with Ordinary Transmission

Gear	"Typical" stock 5 speed[1]	"Typical" close-ratio 5 speed for same vehicle
1st	3.25	2.60
2nd	1.90	1.66
3rd	1.20	1.35
4th	0.90	1.15
5th	0.70	1.00

1. ^ Assuming a final drive ratio of about 4.0.

Note how the ordinary 5 speed transmission has a high 5th gear. This is for fuel-efficient cruising with relatively low RPMs at freeway speeds. Such a high gear is not necessarily useful in a race situation. In order for the car's transmission be in that gear and simultaneously for its engine to be in its high-power RPM range, the absolute speed of the vehicle may simply be too great. In other words, the closeness of the ratios of a close-ratio transmission may be obtained by "squeezing" from both ends.

Application

A close-ratio type of transmission is designed to allow an engine to remain in a relatively narrow operating speed. Alternately, a wide-ratio transmission requires the engine to operate over a greater speed range, but requires less shifting and allows a wider range of output speeds. Close-ratio transmissions are generally offered in sports cars, in which the engine is tuned for maximum power in a narrow range of operating speeds and the driver can be expected to enjoy shifting often to keep the engine in its power band.

The lower gear ratios can be dispensed with because race cars don't have to deal with stop-and-go traffic, intersections, frequent stops, parallel parking, climbing steep hills at slow speed, and so forth. So it makes little sense to have gears that support these driving situations at the expense of insufficient gear ratio variety for the intended use.

In a race, a car is driven at high speeds, with some reduction in speed for taking turns of various severities. Within this range of racing speeds, it's useful to have lots of gears to choose from in order to operate the engine at, or near the maximum power RPM regardless of the race car's speed.

A conventional transmission would effectively offer too few gears, because after being used briefly at the very start of the race, the first two gears would never be used again. The remaining gears have steep differences between them correspond to a large change in RPM.

Suppose that a given engine's high power range occurs in the 7000 to 8000 RPM band. Gearing up from a 0.9-ratio to a 0.7-ratio drops the original RPM by 22% (multiplying it by 0.777). That may be enough of a drop to take the engine out of its power zone. For instance, if the shift is executed at 8000 RPM, the engine falls to about 6200 RPM, where it will have a lot less power. Gearing up from a 1.15 gear to a 1.0 gear represents only a 13% drop in engine revolution speed. Executed at 8000 RPM, the shift will achieve nearly 7000 RPM, just at the low end of the example engine's power band.

Pseudo Close-Ratio Transmissions

It would appear that one way to obtain some of the benefits of a close-ratio transmission without the compromises is to cram more gears into the transmission. Some six-speed gearboxes available in consumer vehicles are labelled as "close-ratio".

Whether or not they are really "close-ratio" depends on whether they add an extra high overdrive gear for leisurly freeway cruising, or whether they keep the top gear about the same as in the 5-speed model of the same vehicle, and squeeze extra gears underneath.