How bad is my gas milage?

[Project1970]

Junkyardjohn - why the Ford 9"? If you're building for fuel economy (and using a 700R4) I don't think you'll be putting obscene amounts of power to the ground...and I've heard that the 9" is pretty inefficient as far as rearends go.

Just wondering, that's all...

FWIW, if anyone's looking for an easy 2mpg boost and doesn't have an HEI yet...get one. Now.

[ckhd]

Originally posted by junkyardjohn
I'VE TALKED TO PEOPLE THAT SWEAR THEY'RE GETTING 20 MPG, BUT I FIND IT ALMOST IMPOSSIBLE TO BELIEVE.

Junkyard, my '69 K10 got 20 (19.5 - 20.2) on the highway (at 65mph-ish). I didn't specify that because i have it torn apart undergoing a frame-off resto. It's engine is in the '68 K20 now. I got those numbers from months of averages. Anytime I would drive out of town, and one hurkin' trip from Eagar to Ogden, UT, I would top it off, then top it off when I got there.

The specs on the '69 were (in case anyone was wondering):
swb K10 fleetside. 4-speed sm465 tranny. Timken (Rockwell) t-case. GM 12-bolt open rear w/3.73's. 31" bias-ply mud-boggin' tires. '79 Camaro 350 (heads too). Flat-top pistons. 4bbl quadrajet carb (personally rebuilt and tuned by your's truly), HEI ignition (from the same '79 Camaro), I can't remember the specs on the cam, but I know I got a grind meant for low-end - I almost never took it to 3000rpm. The cam was timed at 0°. I don't think it had much to do with the mileage, but it also had a steel crank turned .010 under, and a high-volume oil-pump. It had headers and glasspacks.

Now, gears do make one heck of a difference. I said that the cam was meant for low end. I think matching the cam to the situation is key. That engine is now in the '68, it's tuned nicely, backed by a sm465 and a np205 (almost exactly like the '69), but the biggest difference is that the K20 has 4.57 gears. That puts my 65mph rpm up to about 3100rpm. 3100rpm is WAY above what the cam was meant to be cruised at. That makes the mileage drop to about 1/3 of the original mileage... Of course, probably 4 or 5 mpg has to do with the fact that the '68 weighs a lot more.

I'm sticking a 700r4 in the 'burb as soon as Uncle Sam releases my money from his greedy clutches. I don't expect to get 20mpg. But, I am going to carefully calculate my cruising rpm's and also buy a cam that is designed for those cruising rpm's (since this will be my 45-mile one-way everyday commuter). I fully expect to bump it from 10mpg to 15 + mpg.

[jku72]

In response to your inquiry, I am not entirely positive of the cam, but it sure seems either stock, or an aftermarket one meant for low rpm.
I am running a 350, it was rebuilt just prior to me purchasing the truck, it has high compression heads, a little too high for 87 octane if I time it right for mileage, I get a little detonation (just coming in) at max throttle acceleration, somthing I rarely do. I have an edelbrock performer intake, but it is not necessary with my set up, in fact, I might do better with a stock unit. I think that my 19.8 MPG could be improved by going to a headers/dual exhaust setup. I noticed that the trucks mileage falls off radically over 65 MPH. Like at 75 I get 11-12 MPG, which tells me that at that rpm, the engine is being choked up by the wimpy single exhaust. It also explains why the truck got 12mpg with the 3.73 rear gear. I also have gone to full synthetic oil, and have 31 inch tires on the rear, and keep them at 40 psi.
Since I have the 4 speed with the granny low, and I dont tow, I felt like low gear was wasting space with the 3.73, and I hardly ever use it with the 3.08. I am even thinking of going to a 2.67 rear gear to better utilize the low first, and gain a little highway cruise since everybody is now driving 75. I figure if it lugs, I drop to 3rd gear. I could "pretend" my 4th gear was overdrive.

Obviously, this truck is no dragster, but I just putt around with it, and use it for camping, which often means highway miles. No A/C means I get a little warm in the summer, but also means less drag on the engine.

[crazy longhorn]

Most of the 1/2 ton trucks that I drove had 3.73 gears. With a stick, & based on a 28" tall tire 15 mpg from a good running stock sb seemed to be as good as it got@ 60-65mph on the hyway. Hot cams, loose converters, & low gears take a big toll on milage. My longhorn gets 7-8 mpg, & it doesnt seem to matter if in town, on the hyway.......its just a toy that likes its gas!

[Z-72]

Hell, my 72 with warmed up BB is so bad, at the full service pump the othere day I told the attendent to fill her up and after a bit he tapped on my window and told me if I wanted it filled up I would have to shut it off.LOL

[breeh]

Well this thread is making me feel better about my frequent trips to the gas pump. My 68 4x4 gets 10mpg. This is uphill, downhill, headwind, tailwind, whatever. I am running a 700R4 with a 427 big block.

