gas guzzler???

[tkeduk8r]

Hey fellas... what kinda milage you guys and gals get with their 250-6's.... Just took ine cross country and got bout 13-15... its got a 3 on tree with 3.73's and 30x9.5" tires... Ive screwed around wih my carb enuf and i need to know if im close.... thanks fellas....
--mike

[junkyardjohn]

13 TO 15 MPG IS PRETTY RESPECTABLE. THESE OLD TRUCKS AREN'T ZACKLY FUEL MIZERS & MOST PEOPLE DON'T UNDERESTIMATE HORSEPOWER OR MILEAGE

[Brad]

You should be getting about 15-17 depending on tire size, rearend ratio and other factors. I was using a 3.07 rear and a Weber 32/36 that hasn't been tuned yet and I was getting 17-19. I just went back to the 3.73, so that might change, but once I tune the Weber I should still be around 15-17.

[Longhorn Man]

While Brad and a few others are getting pretty good MPG in theres...most ppl only get about 10 - 13 on the highway. Even with the inlines.

[Huck]

MPG is ususally a factor of gearing and driving habits compounded by modifications. My Dad's old Mercury got over 20mpg with a 390cuin. My C5 vette gets 30+ over the road with a 6speed.
My 72 with a crate 350, 96 chev 5speed, headers, HEI, Edelbrock 1406 and a 308 gets 20+ over the road
while my stock 71 with 350/350, HEI, 308 rear gets like 13mpg. I stay with stock rear tire sizes to keep speedo correct and minimize ride impacts.
Converting to a 5 speed or a 700R4 will greatly improve your MPG if you can stay out of the gas pedal. GORDO gets 20+ in his SWB - 350 & 700R4. So gearing is a primary factor. IF you changed your rear to say a 308, you will have problems pulling hills with the 6 cyl. but, you mileage will probably go to 20mpg. Convert to HEI, and tune it the best you can will give you a couple of MPG. Good luck---Huck

[tkeduk8r]

Thanks guys... at leat i know im inthe ball park.... the only problem is i had had a 1980 K5 with a 250-63sp and creeper 3.73's and i pulled 19 on the highway with that and it was a 4x4... i guess i was expecing about the same outta this guy since it is a 2 wheeler and less weight. oh well, i can also tinker some more... next queston... what kind of 5 speed can i use??? will the lesser BW work outta an s-10.. jus looking for cost since any of the newer 5 speeds may be more expensive.... thanks again...
--mike

[busterwivell]

My 402, 4 speed truck is good for 10MPG. Dropped it off this morning at tranny shop. When I get it back, it will be a 402/700R4. Hopefully, it will do more than 10, but the real reason for the swap is to be able to run freeway speeds. 4.56 rear gears are murder on the freeway!

[mrein3]

The mileage question. I've argued this point before in this forum. There are folks out there who claim their stock suburban with 10,000 pounds of payload and 4.56 rear end gears gets 20mpg but I don't believe it for a second.

I have a stock like 4x4. I say stock like because it has all stock components just not all from the same truck from the factory. My truck has a 350 cu. in. engine, 2bbl carb, HEI ignition, Th350 transmission, and a NP205 transfer case. My tires are stock height. My speedo is differentially GPS corrected to within 1mph.

The way I see it a stock 4x4 gets 10 or so miles per gallon and a stock 2wd gets 13. Lets define "stock" as a V8 and 3.73 rear end gears.

I've seen too many other testomonials in this forum to believe my numbers are out of line with reality.

In my case I drove the truck for 18,734.3 miles with a 3-speed manual transmission and have over 2500 miles on the TH350.

My numbers are:
9.57mpg manual
8.35 mpg auto

Those are average numbers over alot of miles, most of which are towing my 3000+ pound boat. On a tank by tank basis I see 8 to 11mpg. 8 towing, 11 not towing.

Now the data you really need...
Assume you get 10mpg with a 1:1 (stock) final gear ration in your transmission and 3.73 rear end gears.
Just switching to a 3.08 will give you 12mpg.
If you get 10mpg with 4.11 gears going to the 3.08 will give you 13mpg.
You SHOULD get 10% better mileage for each numerically smaller rear end gear you go to.

If you want to mess with your transmission most overdrive transmissions that are commonly available have a 30% better ratio in overdrive. So if all your driving is on the freeway at a constant speed you should see 30% just switching to an OD tranny.

