4.10 to 4.56 or 4.88

Discussion in 'Differential & Driveline' started by fallguy, Mar 10, 2017.

  1. crazy4offroad

    crazy4offroad Full Access Member

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    Here's a chart from Grimm Jeeper with the 700R4, NP208 (at 1:1), 35" tires, and on the left is 4.10 gears and on the right is 4.88. Should help you see what to expect, looks like closer to 2300 rpm at 70 mph. If your 4.10 rpms match the chart when it downshifts, I would go ahead and pull the trigger on 4.88s. I would still only expect 10-14 mpg, 10 towing, 14 under the best of conditions. Keep in mind driving these trucks on the highway is like trying to carry a sheet of plywood in a windstorm, definitely hits the mpg hard. Hope for a tailwind.
     

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  2. throttle out

    throttle out Full Access Member

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    I have roughly skimmed through this thread and there seams to be some conflict.
    I daily drive my 87 V10 short bed. It has 3" of lift on 33" tires, 5.3ls and built 4L60E, 12 bolt out back with 4:10's. At 70mph my truck is at 2k rpm exactly. 700R4 and 4l60E are same od ratio.
    Use this calculator, I find it much easier and straight forward to use.
    https://www.ringpinion.com/calculators/Calc_RPM.aspx
    My truck NEVER hunts for gears, and as long as I keep my rpm 2k or lower, I get 16 miles to the gallon. Now I understand the argument that I've installed a newer more efficient engine/trans combo, but if the OP's engine is making an honest 290hp at the crank, he's within 10hp of my engine. As long as the trans is healthy, he should be able to achieve close to my figures.
    If I ran 35" tires I would consider 4.56 gears.
    My speedo reads 2-3mph slow at these speeds
     

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  3. MikeB

    MikeB Full Access Member

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    First of all, make sure your ignition timing is optimal, and I don't mean just at idle. And if the truck doesn't have low restriction dual exhausts, add them.

    If that's the 290hp GM crate engine, then it's not gonna make much torque under 2500 RPM because the cam is WAY TOO BIG for 7.8:1 compression. Yes, 7.8:1 is what the base and 290 engines actually have. Don't know what GM was thinking using 76cc combustion chambers and dished pistons.

    There are a ton of smaller cams out there that will make more torque at much lower RPM, but limit revs to 4500 or so. I can tell you from personal experience that the cam below is a torque monster from 1500-3500 RPM in a low compression 350. In my 69 C10's 350 Goodwrench engine, there was a night & day difference between it and even a 204/214 Edelbrock cam. It was just what the doctor ordered for driving in the Colorado mountains with 29" tires and 3.08 axle.

    Crane series 2020 -- Cam & lifter kit P/N 114112

    Its very early intake valve closing angle makes the engine think it has 9:1 compression or more.
     
    Last edited: Mar 21, 2017
  4. shiftpro

    shiftpro Full Access Member

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    Interesting thread but many apple to oranges examples. 700r4 + 35" ....
    absolutely 4.88 if you have power. If not, 5.13s is not too wild at all.

    If a 700r4 hunts back and forth from 3rd to overdrive you will reach nuclear meltdown temps and say gbye to that transmission.

    Trust the math in the gear ratio website, it's a great tool.
     
  5. 68post

    68post Full Access Member

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    4.88 !
    My last K30 had a 350 ,4.56's, and 33's, and you could pull a steam roller in granny low & 2 High. Overdrive would've been perfect if I'd had one and cruised empty. It could carry a ton and nearly achieve 12 mpg doing it (11.7 - checked), once I ditched the pathetic Holley 600, (a 600 Edel will be no better), for an excellent 750 Q-jet.

    I didn't build it's engine , but I added headers and mandrel bent 2 1/2" exhaust with an H-pipe (already had a performer intake), and a home modified qjet - better mileage AND more power !
    You could hear the cam very slightly - so it likely was a little bigger than the smallest, but worked fantastic with the manual trans. You'll need less cam and to be very careful if you purchase a higher stall convertor - absolutely no cheap convertor will hold-up.


    $.02
     
  6. 5akman

    5akman Member

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    Upon MIkeB's recommendation I went with the Crane 2020 in my new build. 1984 K20 with rebuilt 700R4, Np208, Gm crate engine, 4.88's and 315 (35") Cepek Extreme Country tires. The truck left Chico CA last Friday with my buddy driving it with a load on the flat bed deck and towing two empty boat trailers, one on top the other. He's on his way to Alaska and passed through Haines Junction this morning. Unfortunately, the speedo cable broke 100 miles into his trip so he has nothing to go by but has said that the truck has good pulling power in the hills, loves 60 -65 mph but the tires start to drift at around 65. I have the 700r4 hooked into a vac switch next to the brake booster for lock/unlock. He said for a work truck, the 4.88's are ideal with the 35" tires. He thinks however that he's getting around 10 mpg so I think with the load on the deck and towing, the Qjet is getting into the secondaries more than I anticipated. Its 3100 miles so its going to be an expensive fuel bill!
     
  7. MikeB

    MikeB Full Access Member

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    Looking forward to hearing how it does with a "light" 2000 lb load of fish!
     
  8. 5akman

    5akman Member

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    Me too, I've got some big bills to pay after the minor 15K rebuild on this rig!
     
  9. MikeB

    MikeB Full Access Member

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    Ouch! Of course you can always call it an investment in your business. Also, that beats $50K for a new 2500HD.
     
