Better than expected gas mileage

[mikep]

I took a little road trip of nearly 300 miles today to meet up with another board member to do the old cash for parts thing and I was suprised by how good the gas mileage was and I got an added bonus. Now the specs are as listed below but to sum it up its a 350 with an rpm intake, 750 holley , hei and a solid lifter compcams XS 274S cam using 1.6 rockers and 9.5:1 compression and 36 degrees mechanical timing and an additional 12 degrees vacuum and 3.54 gears in a 4500 lb 3/4 ton. This is the first time out on the hiway with the new cam , intake, carb and automatic transmission with 2200 stall converter.

Before I changed all the above in the last month I was getting a pretty consistant 13.2 mpg on the hiway with a performer eps intake, 600 holley , and performer cam and there wasnt anyway in hell that thing would run on less than 92 octane gas. Now I am getting a surprising 12.4 mpg and the kicker is the extra overlap of the hotter cam is letting me run it on 87 octane without detonation. To be honest I was stunned . I wasnt expecting the added fuel efficiency. Sure the mileage went down by 1 mpg which is a good 8 percent or so but it gained a good 40-50 hp in the process and with the cheaper regular unleaded gas its probably actually cheaper to run now. Of course this is all about to change as I'll be going to a bigger set of heads this year and 4.88 or 5.13 gearing . Its no longer my daily ride so its time to have fun.

[lukecp]

I get about the same MPG you get, 12.4, on the highway when i drive about 70 in my truck. I have 3.07 gears and 27" tires. I was only getting 9-10, until i discovered that my odometer wasn't working properly, so i found my MPG on a known distance. I really wish i could get a little better. With the Q-jet i once had i was getting about 13 with 50/50 highway/city driving, but my Edelbrock 1406 dosen't fare quite so well........

[bigjimzlll]

LOL...I remember them days...today I went on a 100 mile trek...foot in it a lot...I used 14 gallons of fuel..the good stuff...oh well

[Brad]

Da**it Mike, I've been reading all the posts on your progress and I'm sooo jealous, quite honestly. I'm getting the same mileage as you right now with the I6 and 3.73, but without anywhere near the power. Oh how I wish I had your engine!!!

[mikep]

The 12.4 was hiway only and was at 70-85 mph. Return trip was a bit lower as I blipped the throttle a bit more . I'm always amazed how fast that big ass truck will run up to 100 mph. Around town I'm looking at around 9 mpg. I do have my foot in it a lot more now though.

[71-longbed]

10 in town with the foot firmly on the floor

[mikep]

Thats where it belongs !

[1969 GMC]

I just went on a roadtrip, and I got 19-20 mpg with the SM465 and 350, but I drove slow (about 50) and didn't mash it alot. Around town I get worse (duh). Still trying to figure out my gas gauge. With slightly less than 1/4 tank remaining on the gas gauge, I'd only be putting 13 gallons in. Aren't truck tanks 22 gal?

[Longhorn Man]

the tanks are about 20 gallons.

[Fred T]

Tank on my 69 holds 19. I learned the hard way, but at least she went dry in the driveway.

I put a Proform upgrade kit in my HEI last weekend. got it from Jegs. Made a huge difference all around. Of course when I dismantled my Hei, I found the vacuum modulator leaking and the bushings on the advance weights were gone. My milage has improved, how much I don't know yet. Idle is far smoother, I got it down to 500 rpm in gear. Running 8*. advance on idle, 37* at 3,000 rpm.

Brad, your 250 should be getting 15 to 19 on the highway. If you haven't upgraded your distributor, look into it. Iirc, the internal parts are the same as the sbc. You should be able to put in better weights and springs, probably even an adjustable modulator. Pertonix makes conversion box if you don't have an HEI. My 74 with the 250, 3 on tree, 3.73 gear and one bbl got 16, while my 80 van with same setup except had a progressive 2 bbl got 19. No they weren't fast, but they got me there. My best milage out of an inline was in my 62 Nova, 194 ci, one bbl and powerglide. That old gal would get 30+ on the highway running 80 mph.

