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73-87 Chevy _ GMC Trucks => Performance => Topic started by: Irish_Alley on July 25, 2009, 02:04:48 AM

Title: more power = better fuel economy
Post by: Irish_Alley on July 25, 2009, 02:04:48 AM
OK i had water in my oil not a lot but had some so i changed my intake gasket fix it i think for now. but while i had it apart i put a elderbrock performer intake and a carb spacer i think 3" well all is fine till i go to fill up and calculate my mpgs its usually around 7-8 more at 7 than 8 but this time it was around 10. i was like whatever i must of did something wrong so next time i filled up " a couple days later" it was around 9 but that was after letting my wife drive it and then leaving the lights on and killing my battery then letting it idle for a while to let the battery charge back up. now i would expect and it make sense more power would give you better economy since the engine doesn't have to work as hard to get moving but 3 mps that blew me out of the water.
Title: Re: more power = better fuel economy
Post by: VileZambonie on July 25, 2009, 08:26:33 AM
7 mpg isn't very good at all. You probably removed some carbon and got your timing set better.
Title: Re: more power = better fuel economy
Post by: choptop on July 25, 2009, 05:00:24 PM
Any ime you increase the efficiency of a motor, the mileage should improve as well. It just makes sense. But if you build an engine that easily produces 400hp + and drive it to where it uses those horses, then the mileage wont be great. Sounds like you fixed some other problems that needed attention anyway. 7-8 isnt good.
Title: Re: more power = better fuel economy
Post by: gold84gmc on July 25, 2009, 10:12:45 PM
7-8 mpg's isnt that about normal for one of these trucks?
Title: Re: more power = better fuel economy
Post by: 69byrd on July 25, 2009, 10:25:07 PM
I know alot of guys with these trucks who are lucky to get 10mph, but alot of that has to do with there driving habits.
Title: Re: more power = better fuel economy
Post by: Irish_Alley on July 25, 2009, 11:21:27 PM
i had this engine in my 86 4x2 1/2 ton it was getting about 13. then once i dropped it in my 79 it got 7-8 with out any changes. i could of fixed a couple problems that i didn't know about but as far as i could tell there were no signs of any problems except for the water in the oil and that had just started. and my driving habits are about the same cause i never wanted to pull another motor again lol or at least prolong it as long as i can.
Title: Re: more power = better fuel economy
Post by: gold84gmc on July 26, 2009, 06:15:19 AM
ya i dont exactly baby my truck so i probably get about 7
Title: Re: more power = better fuel economy
Post by: Captkaos on July 26, 2009, 10:22:54 AM
My 81 K10 (with a 4 speed) got 12mpg all the time, this was with a 350 Crate.
Title: Re: more power = better fuel economy
Post by: gold84gmc on July 26, 2009, 02:58:14 PM
i got the original 350 with 101k miles all original and a th400 tranny all original and i dont know what the gear ratio in the axle is but its the 9 1/2" 14 bolt...its not exactly in the best shape.....would my engine be better or worse than a 350 crate?
Title: Re: more power = better fuel economy
Post by: TexasRed on July 26, 2009, 04:33:07 PM
Yikes, am I naive to hope for high 18 or 19's with a 383 and a 700r4?

My 305 with th350c got about 18-19 with the original quadrajet. On the highway, City was like 13-16mpg. Closer to 13 with A/C.
Title: Re: more power = better fuel economy
Post by: gold84gmc on July 26, 2009, 06:21:41 PM
Yikes, am I naive to hope for high 18 or 19's with a 383 and a 700r4?

My 305 with th350c got about 18-19 with the original quadrajet. On the highway, City was like 13-16mpg. Closer to 13 with A/C.

