Gas Mileage

[Rubble]

Originally Posted by rustyMUD

I've seen your claim of 21mpg in your sig for a while and will take this as an opportunity to ask you how you do it?



If there is some secret to getting 20+ mpg from a carb V8 gas powered, non-overdrive truck that possesses the aerodynamics of a brick, please share.

Ya,I'm curious about this claim as well,done a quick calculation with your specs.I am guessing tire radius around 15-16" So at 70 mph your turning around 25-2600 rpm.Even more if you have a smaller tire. An explaination would be great...

[nekkidhillbilly]

the od trucks did have to high of gearing. you want to be around 65-70 at around 2k to optimal. a friend had a 87 4wd 305 with od and 342 gears pulled around 18 all the time.

[Jabes1]

I'm super curious as well... 21mpg would be nice. I'm fuel injected, its leaned out to cruise at 16.2 AFR at the moment and running 50 degrees of advance at light cruise. I'm also running a Gear Vendors O/D which cruises nicely at 2100 RPM at 70MPH...

[nekkidhillbilly]

one thing i know you can do to improve mpg and cheaply is electric fans. do you guys have clutch fans or solid state idr what my k20 had?

[JustN]

well i dont know excat #'s and stuff. but when i was in high school my dad drove my truck almost 2 hrs 1 way everyday 350 qudrajet 3.42 or 3.73 and 700r4(od/cruise didnt work) he was gettin 15-20 on highway. was 10 years ago so gas was cheaper but we figured it at one time was very close to that and he kept foot in it

[nekkidhillbilly]

new trucks arent really any less brick shape and even the tbi trucks would pull down 18 mpg on the highway so these should too. gearing and the tires play the biggest part on mpg.

[TheBlueBomber]

Newer trucks have A LOT better aerodynamics that our old trucks do. Our front ends are flat, new front ends are rounded and you can see the effects in the wind tunnel.


I quit caring about gas mileage a long time ago because I know It'll just stress me out

[special-K]

I'm sorry,but if someone is getting 21 mpg at 70 mph in any of these trucks they are the only one in the world. Makes you wonder. Time to recalculate (accurately). The 6.2 diesels are known for being miserly on fuel and that's what they get,with a few rare better reports up to 24 mpg.

[csdineley]

Best I've ever got with my 87 3/4 ton 350 tbi, th400, 4.10's, 31" tires was 14 mpg. Swapped in a sm465 didn't change anything. Swapped tbi for Q-jet and it maybe lost 1 mpg.
But back in the 90's my dad had an 82 1/2 ton lwb, 305 H.O from a monte and a 700r4 we swapped in. Don't remember the gears but he claimed he got 18 mpg driving 80 miles a day for work.

[swb85]

There are sooooo many factors that effect fuel economy outside of the vehicle/drivetrain combo itself.....driving style, air density (elevation), topography, alcohol in the fuel, etc etc.

For giggles, here's the avg highway MPG of my vehicles at 70mph cruise with me driving:

'01 Tahoe 4x4-- LS 5.3, 4l60e, headers/exhaust/tune, ~325hp, 3.73 rear, 33" tires, 5100lbs: 14mpg
'85 C10 swb 2wd-- ~450hp 383sbc, 750 carb, m21 4spd, 3.42 rear, 28" tire, 4000lbs: 15mpg
'98 camaro-- 370ci LSx, ~600hp, t56, 4.33 rear, 28" tire, 3900lbs: 21mpg

I'm knee deep in swapping a t56 into the '85 right now....I expecting to easily pick up 3-4 mpg on the highway over the M21. I'll post up once I have some good data.

[nekkidhillbilly]

Originally Posted by TheBlueBomber

Newer trucks have A LOT better aerodynamics that our old trucks do. Our front ends are flat, new front ends are rounded and you can see the effects in the wind tunnel.


I quit caring about gas mileage a long time ago because I know It'll just stress me out

you really think my 04 chevy is that much better its shaped horrible and flat. most of the cummins swapped square trucks get 20 mpg. my duramax and brothers ram does the same. i just dont feel they are that much better.

[piecesparts]

My 84 GMC SWB, 383 stroker, 700 R4 tranny W/O lockup converter (2400 stall converter in it's place), 3.73 gears W/posi and 28" tall tires gets 14 to 15 MPG on the highway. I run around 2500 RPM at 75 MPH and that keeps me in my torgue converter's optimum operating range.

[Rubble]

Originally Posted by nekkidhillbilly

you really think my 04 chevy is that much better its shaped horrible and flat. most of the cummins swapped square trucks get 20 mpg. my duramax and brothers ram does the same. i just dont feel they are that much better.

I imagine Keith can shed some light on the wind tunnel results comparing the 73-87 trucks to the newer ones.Probably quite a difference.I been driving these old trucks for over 32 years now,had a couple with 305/700r combos,350/700R C10s and was really lucky to ever get 20mpg.That's driving between 55-65mph.Wind resistance plays a big factor,you want to drive faster,your gonna take more fuel to get that square box down the road faster,simple areodynamics.

[Tucson38]

I've been tracking the mileage since I bought my truck in 1984. It's an '85 GMC HD 3/4, 4wd with an SM465 tranny and 3:73 gears. Other than dual exhaust and a 790 cfm Edlebrock, it's stock.

