1991 Chevy V3500 Cummins

[907_Chevy]

Well I got it finished and to say the least I have had a few drivetrain set backs/breakage.
Right now I'm getting 9 to 11 mpg. I expected it to be a little better than that. After seeing this mileage, I purchased an AFC live. I have only been able to play with it for one longer trip and some driving around town but have not been able to get anything better than 11 with easy driving.

Advance timing to 18deg
Mack Rack Plug
Killer Dowl Pin
Boost Elbow
4k Gov Springs
60lbs Valve springs
4” exhaust
Exhaust Brake
Stock Exhaust Manifold
Fast Fuel TS 140G
Delievery Valves 191
Stock turbo

GM NV4500 (MW3)
South Bend Dodge input and bell housing
South Bend Dual Disc Clutch
NP205
Currently 4:10 gears about 1900 rpm at 65mph (Most of driving will be 65-70mph)
37” tires

I'm considering changing to 3.73 that would put me about 1700 rpm at 65mph.
What rpm should I keep at cruzing speed?
Either way, it seems like I should be getting better better milage... any suggestions on changing the engine setup to get better milage.
I do need to upgrade turbo because pushing it's limit but dont expect that will change fuel milage much.

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[mcarlo86]

Considering you are pushing a lifted pickup with the aerodynamics of a brick and running on 37’s, 11 mpg seems about what you probably can expect. If it were a warmed up gas big block, you’d probably be getting 8 mpg. Laws of physics are hard to overcome.

 

[Grit dog]

Considering you are pushing a lifted pickup with the aerodynamics of a brick and running on 37’s, 11 mpg seems about what you probably can expect. If it were a warmed up gas big block, you’d probably be getting 8 mpg. Laws of physics are hard to overcome.

Sort of agree, although I would think OPs mileage should be better as well.
37s on a lifted brick is a recipe for horrible mileage. Going to 3.73 gears is a waste of $ imo. Even if they net you 1mpg better, the roi will take ALOT of miles to break even.
35s on a lifted squarebody brick with stock gen vi 454, 4.10s th400 and a very heavy tire/rim combo is about 7mpg avg.
2016 Ram 3.42/68rfe deleted, tuned 6” lift on 37s pulls down an avg of 12.5-13mpg mixed driving and about the same up to 75mph avg.
That’s why a mild turned up P pump I would think would be better. I would have guessed low teens.

 

[legopnuematic]

What is your boost/egts at highway speeds?

My 02 Cummins (24v, VP44, auto, 4x4) was averaging 18mpg highway with 4.10 gears and 33” tires, now it’s 3.55 gears and 35” tires. Still will average 18 highway. That’s 60-75mph and in the warmer months for reference.

That is one downside of the P7100 pump versus the VP44 or VE pumps, and common rail, they all have the ability to advance the timing, whereas the P pump is a static pump. Great pump for making power and rpm though, that’s where rotary pumps fall short. But a P pump can still deliver fair mileage.

I doubt it makes a difference on a P pump, but VP trucks do better with IAT above 100°f, in terms of mileage.

Anyways, I doubt 3.73s would make a difference. Besides a lighter wallet.

 

[907_Chevy]

Well I got it finished and to say the least I have had a few drivetrain set backs/breakage.

 

[907_Chevy]

Sort of agree, although I would think OPs mileage should be better as well.
37s on a lifted brick is a recipe for horrible mileage. Going to 3.73 gears is a waste of $ imo. Even if they net you 1mpg better, the roi will take ALOT of miles to break even.
35s on a lifted squarebody brick with stock gen vi 454, 4.10s th400 and a very heavy tire/rim combo is about 7mpg avg.
2016 Ram 3.42/68rfe deleted, tuned 6” lift on 37s pulls down an avg of 12.5-13mpg mixed driving and about the same up to 75mph avg.
That’s why a mild turned up P pump I would think would be better. I would have guessed low teens.

Yea, I know its a big brick but also figured low teens based on friends dodge on 37s.
I have a lead on a 14 bolt with disc brakes and 3.73 that might be able to pick up for cheap so would only need to regear front. That was only reason I was considering changing.