how did u get 18-19 mpg with a carb? i must be beating on my truck too much lol
Title: Re: more power = better fuel economy
Post by: choptop on July 26, 2009, 08:31:32 PM
Ive gotten 18-20 mpg with a 305 and carb. My 86 GMC is getting 18-19 mpg, Big Red gets about 14, but with 35's I dont expect any better. I have had several others that got this similar mileage. I havent had any of the 4x4s or 3/4 ton up trucks do it, but thats to be expected. Ive even got 20 out of my 51, so I dont think its a crab thing. One thing to take into consideration is most of us have different driving areas. Mine is mostly flat. The wifes 01 get better fuel mileage at 75 than it does at 65 becaus the taller tires set the gears to high. But at 75-80 mph the engine is around 2200-2300 rpms and likes it there.
 But, like I said the terrain is different. We are flat, and most highway driving is 70-80 mph. I know alot of areas hardly ever see speeds above 55mpg, and have twists and turns with hills. That needs to be considered when comparing notes on fuel mileage with eachother.
  Altitude, and fuel type are considerations as well. We are about 2500ft above see level ( Or close, Im  not sure) so mileage difference will be different in Denver. We also dont have any methanol fuel that I know of, and Ive heard that methanol mixed fuels are notorious for poor fuel mileage.
  That being said, if you live in an area that requires alot of start and stopping and hill climbing, I dont see the mileage being to good.
Title: Re: more power = better fuel economy
Post by: gold84gmc on July 27, 2009, 08:09:45 AM
would a 350 be much different in mpg's than a 305? some of these trucks got around 18 then how come the late 90's trucks with fuel injection couldnt get any better...my dad used to have a 97 gmc 1500 with a 350 vortec and it only got about 16-18 mpg's. and the terrain in maine isnt exactly flat so there are quite a bit of hills
Title: Re: more power = better fuel economy
Post by: eventhorizon66 on July 27, 2009, 01:29:09 PM
These trucks are lighter than any later model chevys and wouldn't even come close to meeting today's crash safety tests.  The only reason I mention the crash safety is the frame is largely resposible for this and I believe our trucks are the last that had full C-channel frame rails (contributing to the lighter weight).
Title: Re: more power = better fuel economy
Post by: SUX2BU99 on July 27, 2009, 03:11:17 PM
For sure, much lighter. I weighed mine at the track recently and it was 4040 lbs and that's with my 270 lbs in it. I think I read that the Ford Lightnings were close to 5000 lbs. Mine seems like it gets attrocious mileage but I only have one saddle tank in it so that's like 15 gallons. But even with the cam, heads and other performances parts done to my motor, it still got 14 mpg on a mostly-highway tankfull back in May. I was pleasantly surprised.

Todays trucks are terrible for mileage. 30+ years of technology progress and they get the same mid-teens they always did. BUT, the flip side is more power, better driveability and more weight they are hauling around all while producing cleaner emissions. The LS-series motors are better than the 1st-gen SBC they replaced though. The 2 Dakota R/T's I had with their injected 360's got 12-16 mpg and the 96 GMC 1/2-ton my friends dad had with the Vortec 350 would have a hard time getting better than 12mpg. The 2003 Tahoe 5.3 we had wasn't too bad. It would get high-teens I would say. Mid-high teens. The 05 Magnum R/T I had with it's 5.7 HEMI would get about 21mpg on my weekly commute. I think the best it ever got was about 26-27. It was a porker though at 4400 lbs!
Title: Re: more power = better fuel economy
Post by: Captkaos on July 27, 2009, 05:10:27 PM
My brothers Silverado gets from what I remember 18 city/ 21 hwy before the turbo, it gets 18/23 now.
We took the same trip he in his 00 Silverado (5.3L/4L60E/turbo) and me in my 87 R10 (305TBI/700R4) and he got 22.9x and I got 22.1 mpg.

4x4s I have never seen get above 17mpg EXCEPT in a diesel app.
Title: Re: more power = better fuel economy
Post by: gold84gmc on July 27, 2009, 05:50:14 PM
on my truck i have 32" mud tires in the back so i take it those dont help with mileage...my mom has an 08 sierra with a 5.3L and it gets around 21mpg
Title: Re: more power = better fuel economy
Post by: exilous on August 02, 2009, 08:14:31 AM
well I'll add my .02. My 05 Dodge ram 1500 quadcab short box AWD with the stock 20s got 11-18MPG before I changed to 17 rims with 31s to try and keep the tire size the same without having to change the speedo and adding a flowmaster super 40 and an AEM brute force intake it upped it to 12-19MPG which still stinks. As for my 85 GMC I dunno yet I can hope to have it in the mid teens, but with all the performance stuff I threw into it on the engine side of things and that I'm adding a second transmission to make it a 12 speed and thats its a dually I will assume 12-15MPG if I am lucky. haha.
Title: Re: more power = better fuel economy
Post by: gold84gmc on August 02, 2009, 08:37:40 AM
a 12 speed in a pickup? i didnt know you could do that lol
Title: Re: more power = better fuel economy
Post by: werewolfx13 on August 02, 2009, 11:51:14 AM
I'm going to assume he means a 3 speed auxiliary, which is doable, but its application is questionable, unless you're running an old 4-53 detroit or a small cat..I've seen a road ranger of some sort shoehorned into one of these trucks, making it a 13 speed, and it was bolted behind a late model cummins, but that motor had no need for so gentle a ratio slope.
Title: Re: more power = better fuel economy
Post by: Irish_Alley on August 03, 2009, 12:00:34 AM
one of my Buddys' was telling me up in NY one of his friends took a motor and Tran out of a cat. just to go 10 mph he was shifting multiple times
Title: Re: more power = better fuel economy
Post by: 76 k10 on August 05, 2009, 10:46:52 PM
ya i dont exactly baby my truck so i probably get about 7

