Modified engine crewcab gas mileage?

[Nick88]

Hey guys I'm new here, I have a 1986 GMC K3500 crewcab that i picked up recently. I tore apart the motor and everything was pretty bad so I'm rebuilding it as a 383: full eagle rotating assembly with dished pistons (gonna have around 9.3:1 compression), flotek 64 cc aluminum heads, edlebrock performer intake, a stock quadrajet, and a very mild cam 212/218 duration- around 454 lift and 110 lsa. Truck has a turbo 400 that I'm putting in a fuel miser lock up torque converter and 4.10 rear gears on what's either gonna be 33s or 35s. Does anyone have a similar set up? While type of fuel economy can I expect?? I know it definitely won't be a prius but I'm trying to salvage what I can hence the cam, converter, and quadrajet. Should I run 33s or 35s?? I figured 35s to keep rpms down on the highway since it will have a lot of highway miles, but will I get better economy with more rpm and 33s?? (Truck has 4 inch lift so I can easily fit either tire) Any help is very appreciated!

 

[legopnuematic]

Lower rpm does not always equal better fuel economy. If the engine is always lugging it’s probably going to drink more fuel than spinning a little more RPM but loading the engine less.

For 35s in a one ton 4x4 I’d think 4.56 would be nice, or 33s and 4.10s would be well suited.

Are you planning on towing or hauling with the truck? That should be considered as well.

Torque rise is not really applicable to cars and light trucks, as it is to stationary power units, heavy trucks, or tractors, but the basic idea I think can apply. If your normal cruising speed is above peak torque and you start pulling a grade, begin loosing speed (rpm’s beginning to drop) you’ll begin falling back into peak torque, thus should be able to maintain the gear and speed. Whereas if you are cruising far below peak torque, come up to the same situation you’ll fall farther below peak torque and have to downshift or lug the engine hard to maintain speed.

 

[TotalyHucked]

I have very little experience with big blocks period, let alone breathed on ones. But my buddy with his stock 2wd '86 CCLB 454/TH400/4.10s on stock 31s got 8mpg pretty much no matter what. He's since swapped on a Holley Sniper fuel injection setup and a 6L80 overdrive and sees 11mpg. For your setup, I'd guess somewhere in the 8-10mpg range. I'd strongly suggest an OD trans so you can use a deeper rear gear but get the OD drop on the hwy

 

[Nick88]

Lower rpm does not always equal better fuel economy. If the engine is always lugging it’s probably going to drink more fuel than spinning a little more RPM but loading the engine less.

For 35s in a one ton 4x4 I’d think 4.56 would be nice, or 33s and 4.10s would be well suited.

Are you planning on towing or hauling with the truck? That should be considered as well.

Torque rise is not really applicable to cars and light trucks, as it is to stationary power units, heavy trucks, or tractors, but the basic idea I think can apply. If your normal cruising speed is above peak torque and you start pulling a grade, begin loosing speed (rpm’s beginning to drop) you’ll begin falling back into peak torque, thus should be able to maintain the gear and speed. Whereas if you are cruising far below peak torque, come up to the same situation you’ll fall farther below peak torque and have to downshift or lug the engine hard to maintain speed.

Do you think it would be lugging that much more? My brother has 4.10 and 35s on his f150, rolls pretty good and has no problem cruising around on the highway.

I'll rarely be towing or hauling but will on occasion.

I chose a very low end torque cam so I can keep rpms low, it's rated 1000-5500 rpm, I think if I keep it when cruising highway around 2k-2500 it will be very healthy in the torque range for going up hills and overtaking.

 

[Nick88]

I have very little experience with big blocks period, let alone breathed on ones. But my buddy with his stock 2wd '86 CCLB 454/TH400/4.10s on stock 31s got 8mpg pretty much no matter what. He's since swapped on a Holley Sniper fuel injection setup and a 6L80 overdrive and sees 11mpg. For your setup, I'd guess somewhere in the 8-10mpg range. I'd strongly suggest an OD trans so you can use a deeper rear gear but get the OD drop on the hwy

The truck is a 383 stroker small block not big block. With the lockup converter the truck should sit around 2400ish while going 65 if I end up doing 35s, which I honestly don't think is that bad, My 88 Blazer with a 700r4 cruises 70 around 2k on a flat road, even higher if going slightly uphill maybe 2200 and that's in OD

 

[TotalyHucked]

The truck is a 383 stroker small block not big block. With the lockup converter the truck should sit around 2400ish while going 65 if I end up doing 35s, which I honestly don't think is that bad, My 88 Blazer with a 700r4 cruises 70 around 2k on a flat road, even higher if going slightly uphill maybe 2200 and that's in OD

My bad, I mis-read. You say you're gonna do alot of highway driving, curious why you didn't go with an OD to start with cuz of the hwy driving. A 700R4 has a deeper first gear so it'll drive better in town and has the OD to bring the RPMs down on the interstate. The calculator I use (that's been very accurate to everything I've owned) shows you'll be over 2500 at 65, that's singing pretty good.