So lets say you are building a truck ONLY for gas mileage. Take a stock 2wd truck and swap the 3.73s for 3.08s. You SHOULD go from 13 to 15.6 mpgs. (13 + (1.3*2)) = 15.6.

Now take the 2wd 15.6 mpg 3.08 geared 1:1 final tranny ratio truck and put in a modern .7:1 OD tranny. You get 30% MORE fuel savings. You SHOULD go from 15.6 to 20.28mpgs. (15.6 + (1.56*3)) = 20.28.

You SHOULD be able to build a 2wd truck that gets 20mpg which is on par with new models.

The only problem is if somebody does all these mods they either burn the tires off where ever they go or they only cruise up and down University Ave. at 30mph and never get into OD.

Another thing to consider is the cost of the above mods. Then divide that by the cost of a gallon of gas. Once you figure out your gallons payback compare that to how long it will take to earn your money back. Then consider the above mods.

Remember you can buy a lot of gas for the cost of a new set of rear end gears and/or a OD tranny.

[busterwivell]

That's true, too.

However, putting in a 700R4 in my truck with 4.56 gears should allow me to travel at freeway speeds, instead of 3000+ RPM at 55 MPH. I'm not looking to improve mileage, just keep up with traffic.

[stelth2002]

I just made a round trip from moscow to pendelton and got over 19 with my truck. I thought that was damn good. The road there is one of the most hillest I have drove in quite sometime. I was expecting 17 as i really need to convert to a 3.07. I never once got pulled down even going up 7-8% grades. Thats what a good inline can do for you. It an numbers matching truck with a build 292 in it.

[busterwivell]

Stopped by the tranny shop on the way home last night. They still think they will be done tonight. the column is switched out, the shifter is removed, the clutch pedal assy is out, the tranny is ready to drop, the 700R4 is ready to put in, still need to shorten the driveshaft and make a crossmember.........can't wait!

[tkeduk8r]

hey stealth.. how did ou build that 292??? Jus curious... and if I were to look for a 5 speed for the old hauler, what should i look for... i could see a medium performance rebulid for mine in the future and if i did what should i do??? My compression on all cylinders is bout 150 across the board.... thanks...\
--mike

[stelth2002]

You could look for a NV4500, that is by far the most popular. I however might look into a ring and pinion change. With the low torque range in the six you can put 3.42s or 3.07s if you want. About the only thing you have to do when you swap to the 5 speed is mount a hyd. clutch. Not that hard, but for me i wanted to keep it orginal. The three on the tree might be a little slow on the start with the worn out 6 and higher gears like 3.07. ??? id do the rebuid and go from there.

[teeitup]

My burb got 8 mpg the first and only time I checked it, 2 inch lift with 33x12.50's on it. 350 4 speed. It has 3.08's in it I think but I have a 3.73 posi getting ready to go in it. Its terrible!!!

[mikep]

mrein3...Thank you

Every time I read about some guy getting 20 miles per gallon with their 350 powered suburban and 4.11 gears I know the feeling . You want to say "BULL****" but the profanity blocker does what it just did. Happens every time.

I was considering going to a overdrive transmission to get better gas mileage but then I realized 90 % of my driving is done under 40 miles an hour . Maybe once a week or every two weeks will I even see a highway. I'm lucky if I even get shifted into 4th gear . In fact the most recent engine I put together for this truck was built with mileage in mind. Its a relatively hi compression free breathing small block with a very mild cam. With the quadrajet I could maybe manage 13 miles per gallon sometimes. The quadrajet didnt have the power I missed so I switched back to an edelbrock 750 and now I'm back to 10 miles a gallon but its got spunk again. Hell, If I'm getting 10 miles a gallon I might as well enjoy myself. The big cams going back in !

[mrein3]

mikep,
I wanted like heck to buy and install a 700r4 when switching to an automatic. However after I ran the numbers for me, even at $1.50/gallon, it would have taken 16+ years for the 700r4 to pay for the difference in rebuild/installation price over the th350. How many 700r4s survive pulling a boat for 16 years???

Don't forget...
If you are running 28 inch tires and switch to 31 inch tires you just improved your final drive ratio by a tad over 10%.

Switch from 3.73 to 3.08 AND do a 31" tire over your 28" stock tire you just gained the 30% on the final drive ratio that a 700r4 gives you. Of course you lose out on the off the line pulling/accelerating part. But then if you have a 4x4 you can shift into 4 Lo and double your ratio for pulling.

Everything is a trade off. Figure out what you want and build it!