man your lucky! im getting 4 mpg no lie
Title: Re: more power = better fuel economy
Post by: eventhorizon66 on August 05, 2009, 11:24:32 PM
ya i dont exactly baby my truck so i probably get about 7

man your lucky! im getting 4 mpg no lie

4!!!  You must have a hole in your tank.
Title: Re: more power = better fuel economy
Post by: gold84gmc on August 06, 2009, 10:07:20 AM
ya i dont exactly baby my truck so i probably get about 7

man your lucky! im getting 4 mpg no lie


wow and i thought my truck was bad on gas lol
Title: Re: more power = better fuel economy
Post by: SUX2BU99 on August 06, 2009, 12:58:58 PM
Well if you treat your gas pedal like an on/off switch, I could see 4 MPG in a 4x4 lol
Title: Re: more power = better fuel economy
Post by: 1980c10 on August 06, 2009, 09:42:23 PM
I recently had a 82 lwb k10 305 quadrajet 4bbl carb and a  4 speed. 31x10 tires and I would always get 18 mpg. I did replace the dist and gave it a full tune up. then i brought to a guy to adjust the carb timing and re-route vacuum lines for the best effect. most of my driving was a combo of city and highway. I now have an 80 swb with a 250 Inline 6 2bbl carb and a 4 speed and get 17mpg after doing the same as i had done with the 305 plus a carb rebuild. it runs great but I wish i would have kept the 82 to use that 305.
Title: Re: more power = better fuel economy
Post by: mean79 on August 07, 2009, 02:24:24 AM
my 79 gets 15mpg its 4x4, but its a 4 speed, and my dad said his 78 with the automatic would suck it out of the tank as if it was goin outta style.
Title: Re: more power = better fuel economy
Post by: jimbo on August 07, 2009, 09:40:09 PM
I've always been a proponent of the adage more power/efficiency = more mpg.  I cannot recall how many vehicles I have swapped 2 barrel carbs for 4bbl's and added an aftermarket intakes as well as exhaust and gotten better mpg nearly everytime.
Title: Re: more power = better fuel economy
Post by: malibu795 on September 22, 2009, 09:11:49 AM
I've always been a proponent of the adage more power/efficiency = more mpg.  I cannot recall how many vehicles I have swapped 2 barrel carbs for 4bbl's and added an aftermarket intakes as well as exhaust and gotten better mpg nearly everytime.
its fact you increase the effecientcy of the motor to burn X amount of fuel.. and the work stays the same you will get more work done for the X amount of fuel.

easier it gets air in and out, and better burn the effcience will go up.  also getting the motor to runs at its most efficent rpm band is key
Title: Re: more power = better fuel economy
Post by: jimbo on September 23, 2009, 04:54:34 PM
I've always been a proponent of the adage more power/efficiency = more mpg.  I cannot recall how many vehicles I have swapped 2 barrel carbs for 4bbl's and added an aftermarket intakes as well as exhaust and gotten better mpg nearly everytime.
its fact you increase the effecientcy of the motor to burn X amount of fuel.. and the work stays the same you will get more work done for the X amount of fuel.

easier it gets air in and out, and better burn the effcience will go up.  also getting the motor to runs at its most efficent rpm band is key
I completely agree.  I am hoping for a decent improvement after this winter when I add my longtube headers and exhaust to the little 305.  On the rpm note, I really need to upgrade my 2.73 rear.
Title: Re: more power = better fuel economy
Post by: Dieselfume on September 24, 2009, 01:26:48 AM
I'm sorta surprised no one mentioned what rear end ratio they had installed until the last post here from Jimbo. Gear ratios (or really the whole trans, axle and tire size combo and resultant hwy speed & RPM) is sort of a key item here.