My overall mileage has been 12.2 mpg. I drop to 10 when towing my boat but other than that there's not a lot of variability.

[special-K]

Originally Posted by nekkidhillbilly

you really think my 04 chevy is that much better its shaped horrible and flat. most of the cummins swapped square trucks get 20 mpg. my duramax and brothers ram does the same. i just dont feel they are that much better.

I see what you're saying. In '73 GM smoothed the top over the windshield,which was the biggest wind resistance reduction ever,so no difference from '73-present. The rake of the windshield is lower,but that's it. A blunt front is a blunt front In '81 the nose was dropped a bit for aerodynamics and air dams had been used since a few years prior. The 88-98s were more trimmed down and aerodynamic than the present trucks and the trucks are heavier now than before. Any increased mileage is mostly due to mechanical advancements.

Originally Posted by Rubble

I imagine Keith can shed some light on the wind tunnel results comparing the 73-87 trucks to the newer ones.Probably quite a difference.I been driving these old trucks for over 32 years now,had a couple with 305/700r combos,350/700R C10s and was really lucky to ever get 20mpg.That's driving between 55-65mph.Wind resistance plays a big factor,you want to drive faster,your gonna take more fuel to get that square box down the road faster,simple areodynamics.

Makes me wonder why so many newer trucks,with all the advancements in timing/spark control and fuel management improvements,as well as roller cams and other advancements,get the same mileage many here have reported.

[Rubble]

The mileage thing with newer trucks has always been a question of mine too,with all the technology of newer engines,fuel mileage should be way better!! Just another reason for me to keep using a 26-32 year old truck for daily driver/work truck.I know a few of the guys in my area doing same work as me with new trucks & their mileage is not much if any better than mine.This is not much highway driving,lots if stop & go on gravel roads & idling in winter.

[dieseldawg142]

you guys are making me jealous with these numbers!
my '85 crew, 4-5 mpg's, maybe 6 if i can catch the jet stream
my newer K1500 step ('90) is a little better, if i short shift & granny it, i can get 15-16 mpg

[Rubble]

[QUOTE=dieseldawg142;6427650]you guys are making me jealous with these numbers!
my '85 crew, 4-5 mpg's, maybe 6 if i can catch the jet stream
Ha,my 81 crew is in the same ballpark!! 5-7 mpg..You got bigger tires than mine too!!! We just had the crew in town all afternoon,60 mile roundtrip,-20 degC.The old 454 probably burnt up more fuel idling than driving,but a warm truck is nice!!

[Ziegelsteinfaust]

Originally Posted by special-K

I'm sorry,but if someone is getting 21 mpg at 70 mph in any of these trucks they are the only one in the world. Makes you wonder. Time to recalculate (accurately). The 6.2 diesels are known for being miserly on fuel and that's what they get,with a few rare better reports up to 24 mpg.

I built a 74 rcsb years ago, and it would routinely touch 20mpg highway at 70.

The engine while starting as a nothing special 307 had full bolt-ons, and was fully detailed when I built it. I used a frap wheel on the heads to clean up the intakes a touch, polish up the exhaust, and port matched the weiand street intake. The engine had a comp 262 cam, 1.6 roller rockers, 9-1 compression with polished pistons, 1 1/2 inch headers, and a pro built Q-jet carb. Then I dialed in the distributor, and vacuum advance to the engine. The carb was what really brought the combo to life. I tried a Holley after to test out the carb I was building, and it never worked 1/2 as well.

When I first got it running, and ready for street trials. My friend and I were cruising around the I-15 to break in the motor. The first run got me 28 mpg, and we were wondering if the the pump was wrong. We drove to another station, and got 29. A couple of days later it settled down, but wow it sure was high for the first 200-300 miles. I always wonder why, but it still meet expectations or goals.

The truck had a 4/6 or 5-7 or so on it, and had 245/255/60/15 tires on aluminum rims. With the rear end being 3.23 I believe. The tranny was a TH350 with about 2200 rpm stall, trick fluid, hd build, and a shift kit.

While not a power house it would beat a stock 350 by almost a couple of truck length's.

We even had similar results with the GM replacement 350 to which we added full bolt-ons. Mine would just do better more often, and was more tolerant of speed or not being in the right zone.

If you want to build mpg for a truck you have to be willing to do alot. A overdrive with the wrong gear will hurt mpg, and you have to consider what tires you have.

Before I got my HVAC job I was going to convert my C20 to a light duty C10, and go for 25 mpg hwy at roughly 65mph. I had planned to use the 4.3 version of the LT-1, with the carb'd intake, and follow the previous build with just a higher compression ratio. Then I would of backed it up by a 4L60 with a 2000-2200 rpm converter, and a 3.55 ratio is what I was calculating at that point. I also considered using Jag suspension front, and rear to allow a lower ride height with out cutting my bed. In theory the engine would of been good for 275-300hp range at the crank, and would of been a nice commuter for traffic too.

Luckily I have my bosses van to do my commuting, and I am making my C20 into a sort of off-roader. I will be dropping in my old 450-475hp 400sbc, and pray for anything close to 15 when I get the rear-end I want.

[nekkidhillbilly]

14 is still better than most new dodges