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Discussion Starter · #1 ·
HI, I have a 1997 k1500 diesel "s' code, it has 3.73 axle ratio, Im currently running 1800rpm @ 60mph, I get about 16mpg out of the truck, and I very rarely tow or work the truck, is it possible to switch to the 3.42 axle ratio, and what kind of benefits would i see in fuel economy, would it be worthwhile, any info would be great, thanks.
 

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It would be cheaper to try a taller tire for fewer revolutions. If the milage doesnt get better then you can change the tires back instead of having to regear again. You should gain 1-2 MPG and only cost you a couple hundred $ for the tires vs the same money for gears and the labor and misc. cost to regear. You will have to change tires sometime anyway.
 

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take time to fill out your sig, depending on your axle(size, bolt pattern) look around and see if you can find an axle with that ratio. good shopping might pay off rather than buying new gear set. larger tires is also good option if you are still running factory tire size. need to recalibrate odometer after.
 

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Discussion Starter · #4 ·
I would like to keep my tire size the same as I already have two sets of almost new winter/summer tires, I dont think the ring and pinion set for the 9.5" rear axle and the front differential are too expensive, I was just wondering if it would pay off in fuel milage to change to 3.42 gears.
 

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It would take alot of miles before you broke even for the cost of doing both axles. Like said above you may find a truck somewhere with the gears you want and just swap axles but that will cost you also. You can buy alot of gas for the cost of new gears or a truck with the gears you want.
 

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Also, you wouldn't be changing far enough to make that much difference. If you had a 4.10 or 4.56 and went to a 3.42, it would make alot of difference. If you do try a taller tire, make sure you don't go wider or the width will negate the height for fuel savings.
 

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MPG vs RPM ??

You do not automatically gain MPG by decreasing RPM I have seen it go both ways ! Test with tires first ( borrow rear tires if you can ) and try it ! I have shot myself in the foot many times on this issue , I even installed a gear vendors unit once and lowered my rpm at the expense of 3-4 mpg less empty ! good luck t
 

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HI, I have a 1997 k1500 diesel "s' code, it has 3.73 axle ratio, I'm currently running 1800rpm @ 60mph, I get about 16mpg out of the truck, and I very rarely tow or work the truck, is it possible to switch to the 3.42 axle ratio, and what kind of benefits would i see in fuel economy, would it be worthwhile, any info would be great, thanks.
How many miles per year do you drive?

Assuming everything else being the same, changing from 3.73 to 3.42 gears will only drop 150 RPM from 1800 at 60 mph.

Since the speed is still the same (60 MPH) and since wind drag is one of the biggest forces the truck has to overcome, I would expect no more that 0.6 mpg fuel economy improvement.

Assuming a cost of $1,000 to regear (front & rear), with fuel at $3.60/gal it would take you 122,963 miles to break even. :(

Even if you were to some how manage a 1.0 mpg improvement (which I highly doubt) it would still take you over 75,500 miles to break even.

Tall skinny tires would be my choice.
 

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I think you might have more mileage luck with a custom tune. Get the timing advanced a bit. That truck should easily do 20 mpg with the gears you have, unless you are going way fast. These guys do it pretty reasonably. www.purrformancediesel.com
 
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