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Old 03-24-2015, 03:58 AM   #1 (permalink)
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Fuel economy of 7.4L (454) powered 3/4 ton suburban

In addition to the little 7.4L engine it has a 3 speed automatic transmission and 4.10 fuel economy gears.
What, 4.10 gears are great for economy compared to optional 5.13 gears.
I don't know what it gets for gas milage but I am going to find out.
I have been told anything from 10 highway to 8 city to half that.
Its got 140,000 miles on it, that means its burned at least 14,000 gallons of gas. Soon it wont be burning any gas.

Well the bad news is the numerically lowest gear ratio I can locate for the full float GM 14 bolt 10.5 inch axle are 3.42:1 gears. The same gears my half ton suburbans 8.5 inch 10 bolt semi float rear has.
I wanted to pick up some 3.23s, but that's not happening.
Edit: It is happening. NitroGear makes 3.21 gears, the numerically lowest ratio known to have ever been produced for 14 bolt full float. But they are $600. NitroGear is the only producer for 3.21 gears, they are aware of this and have priced this gear set accordingly.

The 454 and 3 speed are only going to be in there till I can pull my 4 year old diesel engine out of my rusted out half ton and repair, update and improve its 700R4 transmission and move it all over. The current 700R4 shattered its torque converter lockup, a 27 spline lockup converter on an obsolete 27 spline input shaft and I cant even find a replacement lock up torque converter because everyone has gone to 30 spline which GM started using in 1985.
Edit: I have found some 27 spline options but they are limited to say the least.
I have almost infinite options available for 30 spline torque converters, (almost) nothing for 27 spline. But swapping to a 30 spline input shaft and drum will fix that.

This 3/4 ton suburban has a few important improvements over the half ton. Mainly the 3/4 ton frame, full float heavy duty axle, bigger brakes, class 4 hitch, working trailer brake controller, 40 gallon road warrior fuel tank and its not rusted out.

Its running really painfully rich, I have an edelbrock performer carb on the way and I am going to take the wide band O2 sensor meter out of my Camaro and dust off my advanced edelbrock carburetor tuning kit and get that thing running the correct A/F ratio.

I filled up the 40 gallon fuel tank from about 1/2 tank, took 20 gallons, so it looks like the gauge reads correctly.
Then I am going to weigh it tomorrow and drive it around for a few days and see what it gets for MPG, I am sure the as is FE will be disastrous. I am not sure what day the new carb is getting here let alone when I will be able to put it on and tune it.

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Last edited by oil pan 4; 05-28-2015 at 02:36 AM..
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Old 03-24-2015, 05:28 AM   #2 (permalink)
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This should be interesting!

My brother wound up with a fast C30 Crewcab dually several years back through an interesting series of events. The owner blew up the original 454 in the truck around the same time his 396 powered drag car was wrecked so he plopped the built 396 into the truck. Then he traded the truck for my brother's barely broken in 4-wheeler.

That thing was a beast! It would light up all 4 rear tires! It never broke into the double digits on fuel economy, though. It was a wallet killer at '90's gas prices!
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Old 03-24-2015, 07:54 AM   #3 (permalink)
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Oil Pan,

What a project! What year of suburban, and 2wd/4wd?

In the short term, some 235/85R16 tires would provide some good “rubber gearing” to go along with your 3.42. The only other option I know would be an axle swap for a D60/D61 rear axle with a 3.07:1, but that may be more work than you want to want to for just a 11% reduction.

For the long term solution, are you set on keeping the automatic transmission? I had headache after headache on my 4l60 (which is basically just a 700R4) behind my 350 in my 1991 Chevy K2500, until I finally swapped it over to a NV4500 stick shift. That was without a doubt the single best fuel saving mod I have ever done on a vehicle.

If you want to keep it an auto, you could always try to find an Allison 1000 from a 2001-2007 Chevy 2500HD/3500 with either the 8.1L Vortec or 6.6 Duramax. Those are 5 or 6 speed (6 speed= double overdrive) depending on the year and built like a tank. I would think you could get one out of a wrecked 8.1 gasser reasonably cheap. They make stand alone TCMs to control them, as they are popular for cummins swaps.

