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  #1  
Old 03-28-2005, 09:55 PM
Ryan90K2500
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Default Changing gear ratio

In my new quest for MPG, I wanted to know what the best rear gear ratio would be to provide the most mpg. Also, if you change your rear gear ratio and you have a 4x4, would you also have to change your front differential ratio to match? I'm confused.
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Old 03-29-2005, 07:53 AM
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Default Re: Changing gear ratio

Thats a tough question to answer without all of your specs. Generally you want to install a gearset that coincides with what you are using the truck for. The smaller gear ratio's will make your truck feel sluggish off the line, and won't allow you to tow heavy trailer equipment efficiently. When you step to a smaller number gear ratio, you leave the transmission in charge of torque multiplication which when towing can lead to an overheated and exhausted transmission.

Do this first. Decide what size tires will be on your truck for the long run. Let's say you want to stick with 33's. Now you have to determine your engines powerband. On a stock 350 TBI engine, the powerband (the RPM range where the engine produces the most power) is in the 1800 - 2800 RPM range. The lower end of the powerband is the more efficient range for just about anything. In your case 1800 - 2000 is a good range for MPG. Now you want to choose a gear ratio that will put your truck in that RPM range when you are cruising, pulling, etc...

If you cruise at 70mph down the freeway right now, how many R's does your engine turn? If its in that 1800 - 2000 range then you are at about max efficiency with the gears you currently have. If your engine revs any lower than that range at 70mph, you will be outside of the powerband and the transmission may fight to stay in overdrive or you just may not be able to keep a steady cruise speed when climbing up a grade. Any higher and you just get a bit more power at a cost of efficiency. Got it?

We can help you here if you give us more details about what you want to use your truck for. There are some great tire/gear ratio calculators on the web also that we can use to make this easier for you. Let us know a bit more and we will see what we can do.
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Old 03-29-2005, 08:38 AM
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Default Re: Changing gear ratio

You don't have to change your front ratio immediately when you change your rear.....but you CANNOT use 4 wheel drive until you do or you will ruin stuff.
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Old 03-29-2005, 11:04 AM
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Default Re: Changing gear ratio

Oh yeah, of all things I forgot to mention that you will damage your 4x4 system if you install two different gear ratio gearsets in the front and the rear. Bad idea. I know there are hardcore offroaders that do this on occasion to get more pull from the front of the vehicle than the rear, but it only works in really slick stuff.. as soon as it catches traction, things will break.

Think of it this way, your trucks transfer case does not have its own differential unit in it... which means your front wheels cannot recieve less power than the rear wheels at any time 4x4 is engaged. They get the same power front to back. So if you have 3.73's in the rear, and like 4.56's in the front, and you try to drive on somewhat dry ground the front wheels will move slower than the rear wheels causing binding. When that happens, something in the system has to give... could be a u-joint, axleshaft, ring and/or pinion, or even the transfer case could bust (it only has an aluminum case!). If you want to regear one at a time, just keep the 4x4 lever in 2HI. As long as you wait until both are regeared to use 4 wheel drive, everything will be fine.
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Old 03-29-2005, 08:23 PM
Ryan90K2500
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Default Re: Changing gear ratio

Thank you for your responses.. What specs do you need to know about my truck? I think I might have 3.73s but I am not sure. My tires are 32" and I really like them (I believe they are 32", I measured with a tape measure) I do not have a tach so I'm not sure of my rpm at highway speeds. I hope this helps, I'm currently getting 10-12mpg and gas prices are hurting.
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Old 03-30-2005, 08:42 AM
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Default Re: Changing gear ratio

Personally, 3.73's and 32's arent a bad combination. Thats a good mileage/performance ratio for those tires. You have to consider also you have a fairly large truck. My 93 K1500 got 14 mpg on the best day, and that was with ALL brand new emissions parts, tune up, and new tires! Your truck weighs a little more than my 1/2 ton did... Have you ever experienced better mileage than you currently are getting? How tight is the engine? If you can get your hands on a compression tester, check all of your cylinders for even compression. It only takes one low or dead cylinder to make your mileage go out the window. If you find that the engine is in tip top shape and tune, then you probably won't find yourself getting much better mileage out of it by changing gears, etc..
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