The original post from Irish_Alley doesn't surprise me considering stock manifolds and emissions choking the engine. His follow up about the engine originally getting 13 mpg in the 86 he pulled it out of makes me wonder if there was something noticeably different with trans, axle, or tire size.  The weight difference will have some effect, but not that much. If the pic from his signature is any indication, a lifted 4x4 with big tires is going to make a noticeable difference compared to a 86 1/2t 4x2. Turning 3/4t or 1t running gear compared to a 1/2t will make a little difference too.

Captkaos, that's some pretty good mileage on that R-10 305/700R4. What tire size and rear end gear do you have?  Any canopy or tonneau cover? Lifted/lowered? Stock or mods?

To add to the other data points out there, my stock 87 R-10 stock TBI305/700R4 with 2.73's and 225/75R15, and a 32" tall shell (about 11" above cab) near sea level on Seattle's typical 87 octane brew with unknown % ethanol (guessing 5-10%) gets 16.5-17 mpg hwy. 

Considering how tall my camper canopy is, been toying with the idea of experimenting with VGs (vortex generators) if I can find a good quick way to measure change in power required. I've thought perhaps manifold pressure, but If there was a good scanner to plug into the ALDL and display live injector pulse width or similar fuel flow parameter that could work too.  (eventually will do some basic engine mods for mileage, but I'm an aero guy so...)

I agree with SUX2BU99, today's trucks really suck for mileage compared to where they could be. The trucks are porkers weight wise, and only recently have they approached Captkaos' 22 mpg. There really hasn't been much effort to extract more energy out of the fuel burnt. About 70% of the fuel energy goes out the tailpipe as heat. We haven't even tried to apply 1940's technology to extract more energy out of the exhaust.
Title: Re: more power = better fuel economy
Post by: Captkaos on September 24, 2009, 12:42:45 PM
Mine truck was basically stock with nothing extra.  Stock tire size: 235/75/15.  Stock rear gear 2.73.  Stock 1989 305 engine (original was replaced by PO).  Stock suspension and an open bed.

It looked JUST like this:
(http://www.73-87chevytrucks.com/projects/87R10-Driver/wheelsaftr_2.jpg)

It now has Impala SS wheels and a Extang Tonno:
(http://forum.73-87chevytrucks.com/smforum/gallery/2_31_03_09_5_51_25.jpeg)
Title: Re: more power = better fuel economy
Post by: Lt.Del on September 30, 2009, 07:24:23 PM
First time reading this thread...pretty interesting.  I gotta say, my  79 Big 10 stroked 383 quadrajet (http://www.delbridge.net/install) with rv cam, th350c, dual exhaust, and 2.80 rear gets around 10mpg, 12 if on the highway and keeping my foot out of it. Of course, If i play with the HP and have fun with it, she easily drops to the single digits.



Quote
4x4s I have never seen get above 17mpg EXCEPT in a diesel app.

Well Chris, my 4x4  1991 V2500 5.7 sub tbi (http://www.delbridge.net/suburban) with 33's and 3.73 ratios gave me 18mpg on the interstate last year during a long particular trip to OBX....i purposefully drove it really easy just to see what I could get, even though my whole family and luggage for a week were on board.  I thought that was real good for the sub.   I can routinely get around 16.5 on the interstate not really caring how i drive.  In the city, well, easily take 6.5 off that average. 

I'm leaving Monday for another week trip to Nags Head and Hatteras, so, I'll see again what I get.


(http://www.delbridge.net/obx22.jpg)


 
Title: Re: more power = better fuel economy
Post by: murraygmc on October 01, 2009, 07:30:09 AM
wow iv got a stock 305 and only get 13 mpg lol 85 gmc 4x4 3.72 gears (stock)
Title: Re: more power = better fuel economy
Post by: screamin86 on October 08, 2009, 05:33:20 PM
Well i got roughly 10mpg in my 86 k10 with the le9 and all the emissions stuff except cat removed. It has long tube headers single rin exhaust high flow cat and a factory aluminum 4bbl intake. Im hoping with the new 350 that just went in the added power well help bring my fuel mileage up some. I put the same cam in that 350 as is in my 305 that i have in my 82 ta and when i had 3.42 gears and a 2800 converter and th350 i got ab out 18 being very easy on it in town. The truck has a th350 and a 2800 converter and 3.42s (for now) but it has a 350 with about 9:4.1 so im hoping i can atleast get around 16 being very easy