Modified engine crewcab gas mileage?

[Nick88]

People choose not to believe it because it isn't true. You don't get better MPG with all of that added, in fact you usually get worst MPG. You might CALCULATE a higher MPG because you didn't account for how wrong your odometer is now that you changed your gear ratio and tire size though. I can believe 20mpg for a very well designed and tuned SBC in a blazer sized vehicle. But that would need to be absolute best mileage seen and with an OD equipped vehicle on mostly flat ground at slower highway speed. 17MPG highway is pretty much what those get for real life highway mileage.

I'm not sure how in any reality running at lower rpm, not having a restrictive y pipe into a bead cat, and running better synthetic oils among other things that have been proven to increase mpg would make mpg worse, but ok? And I calculate mpg by going on a known distance trip from a station then going back a known distance then dividing miles by gallons used, even from the factory in sure these odometers weren't perfectly accurate so I don't use it for testing ever. And again 17mpg is the rated standard on a motor choking on emission standards like the bead cat and y pipe mentioned, it's not hard to believe at all better mpg is possible especially when guys spend time and money trying to improve their economy, again I don't believe outlandish claims but thinking a factory rating with technology from 40-50 years ago can't be improved at all is a bit silly.

 

[HotWheelsBurban]

What size tires did you have?

The skinny 16", load range E. 235/75-16? I called em pizza cutters. Once Dad and I got the front end fixed(new tie rods and sleeves, new lower ball joints, new pitman and idler arms) it drove a lot better.

 

[Nick88]

The skinny 16", load range E. 235/75-16? I called em pizza cutters. Once Dad and I got the front end fixed(new tie rods and sleeves, new lower ball joints, new pitman and idler arms) it drove a lot better.

I can imagine with those tires and that rear that truck must have been screaming haha. I replaced the balljoints recently on my blazer and man it drove like a new car, steering us very tight now. Probably an inch of play in either direction before the truck moves and still on the original steering box, truck used to drive like a boat being pushed by the waves.

 

[Grit dog]

It's a blazer, gets 20 pretty easily on the highway if I'm not really in the pedal.

It’s your story. All good. I’d drive that and leave the big truck home if the Blazer is defying laws of physics in the gas milage department.
I can get 20mpg on the highway, easy on the pedal and speed and relatively flat land with my 10speed F150 5.0 and my previous 10speed 5.3 Silverado.
But having driven pretty much every model of light truck from current offerings back to new 80s models in each of the big 3 offerings, those 2 trucks are pretty much the only ones that can post an honest 20mpg in a gasser truck. 6 and 8 speed newer pickups are close.
Edit, talking gassers. 20mpg with the right diesel setup is achievable. And 30mpg out of the baby Dmax engine is actually achievable and about avg.
Gas engines still get about a 30% mpg penalty, apples to apples every time.

 

[Grit dog]

For mention of overdrive, at some point down the road I'll probably end up installing the gear vendors 4x4 overdrive kit on the truck. That way I will have overdrive but keep my th400 that is gonna be very built.

Gear vendors?? Ok

 

[Grit dog]

Truck has a lift already that I would like to keep so smaller tires aren't really an option, smallest 33s but they even look a bit silly on it at the moment. With 3.42s I'd be 400 rpm lower on the highway but I feel like I'd be sacrificing a lot of pickup around town with big tires and that gearing.

You said you were doing “alot of highway”.
I own a lifted 4.10 truck on 35s. Highway driving is basically torture.
3krpms at 70mph. I suppose it would be better if it didn’t have dual 40s barking out the back.
But regardless, you’re supposedly building a pretty stout engine.
Doesn’t matter if it has 400hp, on 35s it’s not going to be “fast”.
Remember the old adage. Cheap fast or good….pick two.
Either you get some more zip around town or you get reasonable highway rpm’s. Can’t have both with a 3 speed trans.
Pick one. Doesn’t matter much for mpgs imo. Lifted CC 1 ton on 35s with that engine is a recipe for 10mpg avg downhill kinda regardless of gears.
10mpg is a pipe dream in my K20 with a good running stock Gen VI 454. Your engine will be drinking a similar amount of fuel.
Just my 2c fwiw.

 

[legopnuematic]

I would think a 4l80e would be a lot cheaper, even gone through with good parts than building a TH400, adding a lockup converter, then adding a gear vendors to achieve basically the same end result. Except you can’t do the gear splitting with a 4l80e like you can a GV.

If you are intending on an overdrive of any kind, that should be considered and factored into your axle ratios.

Just thinking out loud here.