[72jimmy]

Ya know I'm getting in a little late on this post but it is my theroy that the biggest reason that any v-8 gets poor mileage is that the factory does not do enough to tune them in the first place. Several cars that I or a bud of mine have built actually got better mileage with a hot cam, headers, manifold ,carb etc. All we were trying to do was make more HP by increasing the efficency of the engine. The side effect is that under cruise conditions the engine was able to more completely burn the fuel it was given therefore better mileage. Cruise is one thing but when ya get on it or drive it impatiently in stop and go traffic mileage decreases exponentially. So it's all about how you drive it. me thinks.

My 72 Jimmy gets somewhere in the neighborhood of 14-16MPG - the way I drive. It has a fresh 350, 350 trans, 4.10 gears and 35" mud tires (BFG MT). I am amazed on one hand but then I dont drive around like my house is burning down so that is a BIG factor in the MPG rating. This same truck gets 8mpg when I am in a good mood and drive it for fun not caring that I'm "hauling ass to the red light" (which is how most drivers drive). Make sense? My plan for the future is to go with a 383, custom EFI, 700R4 and 5.13 gears. I am pretty confident that I will have 20MPG after this conversion - again the way I drive.

The first time I saw the hot rod effect on MPG was in 1979. My neighbor got a brand new mustang cobra (what became the GT later on) and after a year or so to let the new car feel wear off he started with normal hop up stuff. Cam, headers, intake and carb. When it was a stock two barrel 302 it got 17MPG on the highway. After the hopup we saw 22MPG under the same driving conditions. Now y'all forget that I used a ferd for an example - this stuff applies to all engines regardless of manufacturer.

For us in the real world running a 350 Chevy, a cam that specs out to something like .490 / 225@.050 with headers, and decent dual-plane and TUNED Q-jet is all it takes to at least get high teens :p

I think that for maximum fuel efficiency we need the factory to produce hot rodded small displacement v-8 engines. Like 200 CID with high flowing heads and decent intake and EFI system, real 2.25" free flowing exaust, etc. THen and only then will we see big american cars in the high 20's MPG.... they may not have all the performance we would like but we need to make our daily drivers more efficient for our environment and wallet.

My .02 hopefully spent wisely

[1badz]

I have a 70 4x4 with a 4 inch lift,35 goodyears,4:11 gears,400sb,350 tranny,hei,headers,aluminum intake,with a q-jet carb of a big block with 74 primary jets and the best I can get is 5-7mpg,I thought something was wrong, but I see the rest of your trucks aren't much better,now I don't feel so bad.I have a 96 vortec 350 ext cab 4x4 that I am currently getting 18.9mpg driving it 30 miles round trip to work everyday.

[junkyardjohn]

PROJECT1970--I PULLED A FORD 9" OUT OF A 76 LINCOLN MKIV. IT IS A DISC BRAKE REAR END THAT I REBUILT & PUT A POSSI CENTER SECTION IN. IT ALSO HAS A 5 ON 5 BOLT PATTERN.(SAME AS OUR TRUCKS)NOW BESIDES BEING BULLET PROOF & PRACTICALLY UNBREAKABLE, I THINK IT'S A PERFECT SWAP IN ONE OF THESE TRUCKS, FOR ALL OF THE ABOVE REASONS.

[Project1970]

Disc brakes, I see. I figured as much.

[minibike]

I'm hoping for 18 on the highway with the LT1/4l60E combonation. There is a guy in town with the same engine in a older chevy panel truck getting 18 on the highway running at pretty high speeds. I'll let you guys know in may...

[CE20934]

13 (specs below). I'm happy with that. Beats the bus's 5.

[mooneyes]

...just a thought but sometimes driving "gently" is worse for gas milege..with the performer cam intake etc. your power range is slightly higher than a stock motor...not much but i've found out with alot of trucks and cars i've owned especially automatics instead of putting around and letting the tranny get in high gear real quick to shift it manually and bring the rpm's up a little before shifting...just a thought its worked for me many times

[Southpa]

I worked mine out at less than 8 mpg , by gas consumption. If I get my carb (Q-jet 4MV) rebushed and rebuilt do you think it will improve much? Of course, it depends on how loose the throttle bushings etc. are right now, but I've been told it needs to be done. Also, I suspect I might have the wrong speedo gear installed in the trans so maybe my calculations are off to begin with. Either way, my 1970 - 350 w/ 3.07 gears likes the gas.

[1badz]

I tried a holley750 vac secondary carb on my 70 today it didn't matter. I went 30 miles and burned .25 tank like it was candy.So I don't know if a different carb will help you or not.There's always hope.