  10. 5akman

    5akman Member

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    Many have asked why I put so much $$ into such an old truck. Working on the beach in the salt water kills both the electronics and the sheet metal so I don't want anything with a computer in it if possible. So, if I bought some old 3/4 or 1 tons in AK of the vintage prior to computers, they are all rusted out. Newer trucks have better undercoating but have all the electronics. So I've found I'm better off to buy stuff outside AK, rework them and ship them up.

    I have a few pics posted in the Gallery forum.
     
  11. shiftpro

    shiftpro Full Access Member

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    700r4, 4l60/80
    35" tires
    4.88 or .512 if you want to tow....
    200hp or 500 hp, same deal.
    Beginning, middle and end of story.
     
  12. Honky Kong jr

    Honky Kong jr Full Access Member

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    In a 10 bolt the pinion gear is rediculously small at 4.56 I could only imagine what a 4.88 or higher numerical gear would look like. Do they even make such an animal for a 10 bolt I know the D44 they do. Ok they do and the pinion gear looks to be the same size as the bearing maybe smaller......lol
     
  13. bucket

    bucket Super Moderator Staff Member

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    In theory, the pinion should gain in strength due to mechanical advantage, despite it's smaller size. At least I think that's correct, lol.
     
  14. HotRodPC

    HotRodPC Administrator Staff Member

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    Has @fallguy ever come back to update this thread recently? I don't think so. :shrug:

    For purpose of conversation, 2 big things IMO are being overlooked.

    #1, What is the purpose of this truck? I didn't see anything about it being a tow rig. I did see he's asking about 80+ mph. Which is highly possible the plan in Montana driving. But first things first, before deciding to do to fix or improve this, knowing what the most intended use of the truck should be known. So does it need 35in x 12:50 tires? Why? Just to be cool? Need for higher ground clearance? Thought of more traction? 33in x 10.50's could be PART of the answer if the 35's aren't needed. Less ground friction with 2in x 4 tires is another 8 inches not dragging the pavement. 2 inches shorter x 2 inches narrower is less tire absorbing parasitic hp loss. Need more traction? Go with the smaller tire next time to buy tires and add a lunchbox locker to the rear axle. 2 wheel at 33x10.50 is better than 1 wheel at 35x15.50.

    #2 This 290hp.. How do you figure? And is this to the flywheel or to the wheels? I have a feeling it's to the flywheel. Everyone sees the GMPP 290hp engine with Eddy Intake, Carb and Headers, so that's going to be to the flywheel. IF, and I mean IF you're actually getting that 290hp, someone has pointed out, at what RPM. What are the odds you're spinning the engine that fast when you need that 290hp? And what about the torque? Torque is the power or the lack of in this case, because really, that's what the complaint actually is. Lack of Torque !!! So if you have 290hp, consider parasitic hp loss. You're going to lose 30hp right off the bat for the 700r4. That's not even considering the loss for turning a PS and Water Pump, Alternator, and then add a transfer case to this loss too as well as pushing another axle. The big tires etc etc. Add wind drag cuz afterall, these trucks are called a Squarebody for a reason, not round or aero body, and then it's lifted higher up in the air. All factors that are to be deducted from the 290hp IF you're really getting that 290 to the flywheel which I highly doubt and even if so, it's at the wrong rpm.


    So, factor in the actual complaint, Low Torque and at what rpm is the max torque achieved? True, gear ratio is nothing more than torque multiplied to the ground. Don't forget 0 x 2.73 = 0. 0 x 4.10 = 0, 0 x 4.88 = 0. Point is, add all the gear you want, if it's little to no torque to multiply don't have many expectations and maybe gear ratio isn't the first thing to consider. First thing I always consider is how much of this FREE hp/torque that I'm throwing away, can I get back? Un-needed weight, un-needed accessories, wind drag, brakes dragging and anything else that can be checked or changed. Next, consider this gear ratio multiplication. If 4.10 isn't getting the job done, is another .78 which is less than 25% more multiplication and that is also taking away from my top end cruising so is it really worth $1000-$1300 for regearing? And to accomplish what? Going from 2.73 or 3.08 to 4.56 or 4.88 maybe so. From 4.10 to 4.56 or 4.88 I don't think so. If I was running 4.10's in 10 bolts and I wanted stronger axles anyway, and I was able to score some good used CUCV 4.56 axles ready to bolt in for $1200 or so, then sure, I'd give it a try, but I'm spending money more for the strength than the gear ratio and the little added gear is just a little bonus.

    It was already mentioned above, but in this case, where there's not a huge change for the $1000-$1300 to re gear, I think I'd be spending $1000-$1300 on heads swap to raise compression ratio, better breathing heads for more performance and a cam swap to put the power band more into an rpm range where I intend to work the truck and use it, whether that be low end, mid range or top end. By doing this, It just might improve my mpg too and by not doing a gear change I'm not giving up my top cruising speed so I can continue to cruise 80+ on the interstate. If I still wasn't 100% happy but I think I would be, but just in case I wasn't, my next change would be dropping down to 33in tires after I wore out the 35's and getting back some of that parasitic hp loss. If I needed tires anyway, it's not money spent towards the poor performance. If anything, the 33's will cost less than the 35's and I know my mpg will improve at least a little. Maybe 1/4 mpg but its still gain rather than loss.

    Whew, my hands hurt. Sometimes I should keep my opinion to myself. :shrug:
     
  15. shiftpro

    shiftpro Full Access Member

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    That was an excellent post Hotrod, thanks.
     

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