 

[Nick88]

You said you were doing “alot of highway”.
I own a lifted 4.10 truck on 35s. Highway driving is basically torture.
3krpms at 70mph. I suppose it would be better if it didn’t have dual 40s barking out the back.
But regardless, you’re supposedly building a pretty stout engine.
Doesn’t matter if it has 400hp, on 35s it’s not going to be “fast”.
Remember the old adage. Cheap fast or good….pick two.
Either you get some more zip around town or you get reasonable highway rpm’s. Can’t have both with a 3 speed trans.
Pick one. Doesn’t matter much for mpgs imo. Lifted CC 1 ton on 35s with that engine is a recipe for 10mpg avg downhill kinda regardless of gears.
10mpg is a pipe dream in my K20 with a good running stock Gen VI 454. Your engine will be drinking a similar amount of fuel.
Just my 2c fwiw.

Yeah the gearing I know will murder me, I'm gonna be on the highway I'll probably cruise 55 in the right lane, much above that and I think it will just suck, also why I'm trying to figure a good exhaust for the truck, the blazer has duals plumbed into a dual in/out 17 inch 2 chamber muffler but the truck is basically silent, sounds about the same as a stock truck, which I would prefer louder on the gmc but maybe not 40s lol. As for it being fast I know it won't be, and honestly I don't care for it being fast, my goal isn't speed but rather making it move under its own weight pretty easily, the blazer is mostly stock and cruises like a brand new truck but out of stops it's a bit of a dog. As for the 3 speed trans, I know it's gonna suck hence why my other post saying at some point I will get gear vendors OD. My whole thing is not to have both good pep in town and reasonable highway rpm at the moment (before od setup down the road) but rather right now I'm tryna lessen the hurt from the highway rpms, which is why I'm putting in a lockup converter and what not. Once it's done I'll report back mileage of what I'm getting, I'm thinking city will be bad, but I think on the highway if i stay off it and cruise, I don't see any reason why the truck can't get ok mileage (obviously as long as I keep rpm low and aren't tryna be in the left lane the whole time)

 

[Nick88]

I would think a 4l80e would be a lot cheaper, even gone through with good parts than building a TH400, adding a lockup converter, then adding a gear vendors to achieve basically the same end result. Except you can’t do the gear splitting with a 4l80e like you can a GV.

If you are intending on an overdrive of any kind, that should be considered and factored into your axle ratios.

Just thinking out loud here.

The whole way I see it is to have a 4l80, modify everything to make it work, put in a trans controller, build it to the same strength my th400 will be, get a new converter, and everything just for OD, I can save up and spend a little bit more on a gear vendors unit i can bolt up and then my 3 speed behaves closer to a 6 speed because of gear splitting.

 

[Nick88]

Gear vendors?? Ok

What's wrong with gear vendors?

 

[TotalyHucked]

I definitely would've gone the 4L80E route, that's really the better answer here. Cost to rebuild/upgrade is roughly the same as a TH400 (an 80e is a TH400 with an OD, same strength), converter's the same cost, controller is $500. A Gear Vendors is like $3500+ by the time you're done. It'll drive much better, have better pep and has slightly more OD than a GV. You'd probably gain ~2mpg on the hwy.

But you're clearly hell bent on the TH400 route. You're still gonna get 10. Grit Dog is right, it's gonna get 10ish, plain and simple. At the end of the day, it's still a carb'd, hot 383 in a heavy ass 1-ton 4x4 truck. Unless you put a diesel in it, it's gonna be 8-12 no matter what setup you have. It's a moot point.

 

[Nick88]

I definitely would've gone the 4L80E route, that's really the better answer here. Cost to rebuild/upgrade is roughly the same as a TH400 (an 80e is a TH400 with an OD, same strength), converter's the same cost, controller is $500. A Gear Vendors is like $3500+ by the time you're done. It'll drive much better, have better pep and has slightly more OD than a GV. You'd probably gain ~2mpg on the hwy.

But you're clearly hell bent on the TH400 route. You're still gonna get 10. Grit Dog is right, it's gonna get 10ish, plain and simple. At the end of the day, it's still a carb'd, hot 383 in a heavy ass 1-ton 4x4 truck. Unless you put a diesel in it, it's gonna be 8-12 no matter what setup you have. It's a moot point.

Main reason for no 4l80 is because I'm just rebuilding the th400 with a kit to build it up for a few hundred, I'm in a time crunch on the truck so doing a full swap isn't really an option at the moment. I'm not hell bent on the th400 route and honestly I'd much rather have a 4l80 from the beginning but it's not an option now. Here's the way I see it though: a reman complete 4l80 with converter is gonna be a bit over 2k+ before tax and shipping for a 4x4, after that I need to still get an adapter for the transfer case (unless the th400 adapter is interchangeable, I'm not sure) and that would be a few hundred, plus the controller, and at that point after everything I'd probably be around 3k for a setup anyway, so why not have a gear vendors set up with then basically 6 speeds and much better pickup because I'm sitting in the power band longer. I see where your coming from I just can't really go that route right now and by the time I can it just makes more sense to get more gears with gear vendors. 10 I'm fine with, I was aiming for 12-14 which is why I chose the cam, tc, quadrajet and stuff but if it's 10 oh well. Either way just for experimenting, when the truck is on the road I'll post what I'm getting city/highway for anyone in the future.