[SCOTI]

Fred T, You got some good #'s w/your 6cyl combos.

I have the same prob w/my I-6 in my beater. Around town it's about 14mpg, but get on the hwy & it drops to about 12+/- mpg. That's w/updated HEI, 275/60's w/3.73's @ 60mph/3000rpm's.

[lukecp]

I know a guy that had a '71 GMC, with a 250, 3-spd, and 3.73 gears. The best he ever got was 12 mpg on the highway. The motor proably had 200k on it, but whats the point of having a I6 that gets the same mileage as a healthy V8 but has far less power?

[mikep]

my last 69 with 3.73 gears and a 250 L6 got 18 mpg and it was tuned pretty well . Ive owned 24 trucks over the years without about a third of them being 67-72 chevies and another third being chevies in general and have a pretty good idea of what they are capable of and not capable of . When I see claims of 20 mpg out of a 4500 lb V8 3/4 ton I dont believe a word of it.

[ChopTop]

The best mileage I have ever got on any carb truck was 21 mpg. 1980 short step, I6 3on the tree, had power steering that was it. Stock engine for year was two barrel, split manifold (special head). 8" chrome wagon wheels with bias ply Cooper 70's. That was solid highway driving on super with factory muffler (dual) and immediate turndowns. Once with wooden topper on and loaded with household good (moving) truck clocked 23 mpg. Of course that brought up an argument on who was the better driver!

[mikep]

I'm always hearing about guys with 3/4 ton big blocks who get over 20 mpg. Must be my driving style. while I cant see a Chevy doing it , I'd expect that kind of mileage out of a Ford truck. Amazing how good your gas mileage will get when you are getting towed.

[Brad]

I was getting 18 when I had the 3.07 out back, but I went to a 70 housing and left the 3.73 in there for now. I'm still looking for a T-5 trans at a reasonable price. Around here, the yards want $300 or more for a 2.8 version. Yeah right!

I did rebuild my Weber 32/36 this past week (running crappy), but discovered a cracked exhaust manifold and need to replace it before I fire it back up to see if I get better mileage.

Fred T, I've got one of those adjustable vacuum cans and spring kit, just have never taken the time to put it in. I really don't know what to look for in terms of improvements or how to test it all once it's in. Will I see noticeable improvement in performance with the I6 or will it just be mileage increase??

[jku72]

Compression ratio has a big effect on combustion efficiency.
A low compression 6 will get worse mileage than a higher compression 350.
Gears are the second biggest factor, or perhaps even bigger than compression ratio.
Next in line is weight, ignition performance, tire pressure, and a whole list of things that gain half a point by themselves but added together yield results.
I have a high compression 350 with hei, backed up by a sm465 4 speed and 3.07 gears, and it gets between 16 and 19 mpg. 19 on the highway, 65 mph cruise. Not too difficult to do.
I never understood why people tolerate crappy gas mileage.
Even my big block longhorn gets 12, and has gotten 14 on the highway. On the other hand, I have a friend with a 65 fo** 3/4 ton, with a 360, its low compression, and that trucks sucks gas hardcore. Like 7 or 8 mpg tops. He still runs points, has 1974 low compression heads, and a puny single exhaust, as well as a auto trans.
In contrast to the guy who says everyone who gets fair gas mileage is a liar, I will say that anyone who gets less than 10 is doing to themselves, through either poor planning, or a hot rod streak running through them. I know I wont run a truck that does worse than my longhorn, I can barely afford to feed it!

Flame away.

[crazy longhorn]

haha! the thing about liars, is the 1st 1 dont stand a chance! (my daddy told me that many yrs ago ). I have seen 14-15 mpg on a sb powered truck pulling @ mid 3 's gear(3.40). with the 3.73, & figured on a 28" tall tire.......you have to roll em easy on the road to see that 14 mpg! My longhorn falls into the other category.......I have a hotrod streak about a mile wide! The old truck is on 1/2 ton suspension, & gets right at 8 mpg(if i drive it easy).....it doesnt seem to matter if in town or on the hyway, its the same Its a lot of fun to play with tho.....specs at the bottom in the sig..............crazyL