 

[907_Chevy]

What is your boost/egts at highway speeds?

My 02 Cummins (24v, VP44, auto, 4x4) was averaging 18mpg highway with 4.10 gears and 33” tires, now it’s 3.55 gears and 35” tires. Still will average 18 highway. That’s 60-75mph and in the warmer months for reference.

That is one downside of the P7100 pump versus the VP44 or VE pumps, and common rail, they all have the ability to advance the timing, whereas the P pump is a static pump. Great pump for making power and rpm though, that’s where rotary pumps fall short. But a P pump can still deliver fair mileage.

I doubt it makes a difference on a P pump, but VP trucks do better with IAT above 100°f, in terms of mileage.

Anyways, I doubt 3.73s would make a difference. Besides a lighter wallet.

Boost was about 20psi and egts were about 700-800 if i remember correctly. They didnt seem to bad.

Forgot to mention, we have winter fuel so I do expect to pick up a little in the summer.
As mentioned above only considering becasue might have cheap rear with disc brakes with 3.73

 

[77Dmax]

For comparison my dmax swap gets 16mpg highway and country driving. Thats a stock LLy and Allison with 3.73s. 33x12.50 tires. The 37" tires are killing you. Way too much rolling weight.

 

[legopnuematic]

20psi just cruising along unloaded? HX35?

Your egts are inline, seems like a lot of boost at cruise though.

My 02 cruises on flat ground about 5-7psi, pulling a hill 10-12psi. Stock injectors, and HY35 turbo, edge JWA on level 3 (+85hp according to edge, should be more like +65hp as its can bus only fueling, 02 auto is 235hp, so around 300 crank with the edge give or take). Even towing 5000lbs behind the truck on the same route, flat, with some hills and one steep grade maybe hit 15psi. On cruise control at 65-70mph. Egts in the 7-800 degree range.

Of course that isn’t a direct comparison to your truck, but just some data points.

 

[907_Chevy]

20psi just cruising along unloaded? HX35?

Your egts are inline, seems like a lot of boost at cruise though.

My 02 cruises on flat ground about 5-7psi, pulling a hill 10-12psi. Stock injectors, and HY35 turbo, edge JWA on level 3 (+85hp according to edge, should be more like +65hp as its can bus only fueling, 02 auto is 235hp, so around 300 crank with the edge give or take). Even towing 5000lbs behind the truck on the same route, flat, with some hills and one steep grade maybe hit 15psi. On cruise control at 65-70mph. Egts in the 7-800 degree range.

Of course that isn’t a direct comparison to your truck, but just some data points.

Typo. Should have been 10psi or little less. When I was loaded with about 7k, I was seeing 15-20 pulling some hills with egt 700-800 at 65mph.

 

[legopnuematic]

Ok, that’s much better.

I’d say then @77Dmax has got the right idea, those 37s can’t be light, I’d imagine the tires themselves would be around 80lbs apiece. Add the wheels themselves, rotors and drums that is a fair chunk of weight rolling.

May not be where all of your potential mileage is going, but some of it.

 

[bluex]

Oh man, I am considering a cummins for my dually to get the mileage up away from 10 with the 454. You guys aren't making it sound worth the effort lol.

I know it's not a fair comparison to these lifted big tire 4x4s though considering the completely different style of truck although the NV4500 an 4.10s are the same

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[squaredeal91]

Haven't read every post but what a cool ride you have! Similar to big yellow. She gets 18 mpg with 373s and the ZF5 speed. Ve pump. 32 gov spring and stage 2 pin. 7,350# lol

 

[77Dmax]

In the drag racing world it's generally accepted that rotational weight is anywhere from 5-10x heavier than "normal" weight at speed.. then add in the aerodynamics of a lifted brick.

 

[907_Chevy]

Haven't read every post but what a cool ride you have! Similar to big yellow. She gets 18 mpg with 373s and the ZF5 speed. Ve pump. 32 gov spring and stage 2 pin. 7,350# lol

What tires are you running?