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Old 10-11-2010, 03:58 PM
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question regarding gear ratio

Ive got an 84 chevy k10 4x4 350, 700r4 4speed overdrive tranny and 208NP transfer case and i believe that my rear gear is 3.10. I just took it for a long drive up through the mountains. this was the first time the truck had actually put some miles on it since i got my 700r4 tranny rebuilt. im running 33's on it and there is no lift. on the highway its hard to maintain 70mph because my foot is practically on the floor and if i go any further it downshifts out of overdrive. what would you say the gear ratio should be? i dont really off road and the only time i use my 4wheel drive is when the weather permits it and i dont tow anything at all. im pretty much looking to not have my foot down so far on the pedal and i would like to go up hills maintaining speed at least, currently i tend to lose speed even when i floor it up hills. if i have forgotten anything i will include it in the further posts. thanks

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Old 10-11-2010, 04:16 PM
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There is no 3.10 ratio for these trucks but there is a 3.08, probably what you have. You are likely going to need something like 3.73-4.10 ratio with 33" tires. Most stock GM 1/2 ton trucks from the 80's with the 700R4 have 3.42 or 3.73 rear gears, depending on stock tire size and whether towing package is ordered or not.
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Old 10-11-2010, 05:14 PM
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so what would the higher gear ratios offer? since im not towing anything should i even have 4.10? im not too familiar with the rear end ratios so any info would be helpful
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Old 10-11-2010, 05:26 PM
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Problem you have now is the overdrive ratio and rear axle ratio combination puts your cruise rpm too low, out of the efficiency zone for the engine and forces you to bear down on the gas just to maintain speed. You're geared too low for the overdrive trans, and the larger than stock 33" tires just make it worse. With 3.08 gears and overdrive ratio of .70 it is like you have 2.15 rear gears, and the larger OD of the 33" tire compared to stock size makes that even worse, like a 1.95 ratio. No wonder it is awful at cruise speeds.

I've got a non overdrive 4-speed in my '84 Chevy 4x4 with 33" tires and it would be terrible if it didn't have 3.73 gears in it, and that is with no overdrive. With overdrive I'd bet it would need 4.10 minimum to drive well, and 4.56 to pull a trailer.
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Old 10-11-2010, 06:43 PM
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that makes complete sense thanks for that. So if i wasnt planning on ever towing (i dont have a tow package anyway) should i go for 3.73 or for the 4.10 and which do you think would be the best choice just to not waste so much gas?
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Old 10-11-2010, 06:53 PM
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gear ratio

Through buying many tires for myself and friends trucks, racing and street driving, sometimes driving to races, I would go with the 4:10 ratio because of the 33" tires and also the 700r4. 4"56 will be a bit to much for normal useage and will be a bit more costly on fuel. Good luck.
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Old 10-11-2010, 07:06 PM
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With the 700R4 and 33 inch tires I would go with the 4.10 too.
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Old 10-11-2010, 08:03 PM
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The 4:10 gear, 33" tires and the 700R-4 would put you at 2000 RPM @ 70 MPH. A good power range.

A 3:73 at same speed would tach 1800 RPM @ 70 MPH

A 4:56 at same speed would tach 2200 RPM @ 70 MPH

If you're running a smallblock I'd use the 4:10 or 4:56
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Old 10-11-2010, 08:15 PM
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so would you say that there isnt much difference between the 3.73 vs the 4.10? or is 4.10 going to be the best bet for overall performance and gas mileage. I appreciate everyone who has assisted me, thank you
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Old 10-11-2010, 10:17 PM
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Hi
I currently have an 84 4x4, v8 350 sbc short bed 33 inch tires 700r4-208 tcase running 4.10 65 mph 2200 rpms, truck originally had 3.08 and would do exactly what you described with the down shifting, sometimes down to 2nd gear at 4000 rpms or greater even up a slight incline. Your tranny will love you if you changed ratios but remember to do both frt and rear ( Im sure you already knew).!!!
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Old 10-12-2010, 09:28 AM
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thank you for your input, once i get the money i do believe that i will change my gears out.
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