 

[Bextreme04]

Main reason for no 4l80 is because I'm just rebuilding the th400 with a kit to build it up for a few hundred, I'm in a time crunch on the truck so doing a full swap isn't really an option at the moment. I'm not hell bent on the th400 route and honestly I'd much rather have a 4l80 from the beginning but it's not an option now. Here's the way I see it though: a reman complete 4l80 with converter is gonna be a bit over 2k+ before tax and shipping for a 4x4, after that I need to still get an adapter for the transfer case (unless the th400 adapter is interchangeable, I'm not sure) and that would be a few hundred, plus the controller, and at that point after everything I'd probably be around 3k for a setup anyway, so why not have a gear vendors set up with then basically 6 speeds and much better pickup because I'm sitting in the power band longer. I see where your coming from I just can't really go that route right now and by the time I can it just makes more sense to get more gears with gear vendors. 10 I'm fine with, I was aiming for 12-14 which is why I chose the cam, tc, quadrajet and stuff but if it's 10 oh well. Either way just for experimenting, when the truck is on the road I'll post what I'm getting city/highway for anyone in the future.

4l80 will just bolt up to the T-case if the T-case is already behind an '85-later TH400. They use the same round bolt pattern after 85. You will want to get a 4wd 4L80 and get the adapter that was on the 4l80 at the same time. Honestly, most people just get a decent junkyard 4L80 that doesn't have burnt fluid for a few hundred bucks and send it. There are guys running straight from the junkyard 4L80's behind turbo LS's for years with no issues. I decided to rebuild my donor because it had 288,000 miles on it behind a Gen VI 454.

It cost me ~$750 for ALL new frictions, plates, bushings, bearings, pistons, sonnax no walk bushing, sonnax lube regulated pressure valve, Transgo TCC and AFL valve kit, internally dual fed the direct drum, and reman torque converter. $10 for a new 2wd output shaft because my NP205 is a figure 8, and $20 for a rear drum reluctor gear to drive the OSS sensor. I also needed a $250 3/4" adapter plate from Northwest fab because I needed to use my original figure 8 adapter. You shouldn't need to do any of that adapting because yours is newer.

The biggest issue is going to be controlling it. You can do a full manual conversion for next to nothing electronically. You can actually buy a harness/box for under $50 to do that, or just build one yourself. There are various instructions on how to do that online. There are also various electronic controller out there that can do the controlling for ~$500-1000 dollars and are pretty good. Best solution would be a GM controller and wiring harness from an LS engine with a few required inputs. Namely TPS and RPM. It will behave even better if you can give it a 4wd lo signal and MAF signal, but that is harder with a carb setup.

 

[TotalyHucked]

I definitely get the time crunch aspect of it. Not trying to rain on your parade or anything, just discussion. Maybe you will get lucky and see closer to the 12-14 end of things keeping speeds down. Sounds like it's gonna be a badass truck either way

 

[Nick88]

4l80 will just bolt up to the T-case if the T-case is already behind an '85-later TH400. They use the same round bolt pattern after 85. You will want to get a 4wd 4L80 and get the adapter that was on the 4l80 at the same time. Honestly, most people just get a decent junkyard 4L80 that doesn't have burnt fluid for a few hundred bucks and send it. There are guys running straight from the junkyard 4L80's behind turbo LS's for years with no issues. I decided to rebuild my donor because it had 288,000 miles on it behind a Gen VI 454.

It cost me ~$750 for ALL new frictions, plates, bushings, bearings, pistons, sonnax no walk bushing, sonnax lube regulated pressure valve, Transgo TCC and AFL valve kit, internally dual fed the direct drum, and reman torque converter. $10 for a new 2wd output shaft because my NP205 is a figure 8, and $20 for a rear drum reluctor gear to drive the OSS sensor. I also needed a $250 3/4" adapter plate from Northwest fab because I needed to use my original figure 8 adapter. You shouldn't need to do any of that adapting because yours is newer.

The biggest issue is going to be controlling it. You can do a full manual conversion for next to nothing electronically. You can actually buy a harness/box for under $50 to do that, or just build one yourself. There are various instructions on how to do that online. There are also various electronic controller out there that can do the controlling for ~$500-1000 dollars and are pretty good. Best solution would be a GM controller and wiring harness from an LS engine with a few required inputs. Namely TPS and RPM. It will behave even better if you can give it a 4wd lo signal and MAF signal, but that is harder with a carb setup.

Thank you for those numbers, I'll definitely look into it more, either way 4l80 or gear vendors it's not happening for a bit after the dent the motor made in my pocket. I appreciate the numbers so if i go that route I know what I'm looking at after I find a transmission, main thing for me would be just finding a junkyard 4l80 near me, not too many yards that have anything like that near me, rebuilding one wouldn't be a problem at all, I've rebuilt plenty of transmissions, it's just about finding one haha.