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Differential & Driveline Front and Rear Axles, Lockers, Limited Slip, Yokes, etc

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Old 03-12-2017, 05:25 PM   #16
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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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Montani Semper Liberi
77 SWB "ONE-TON K10" - 6" Front Superlift Springs/Rear ORD Shackle Flip 38" TSLs on 15x10s
Welded 14-bolt FF w/Disk Brakes - D60 Front w/ORD X-over Steering, HD Shackles/Grease Bolts
TH350 dual-feed HD trans & NP205 tcase with 32-spline front output mod
79 SWB K10 "Dad's Truck" - Pretty much stock except front push bar, new sliding rear window and bedliner
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Old 03-20-2017, 07:38 AM   #17
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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.
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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Old 03-20-2017, 04:51 PM   #18
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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.

Mike B
1982 C-10 SWB, basic Custom Deluxe with original paint
355 w/ RamJet roller cam, LS6 valve springs & Vortec heads

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Old 03-24-2017, 08:48 PM   #19
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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.
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