Truck Rear Axle ratio?

[Buckfever]

Could you guys educate me on the deal with the rear axle ratios?
Don't know a thing about it.

[ncboman]

probably should be in shoptalk but anyway, what is it you want to know?

[eshoremd]

higher the #, ex. 4.10, more torque to the ground, better for towing, more power off the line feel, worse gas mileage. lower the #, ex. 3.73, better for highway driving, better for gas mileage, motor revs lower at same speed as compared to the higher gear ratio, typically doesnt feel as powerful off the line. the higher the # the faster the motor will rpm. if you mark the rear tire and driveshaft and spin the rear tire 360 degrees with 4.10's the drive shaft will turn 4.10 times. hence the #. i have 373's in my chevy 2500.

[Buckfever]

Sorry, I missed the shoptalk section. So is the difference is gas mileage proportional to the difference in the ratio?

[Rock Chuck]

Not necessarily. Get out the specs on your truck and look at the max horsepower rating. Note the rpm's where the rating is the highest, say 4000 rpm. Multiply that rpm by 2/3, getting approx 2700. Thats the engine speed where you best mileage will be. If, at 60 mph, you're cranking 3000 rpm, you're running a little fast for maximum mileage. If your axle ratio drops your highway rpms below 2700, you'll probably start losing mileage. A lot of guys put on larger tires thinking they'll cut engine speed and get better mileage, but actually hurt mileage because they're getting out of the ideal rpm range.

To find the gear ratio, look for the metal tag on one of the differential cover bolts. It will have either the ratio, say '4.11', or the number of teeth on the ring and pinion gears. If it says '37 9', that means that it has 37 teeth on the ring gear and 9 on the pinion gear. Divide: 37/9 = 4.11, your axle ratio.

Dick

[eshoremd]

ohhh! so i go down the highway at 1700 rpms at 60mph with my 3.73's so id get better gas mileage with 4.10s. yeah ok!

why do they put 6 speeds in cars? lower rpms on the highway=better gas mileage at cruising speeds and higher possible top end.

[Buckfever]

Why the 2/3rds? Is that a rule of thumb based on torque, or engine efficiency?

[Rock Chuck]

That's what I've read in some of the car & truck mags. Supposedly, it's based on engine efficiency. While I've read it several different times in different places, it's possible that new engineering techniques have made it obsolete.

Dick

[Twanger]

I think 6-speed transmissions in cars are a gimmick. Car engines have very wide torque curves and don't need that close a gear ratio. On peaky motorcycle engines (typically 2-strokes) a 6-speed can be useful to keep you revving in the power-band. Mostly needed to go-fast. These days even motorcycles have wide torque curves. I've got a Honda 700 interceptor and it really only needs two forward speeds. I can do 0-60 if first gear, and then drop it into 5th.

[eshoremd]

twanger said:

I think 6-speed transmissions in cars are a gimmick. Car engines have very wide torque curves and don't need that close a gear ratio. On peaky motorcycle engines (typically 2-strokes) a 6-speed can be useful to keep you revving in the power-band. Mostly needed to go-fast. These days even motorcycles have wide torque curves. I've got a Honda 700 interceptor and it really only needs two forward speeds. I can do 0-60 if first gear, and then drop it into 5th.

its definetly not a gimmick walt. that sixth gear is what enables cars such as the 500hp chevy corvette ZO6, which runs sub 11.5 second quarter mile times, to get 28 miles to the gallon on the highway.

[Twanger]

Todd - What I'm saying is that in the real-world there's not much payoff for having a 6th gear. A big cubic-inch muscle car could simply have a 5th gear at the high gear ratio, and do just fine with the other 4 gears getting up to speed. My truck has a 4-speed automatic, 350CI 300HP V8 and rev's VERY low in top gear - to get good mileage. The other 3 gears are used to get me up to speed. These engines make so much torque over such a wide rev-range that many gears are superfluous... just like on my motorcycle. I guess if you want to shave half a second off your quarter mile time then it might be worth having another gear. In the real world of traffic driving, 70MPH speed limits, and driving below 50% throttle settings the additional gear is wasted metal.

[eshoremd]

coming from an overall performance standpoint i just have to disagree. i think everyone in the automotive industry would too. mercedes just came out with their 7 spd auto, guess theyre wasting two gears. i understand what your saying walt but more gears are just better for overall performance. its not necessary for going from point a to b, but cars these days arent made to just go from a to b. theyre suppose to be fun. i think we're both right in a way. in support of your side of the argument do you know that 93-2002 6 spd camaros, under normal driving, make the driver shift from 1st to 4th. wont let you use 2nd and 3rd unless your "getting on it". most guys hate this and buy whats called a, "skip shift elimanator" from slp. this is an excellent feature for saving mileage but would suck if it was permanent from a performance standpoint.

[Buckfever]

What the hell are you guys talking about? LOL

[ncboman]

the topic got hijacked.

buckfever said:

Sorry, I missed the shoptalk section. So is the difference is gas mileage proportional to the difference in the ratio?

Generally, figure a 4:11 rear end is drag racing stuff, high torque off the line and/or good for towing heavy loads. 3:73 is more of a highway rear end, better for gas mileage on the road.