Gear Ratios in Automobiles--How Do They Work?

My friend bought 3.73s. What does this mean? Is the car good for interstate driving now?

Your question is so terse it’s hard to grasp what you are talking about. The number you quoted sounds like the gear ratio for a differential. That’s the gadget that allows gear-driven wheels to turn at different speeds, as when turning a corner.

If that is the differential gear ratio that is within the usual range, although maybe a little low for around-town driving but should be fine on the highway.

For a good understanding of transmissions and differentials go to the web site HowStuffWorks - the automobile section.

Mostly out of curiosity, is there any way to easily change the gear ratio in a passenger car? My Subaru appears to have a gear ratio in the lower gears so that the car seems to jerk while driving. I’ve mostly seen it in 3rd at about 25 mph. I probably will never ever do it, but I am a little curious, especially because that jerking is really annoying. (Unless it was an effect of my clutch almost being completely worn out; I haven’t really noticed it since I replaced the clutch but then again I wasn’t really paying attention.)

The gear turns 3.73 times for every revolution of the wheel, right? How does this relate to acceleration, top speed, fuel economy, etc.? What is high/low?

If the ratio you cited is the differential ratio it means that in the top gear, other than overdrive which is another story, the rear wheels make 100 revolutions for every 373 revolutions of the engine.

Ordinarily, high gear means direct drive, i.e. the transmission output shaft turns at the same speed as the engine.

Overdrive is a gear in which the transmission output shaft turns slightly faster than the engine.

With a gear ratio that is low you get good mileage but acceleration and pulling power is low. Going up hill or against the wind or if you need to pass, for example, you would need to shift down to a lower gear. Confusing terminology isn’t it? A “low gear” means that the engine has to turn faster to get the same rear wheel rotation speed as with a “high gear.” So a “low” gear has a higher gear ratio than a “high” gear.

Again, I say. Go to web site HowStuffWorks to get a good primer on gears, clutches, transmissions and so on and so on.

acceleration and pulling power also are low.

It depends more on what his old ratio was. If his old ratio was for example 2.73, then the engine will be turning more revs at interstate speed, and he’ll get worse gas mileage. This will lead to some increase in noise in the car, and will possibly lead to a somewhat shorter engine life. He’ll also have to probably change speedometer gears. If he went from say a 4.10 ratio to the 3.73, then everything would be reversed from what I said(except he’ll still have to change his speedo gear).