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Thread: The right gearing?

  1. #1
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    Default The right gearing?

    I drive a 1983 C20 6.2 700r4 OD with gear 4.10 ratio. Where we live we have not any hills and normally we dont do much towing. But we do have an 8.5 ft camper we love and we can be out for long trips down in Europe with up to 2000 miles and yes, when we are abroad there can, of course, be some hilly traveling. The truck runs lovely and not noisy at all. I have done thoroughly sound insulation of the cab and the 2,5 inch dual exhaust with X-cross just behind the transmission makes a very good job, so noise is not an issue at all. I'm just thinking of fuel consumption when I evaluate to change gear to 3.73 ratio. Anyone, that has advice on this? Is it a good idea or shall I keep the 4.10? Happy for all tips.

  2. #2
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    You may or may not see an improvement in economy. If you do, it won't be much, and quite possibly a loss. There's only so much you can do with 130 HP, so the optimal RPM range is critical to economy and maintaining a desired speed. Increasing the drive ratio will likely require a lot less time/distance in OD, and/or deeper in the skinny pedal (more fuel) longer, especially with the camper on. With a headwind or a modest grade, you won't likely be using OD at all. Also consider, a higher load at a lower RPM often results in more engine heat to manage, which can be marginal in many situations. More fuel at a lower RPM also means less RPMs to manage heat (slower water pump and fan). None of this takes into consideration the cost and inconvenience of actually doing the gear change. Ultimately, we may not live long enough to recover the loss, if it is possible at all. To get some idea of what you may be seeing as a result, upsize the tire height about 10% as a test. Borrow a set, if you can. I think it won't take long to find your conclusion.

    As an example, I had to lower the drive ratio of my Blazer to recover lost power and economy. After switching from 31" tires to 33", power disappeared and economy dropped by almost half. This was with 3.73 gears. Changing to 4.10 gears recovered almost all of the power and most of the fuel economy I enjoyed before.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  3. #3
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    Thank you for your useful thoughts. Much appreciated.

  4. #4
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    To DmaxMaverick
    One more question. Before I finally drop the idea to change to a taller gear I should appreciate your comment to the statements I read in the TDP book "Troubleshooting and repair guide" (which I value very high) chapter "Low Fuel Economy". On low left page 25 one can read "...if you are mostly interested in the best highway fuel economy and high-speed interstate cruising, buy a truck with taller gears. You might not be happy with 4.10 gears otherwise." It was actually after I read that chapter I started to think about a change. But I must say that after reading what you said about having tall gear for 130 hp, I can easily see a risk to end up in situations where I have to switch to a lower gear to keep speed while driving in strong counter wind or uphill.

  5. #5
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    Too much depends on the vehicle in question, and the application. You're already starting with a heavy truck, and an occasional heavy load. Simply stated, it is not going to compete with smaller/lighter or more aerodynamic vehicles in regards to fuel economy, and changes you make are less likely to benefit in any significant amount. It has an OD transmission, which is more than most of that weight rating, which allows you to choose to use or not. 3rd gear is direct (1:1 ratio), which is the highest gear for non-OD transmissions. Fortunately, the 700R4 (all other arguments about it aside) has a very low first gear, well matched higher gears, and a locking torque converter. If you haven't already, look into manual torque converter control. Getting it locked up earlier, and keeping it locked (such as during coasting, compression braking), helps with slowing, transmission longevity, and transmission heat generation. Heat generation is power that didn't go to the wheels. Also, the 700R4 is a rather weak transmission for heavy duty work, so greater loading at lower RPM's is not helping it.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  6. #6
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    That 8-1/2 foot camper is factor in determining gearing options.

    My 1982 GMC 6.2/TH700R4 had 3.42 gearing, and I towed a 14' camper occasionally with that truck. The camper was a parachute, and any headwind made towing a chore. The Banks turbocharger I eventually installed solved the problem, but...

    Without a turbo, the 3.42 gears made interstate driving pleasant, but towing that camper into any sort of headwind made it unpleasant. 4.10s would have been a better choice for that amount of wind drag and no turbo... if I'd towed more often than I did.

    While not towing, my truck with 3.42 gearing and with or without a turbo would deliver 24-25 mpg at 65-mph. The engine noise almost completely disappeared at 65-mph. Was great. Question that only you can answer is... is a few more mpg and less noise worth the cost of a re-gear?

    Jim

  7. #7
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    Thank you both- I decided to stick to my 4.10. I'm grateful for all the good advice this page gives. Keep on truckin.
    Attached Images Attached Images

  8. #8
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    That's a nice brochure picture. Now, post a pic of your 38 year old, run down, used up old truck. (I'm kidding, and envious).
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  9. #9
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    I have owned the truck for 16 years now a rebuilt it twice. Second time two years ago and that time it was a total frame off restoration. I love the truck and so do Ella o (on the previous picture). She is 15 years now so the truck has been here during her entire life. I bring some pictures from the project.
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  10. #10
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    I had an 81 C20 that I converted to a 6.2L diesel turbo with a 700R4, 4.10 gears, and 31" tires. I felt it was the perfect combination for power and economy. Like mentioned above, the 700R4 has a steep overdrive, so it makes the 4.10 gears happy on the highway but still leaves you with the proper gearing for towing. I think sticking with your current combo is a good idea.

    Casey
    1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter

  11. #11
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    I have really been convinced that this is the right way. One factor that makes me thinking in that direction was the reminder I get that the 130 HP I got isn't that much actually. And I have already noticed that as soon it's a bit hilly and the camper is on, OD isn't to think of. So yes, with the current combination I can see I can have the best of two worlds.

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