The gears are what go in your axle and connect the driveshaft to the axleshafts...basically. There is a ratio there of how many times the driveshaft will spin to make the axleshaft spin once. With 4.56 gears, the driveshaft will spin 4.56 times for every 1 time the axleshaft/wheel goes all the way around. Having the proper gear ratio is necessary for a number of reasons. First of all, if they are too low, your engine will always be running high rpm's and give you shitty mileage etc. Too high a gear and the truck will be slow and your transmission will hate you because it will have more stress on it, along with other stuff.
lower gear ratio (4.10)=better highway mileage, slow acceleration, harder on tranny.
high gear ratio (4.88)=better city mileage, faster acceleration, more low end power.
a 4.56 ratio will give you the best "all around" performance. You won't have the fastest accelerating truck, but you'll still get decent mileage. If you do both city and highway driving, this is an all around good choice.
I'm not sure if i'm explaining this correctly, but I believe the gear ratio determines how many times your driveline rotates, before your differential makes one full ratation.
For example.. with 4.88 to 1 gears. Your driveline will turn 4.88 times for your differential or axle to rotate once.
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