OK can somebody that is real knowledgeable on gearing please explain how different gearing and tires sizes helps or hurts towing power, take-off power, mpg, etc. I really don't know much anout it and would like to learn. I have a 4.7 4X4 with the factory 3.55 gears, I am running 265/75-16's and would like adjust the gearing for a little better towing w/o destroying my mpg. I was thinking maybe I could get the gear ratio al least back to 3.55 with the tire size I am using. I know it can be done, but don't know what ratio to use.
Thanks Guys (& some Gals)
Ron

 

i dont know all the different calculations and stuff but i have enough knowledge i think i can help you out a little. The numberically higher the number the better towing and take off you will have this is a lower ratio but numerically higher. I am running 31's with 3.92's and still get 19-21 highway in the summer time. I have also heard of some guys going to 4.10's for the acceleration and having them say it hasn't hurt there mileage to much but i think they were 2wd. For me the 3.92's are perfect good take off and good gas mileage so im happy they are also the lowest you could get factory. hope this at least helps you a little in better understanding.

 

Thanks for the response it did help a little.
I would still like to hear some educated mechanic info on how this works. I am an info nutt.

 

i just found this site its really interesting it shows you how everything works, gear ratio's, auto and manual trannys, ect

http://auto.howstuffworks.com/car-tr...ms-channel.htm

 

this might also help you out a good bit with your tire size and what gears you might wanna go with.

http://www.4lo.com/calc/geartable.htm

 

Thanks for the links, good info.
Does somebody w/slightly oversized tires like mine have the 3.92 gears and if so did it affect you mpg? Do you really notice a difference in take-off power?

 

The best way to think of axle gearing is like the "gearing" on a bicycle.

It's true that, to a point, you may increase acceleration with a numerically-higher gear ratio (like 3.92 vs. 3.55 for example). But there's also a point of diminishing returns. Consider first gear on your bicycle. You can REALLY acclerate fast with that gear. But in no time, you're pedaling much faster than you can manage, and you've gotta shift gears already. You have a "powerband" where you're most effective. If you're pedaling too slow, it's hard to turn over the crank. But if you're pedaling too fast, you're winded and you can't keep up.

Engines are the same way. They don't make a whole lot of power at idle. And they (usually) don't make their peak power at redline either. It's somewhere in the middle where the powerband is thick and juicy. If your gearing is too short (numerically-high), you might be able to accelerate quickly in any given gear, but before you know it, you're at the "top" of that gear and you have to shift already. The engine gets into, and more importantly, out of, its powerband too quickly. You spend all your time shifting. But by the same token, if your gear is too tall (numerically-low), accleration can suffer because it takes some time for the engine to getup into its powerband. Imagine starting off with your bicycle in 4th or 5th gear. You'll eventually get pedaling pretty good, but it'll take you a minute to get there because the gearing is so tall.

Transmission gearing plays into this as well, and it can get complex. In general, short gearing (numerically-higher) gets you faster acceleration and worse fuel economy, but that's just a very vague generality. And in general, taller gearing will get you better fuel economy, but not always.

You asked about changing the gears in the axles (remember, you'll have to do front AND rear for a 4x4) to get back to a 3.55 equivalency. For that, we'd need to know the stock tire sizes. Assuming the 16" wheels are stock on your truck, I looked up a 2002 Quad Cap 4x4 (for example), and it showed 265/70R16 tires as the stock size. Is that your stock size? That's very, very close to what you have now, and I wouldn't at ALL consider the expense of changing gears. The difference is only about 3-4%. Which means you'd need a gear ratio between 3.66 and 3.69. I doubt they even make that particular ratio for our trucks, and it wouldn't be worth it anyway.

Now, if your truck originally had 235/75R15 tires, those were a good bit shorter than what you have now (close to 10%). In that case, you'd need a gear ratio close to 3.91...and that does exist (some trucks had 3.92 gear ratios). If this is was your stock tire size, you'd have to determine for yourself whether the expense is worth it. To me, it would not be...it'd probably be close to 1000 bucks for front and rear.

Edit: mine has 3.55 gears, but the current tires (32" BFGs) are about 15% taller than the stock tires. So my effective gear ratio is about 3.01 or so. It still has good acceleration (it'll spin the tirespretty easily), but I doubt that fuel economy has improved,largely because of the huge rotating mass (large tires). If I wanted to correct, it'd be a 4.10 gear ratio, but I'm not all that concerned about it, and I'donly have one axle to fix (2WD). I do reckon that my fuel economy would suffer if I changed to a 4.10, but I'm not sure by how much. I like how the truck drives now, so I figure I'll leave it.