67BETE

I know I'm really putting my stupidity out there for everyone to see, but since this forum has the potential to educate us idiots, I thought I would ask anyway. Everyone tosses around their gear ratios like their war trophies 3.45:1...3.35:1. What the heck does that mean?? I assume you can change these ratios and thereby alter the cars performance - which direction does what?? How hard is it to do? Finally, is there a good "old" book about 1960 style combustion engines that I could get my hands on to learn a lot more about this kinda stuff?? I'll put this here since it's probably what you're doing as you read this. Be kind!

Allcoupedup

http://science.howstuffworks.com/gear-ratio.htm

Allcoupedup

About your 2nd question... an older gentleman I worked with gave me a text book from the late 60s that he used in high school. It was as basic as they come. It talked about far flung technologies like over head cams, front wheel drive, and wankel (sp?) engines too. Check half.dom or EBAY and I'm sure you'll find something.

brian

Johns_65Vette

The lower the number, the higher the ratio. In other words a 3:08 (higher ratio), will be better suited for the highway (lower engine RPM's), than a 4:11 (lower ratio). However the 4:11 will get you off the line quicker (better torque multiplication).
It always seems to be a compromise. My car has 4:11 ratio (driveshaft turns 4.11 times to the rear wheels one turn) and they were killing me on the highway, bacause the engine revved so much.
Instead of changing the rear gearing, I opted to put in an overdrive transmission. Now I have the best of both worlds.

Grey Ghost

Kirk - you can also determine your axle ratio by jacking the car up, placing an index mark on the tire and drive shaft. Then rotate it and count the number of turns the driveshaft makes. You then divide to get the ratio.

Don't forget that transmission gears can also be changed.

kellsdad

The gear ratio number (e.g, 4.11) indicates the number of times the engine rotates each time the rear wheels rotate once when the transmission is in direct drive (typically 4th gear in a four speed transmission). Thus, the higher the gear ratio, the faster the engine must turn to achieve a given wheel speed. Likewise, the higher the gear ratio, the lower the wheel speed will be for a given engine RPM. Higher gear ratios generally permit quicker acceleration, while a lower gear ratio may permit a higher top speed and will permit lower engine RPM at cruising speeds.

63Corvette

A gearbox (and rear axle) gears are functionally a lever with which to multiply the torque of the engine, in order to enable you to pull away from a stop at low engine speed, and still have a useful cruising speed within the safe speed range of your engine.

Allcoupedup

A better way to describe the ratio is the ratio of input revolutions to output revolutions. In any given car, there are two ratios that need to be taken into account - the rear end ratio and the transmission ratio.

So a "4.11:1" rear end ratio actually measures the number of driveshaft revolutions to the number of wheel revolutions. When transmission is added into the equation, the final drive ratio is a function of which gear the transmission is in.

Ratios can be combined through multiplication - if I was using a close ratio muncie with a first gear ratio of 2.2:1 and a 3.36:1 rear end, the final engine to wheel rotation ratio would be 7.39:1. In other words, for ever 7.39 crankshaft rotations, the tires would make one rotation.

Brian

67BETE

Thanks, this has been awesome. I've read all the links and then some and finally have a handle on it. At least now I will understand a little better when people toss the numbers around.

2toys

3:70 Posi for me. Gee now I know what the 3:70 stands for.... wonder what "posi' is. Hmmmmmmmm

SPLITRAY

Posi-Traction or positive traction to both wheels... I think!