I have dreams of a future ¾ ton diesel suburban project, so I will be watching with interest. Oh, and we want pictures!
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Old 03-24-2015, 08:00 AM   #4 (permalink)
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Old 03-24-2015, 11:17 AM   #5 (permalink)
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200-4R converters work for 27-spline 700R-4s, even RockAuto.com sells reman Dacco converters for the 200-4R, use an '86 Monte Carlo SS version for a high-ish stall, pick some version that bolted to the disastrous Olds 5.7L diesel for a low stall.
3.21:1 gears exist, but can't be had new anymore, for the 10.5" axle. Cheaper to swap a D70 with 3.07:1 from an '89 Cummins, or a 3.08:1 Ford 10.25", both are full-floaters.
The 454 is a lame 7.9:1, but swapping the '96-'00 heads bumps that up to 9.0:1, which is good for those at higher elevation.
I suggest a B&M TorkMaster 2000 converter for the TH400, it will reduce slip at cruise from 200 RPM down to 50 RPM.
Weiand makes an intake manifold specifically for the "peanut-port" heads, but if you want to keep the EGR then Edelbrock is still the only choice.
The absolute best MPG cam for a 454 is Comp's HighEnergy 252. Hedman makes the cheapest headers, but the Doug Thorley's are legal.
For tires, BFG makes a LT255/85R16E you might consider.
Be sure you have the GM 5" airdam that matches your front bumper, then lower the 'burb as far as practical.
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Old 03-24-2015, 02:26 PM   #6 (permalink)
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Although my big hauler is a completely different make/ model etc, it's still a heavy low mpg beast nonetheless. I don't have the $$ to do much as this time for mpg improvements but I will be watching your thread for inspiration

And cosmick, I see you are UAW. My father was a member for almost 40 years.
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Old 03-24-2015, 09:50 PM   #7 (permalink)
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Really interested what you can eek out of the 454 before doing the diesel swap. If you can pull down 12 or 13 on the highway that would be a major victory in my eyes...what with the weight, gearing, and lack of OD.

What year is the "new" suburban?
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Old 03-24-2015, 10:12 PM   #8 (permalink)
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Had a 02 1/2 ton 4x4, got 18ish on long HWY trips with a 5.3, 15 on E10 was normal, 12.5 on E85. With E85 prices I'd tune the Elebrock to run that. almost $1 cheaper a gallon around here, well $0.80 a gallon cheaper. is close to $1. I have rods and seat kit in the garage left over from putting a 600cfm on a 350V8 S-10 if you need them doubt I'll ever use them, cover shipping and it's yours.
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Old 03-25-2015, 01:27 AM   #9 (permalink)
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3:42s in a 3/4 ton Suburban with a big block / diesel? Good luck with the 700R4. Those transmissions aren't bad, but with that much weight, that much torque, and that little gear, whatever trans you put in it is going to have to be well built and well cooled.
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Old 03-25-2015, 11:15 AM   #10 (permalink)
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a 6.2L diesel only makes a truly lame 240 ft/lbs TQ, no more than an equally lame 305 gasser. The 6.2 feels torquier to drive than the 305, but that's only because the torque comes on quicker and stronger off idle.
A 700R-4 does need to be "built" to survive a 6.2, but even stock 305s break the best of stock 700R-4s, however, the 700R-4 isn't inherently weak, most of the guts are as strong as TH400 guts. The aftermarket has addressed the rest.
Cruise RPM for a 700R-4, 3.07:1 gears, and 33" tires is fine for a 454, the 6.2 probably can't pull it, however.
If you want MPG with a 454, then swap it into an '84-'90 Trans Am. The math says 25 MPG is possible. If I had money I'd do just that, first trying to pass emissions, then trying for 30 MPG.
The Viper V10 started as 488 cubic inches, they gave it a 0.50:1 sixth with a 3.07:1 rear and 26.2" tires. Only a bit of researching uncovered the '87 GM G-body was able to pull similar RPM with similar drag from only 262 cubes. It was a 200-4R with 0.673913:1 OD, a 2.2857:1 rear, and 25.5" tires. So if a 262 can pull that, a 454 should pull much less RPM. Probably need a divorced NP205 to get enough launch ratio with an even taller rear gear, even if swapping a 2.97:1 first, a la Mustang Cobra, in place of the Viper's 2.66:1.
This may all seem a bit off topic, but I think it worth considering what's possible, drag permitting. Or lack thereof.

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4.7l, 454, electric water pump, hot rod, intake

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