mileage and gear change

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I am seriously considering swapping my 4. 10's out for 4. 56's. After installing 34. 5 inch tires (with no lift, and thus minimal aerodynamic change) I am getting 15 mpg highway at 65 VS about 19/20 before the tire change, before at 65 my rpm was 1960, after the tire swap 65 mph = 1790. the 4. 56 swap would put me almost exactly where I was with stock tires 65 would equal 1990. 65-70 is my cruising speed with the big non speed rated tires, I don't need to be going 80+. just my opinion but if I increase the rpm's a little I may regain a couple of mpg's and yes i do take in consideration the taller tires when making my mpg calculations. this would also bring back a little power, I lost 1 second on my 0-60 times, and the lower gears would help towing, since I'm not driving close to the speed of light like HVAC #ad
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I don't think my mileage will decrease after the gear change. wht do you guys think? has anyone gone this low in gearing a cummins ram? if so tell me the results.

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BLACK BEAST: 2K1 QC, SWB, Auto, 4X4, 4. 10 LSD, SLT, Black, Camel leather, Tow pckg, nv241HD, cab lights, 2 inch daystar coil spacers, 35 inch Mickey thompson claws, bushwacker flares, rhino lined bed and rocker panels.

soon to come; DD stage 1's, boost module, possibly van aaken.

GREEN MACHINE: 93 Jeep Wrangler 4. 0L, T-18, 38X11 Boggers, Dana 44's, AtlasII T-case, ARB's, warn and moser shafts, Ramsey 5K winch... . NEEDS A DIESEL!!!!!!

 

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I can't help much with the gearing swap, but as far as the milage goes... I'ld kill to get that. I average around 13. 5 in my truck and that is without towing or stop and go traffic.

If I were you I would leave it as is and enjoy.

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2000 Ram 2500 QC 4x4 SLT with Cummins and 5 spd transmisssion.
Stock so far but considering bedliner and louvered gate... then?

 

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That sounds like my story. I've got 3. 54 gears. I put on 285-75R16's and my mileage hovers around 15-16.

However, I don't think lowering your gear ratio will give you back any mileage. At least that's not the case for me. It's easy enough to check out. Run a tank with O/D off at the rpms you think you want to run. Then run a tank with the O/D on at the same speed that you ran before. See which tank gets the best mileage.

I found that my truck gets the best mileage around 1600-1700 rpm. Your mileage might be being eaten up with extra rolling resistance (wider tread / heavier tire / more road contact / more flexible side-walls) -- I think that's what happened to me. You could play around with the air pressure and see if that makes a difference. I can get 2-3mpg better by adding air pressure BUT it causes my tread to wear unevenly so be careful.

Right now I'm considering 315's and an overdrive.

Good luck and keep us posted.

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1999 24V, NV4500HD, QC, 4x4, 3. 54, 8' Bed w/ Rhino Lining, 285/75R16s on 16x8 Eagles, Glasstite high topper

[This message has been edited by Kyle (edited 06-20-2000). ]

 

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Turbo-D: Them are some big shoes you're running. With 34. 5" tires and 4. 10 ratio you are at almost the exact same effective ratio as a truck with 245/75-16's and 3. 54's. With the tall overdrive ratio of the automatic I think the 4. 56's would help your driving performance (0 to 60 times and towing) while still having reasonable RPMs at the cruise speeds you're looking at.

I think however you might be optimistic about getting your MPG back. With the 2" coil shims and larger tire diameter the front of your truck is about 4" higher than stock. Add the wider profile of the tires and the fender flares and you've added some considerable air drag. And the Claws, probably running at a lower pressure, and with their aggressive tread are a significant increase in rolling resistance. (They must look way cool!) If you can change the ratios with gear sets only the cost should be pretty low (even with 4x4). Do it to feel good, but I wouldn't count on getting a payback from the fuel budget.

By the way, I congratulate you on realizing that you need to keep your highway speeds reasonable when running big off-road tires. They are a compromise, giving up some high speed stability for increased off-road traction. If more of the kids in their built 4x4 trucks kept that in mind we would see a lot fewer highway rollovers.




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Wait'n for my 2001 2500QC, SB, 4x4, HO 6-sp, 4. 10 LSD, SLT++

 

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I get 18. 5 - 19. 0 mpg in the summer months. I have a 99 4x4 3. 55 auto with 35x12. 50x16. 5 on 16. 5x9. 75 wheels