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

purperformance.com

With the current setup, I'd wanna stick with 35s at least. 33s will be over 2700 at 65.

 

[HotWheelsBurban]

My bad, I mis-read. You say you're gonna do alot of highway driving, curious why you didn't go with an OD to start with cuz of the hwy driving. A 700R4 has a deeper first gear so it'll drive better in town and has the OD to bring the RPMs down on the interstate. The calculator I use (that's been very accurate to everything I've owned) shows you'll be over 2500 at 65, that's singing pretty good.

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

purperformance.com

With the current setup, I'd wanna stick with 35s at least. 33s will be over 2700 at 65.

That was the one thing I didn't like about driving the '90 Burb on the road. No overdrive, just a turbo 400 behind a TBI 350, and 4.10 rear end. Originally ordered to haul a boat, and probably not real far.... It would max out at around 65 on the highway and it was screaming then. This was an R2500 Burb BTW..... would haul plenty, just don't be in a hurry to get there.

 

[legopnuematic]

calculator I use

Side note, Pur Performance is in my neck of the woods.

I've spent more time in their parking lot than in the store lol. They used to to car shows and then swap meets there.

It's literally just out of frame in the picture

You must be registered for see images attach

 

[Bextreme04]

The truck is a 383 stroker small block not big block. With the lockup converter the truck should sit around 2400ish while going 65 if I end up doing 35s, which I honestly don't think is that bad, My 88 Blazer with a 700r4 cruises 70 around 2k on a flat road, even higher if going slightly uphill maybe 2200 and that's in OD

I have 35's, 4.10's, 350/TH400/NP205. I get around 10mpg. I'm swapping to a fuel injected 454/4L80. If it was me and I had a already round pattern TH400 truck, I'd be looking really hard at a 4wd 4L80 and 4.56 gears with the 35's. The 4.10 gears will be perfect for the 454, but that 383 is gonna want more gears in it to be happy. The OD will make up the difference. My setup currently feels like the 350 is SCREAMING at 65mph on the highway.

 

[TotalyHucked]

Side note, Pur Performance is in my neck of the woods.

I've spent more time in their parking lot than in the store lol. They used to to car shows and then swap meets there.

It's literally just out of frame in the picture

You must be registered for see images attach

I've literally never heard of them until I found their site trying to find a good RPM calculator lol

 

[SquareRoot]

New guy asking about fuel milage... 10....like everybody else.

 

[Nick88]

My bad, I mis-read. You say you're gonna do alot of highway driving, curious why you didn't go with an OD to start with cuz of the hwy driving. A 700R4 has a deeper first gear so it'll drive better in town and has the OD to bring the RPMs down on the interstate. The calculator I use (that's been very accurate to everything I've owned) shows you'll be over 2500 at 65, that's singing pretty good.

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

purperformance.com

With the current setup, I'd wanna stick with 35s at least. 33s will be over 2700 at 65.

I just want to avoid the 700r4 because the 400 is so much stronger, if I did od swap I'd probably do a 4l80 at some point, yeah it would be over 2500 but I have a lockup converter so it should drop it a couple hundred rpm which will help a lot.

 

[Nick88]

I have 35's, 4.10's, 350/TH400/NP205. I get around 10mpg. I'm swapping to a fuel injected 454/4L80. If it was me and I had a already round pattern TH400 truck, I'd be looking really hard at a 4wd 4L80 and 4.56 gears with the 35's. The 4.10 gears will be perfect for the 454, but that 383 is gonna want more gears in it to be happy. The OD will make up the difference. My setup currently feels like the 350 is SCREAMING at 65mph on the highway.

I'm shocked mpg is so bad my 88 blazer gets around 20 on the highway cruising at only a couple hundred rpm lower than the crewcab will be cruising

 

[Nick88]

New guy asking about fuel milage... 10....like everybody else.

I just thought it would be better because my tbi gets good mileage

 

[Nick88]

That was the one thing I didn't like about driving the '90 Burb on the road. No overdrive, just a turbo 400 behind a TBI 350, and 4.10 rear end. Originally ordered to haul a boat, and probably not real far.... It would max out at around 65 on the highway and it was screaming then. This was an R2500 Burb BTW..... would haul plenty, just don't be in a hurry to get there.

What size tires did you have?