Am I crazy to expect more than 10 MPG

[longbeach]

I have a 1986 K20 with a crate engine with fewer than 5k miles. 350/TH400, 4:10 gears, Edelbrock 1406 carburetor. Truck runs fine, Starts fine. Stops fine. Carb is not that old and I tried a brand new one. All I get is 10 MPG, no matter what. New plugs, new wires. Timing is about 10 degrees BTDC. Any ideas?

 

[Frankenchevy]

Taller gearing, trans with lock-up and/or OD, tuning the carb based on AFR & vacuum, tire choice, etc.

EFI is better for fuel economy most of the time, but $2000 takes a while to make up.

 

[gmbellew]

just for comparison, my 1990 v1500 suburban with 350tbi, 700r4, 3.73 gears, and 33x10.5 tires gets 12-13mpg on the highway running ~75mph as long as it isn't too windy. This would largely be driving across the state of Missouri on I-70.

During mostly city driving, I get around 10mpg.

that will give you some idea on what OD may do for you.

 

[Snoots]

With 4:10 gears, good luck on MPG.

 

[dvdswan]

Get a vacuum gage and try to keep the vacuum around 20. You'll drive like a grandma.

 

[Blue Ox]

Not crazy, but perhaps overly optimistic.

 

[bucket]

10 mpg is normal. You can try for 11 with tuning. Overdrive might get you to 12mpg.

 

[bucket]

just for comparison, my 1990 v1500 suburban with 350tbi, 700r4, 3.73 gears, and 33x10.5 tires gets 12-13mpg on the highway running ~75mph as long as it isn't too windy. This would largely be driving across the state of Missouri on I-70.

During mostly city driving, I get around 10mpg.

that will give you some idea on what OD may do for you.

I had the same combo for a while, I could never get over 11 mpg. Even flat interstate travel was a real bitch to do efficiently, as the gearing was taxing on the little 350 and I couldn't keep it locked up in 4th. If it ran unlocked for too long, the trans got hot.

It wasn't worth the 1 mpg for me. I normally would just drop it to 3rd and happily cruise 75 mph and get 10 mpg.

 

[mtnmankev]

Keep in mind, the 350 engine was designed and built to be a serious small block workhorse, it wasn't designed with fuel mileage as a big concern.
And, as cubic inches increase, so does fuel consumption.

 

[gmbellew]

I had the same combo for a while, I could never get over 11 mpg. Even flat interstate travel was a real bitch to do efficiently, as the gearing was taxing on the little 350 and I couldn't keep it locked up in 4th. If it ran unlocked for too long, the trans got hot.

It wasn't worth the 1 mpg for me. I normally would just drop it to 3rd and happily cruise 75 mph and get 10 mpg.

I do wish I had 4.10, and if I ever have a reason to do major tranny/transfer case work, I will strongly consider it. I don't think I'd lose much mpg and it will help on hills.

flat ground and slight uphill are fine and generally the converter stays locked. moderate hills force an unlock and anything much more than that forces a downshift. once it downshifts, I try to keep my foot in it to keep it in 3rd until conditions are right for it to up shift back to 4th. I don't want it hunting and shifting back and forth. if i am facing a strong headwind, it makes it worse and I usually have to slow down to 70-72 to keep it locked.

I do have the factory aux cooler....although no temp guage to verify my temps. but I check the tranny fluid every month and change the tranny fluid every 30k miles, which is every other year, and it never seems burnt up.

 

[longbeach]

Thanks for all the replies. I like the idea of the vac gauge. But, guess I'm stuck with what I have. ****..

 

[Frankenchevy]

Thanks for all the replies. I like the idea of the vac gauge. But, guess I'm stuck with what I have. ****..

If the th400 ever needs replacing, an OD transmission would go nicely with your 4.10s.

 

[longbeach]

Good point thanks. I think I'll go to a straight six at that time also

 

[Grit dog]

Thanks for all the replies. I like the idea of the vac gauge. But, guess I'm stuck with what I have. ****..

Looks like you're "stuck" with a pretty damn nice truck!
Sounds aboot normal to me.
I haven't put many calculable miles on the 86. Same as yours but with a thirsty Eddy carb on top of a stock roller 454 on bigger tires with a lift, but it "feels" like it's at around 7mpg, from the dozen or so tanks of gas we put thru it before I took it apart. (Speedo is off, mostly in town driving, some burnouts, couple loads of beauty bark, etc)
Looking back to dad's '74 C20 454 camper special 4.10s, bone stock in 1980, I recall us filling up for the first drive home after we bought it (IA to IL drive). Remember dad being giddy about having "2 tanks". "Will be able to drive all the way home with this thing without filling up." (I was 8 years old, rode b!tch most of the way and ran the CB!) I remember NOT making it home without filling up and where we filled up. (Rockford IL, the useless info I retain.....)
Fast forward 40 years, google tells me its 376 miles from where we left from to where we lived. Doing some rough math, assuming getting 16-17 gal out of each tank, that truck was right about, wait for it.....10mpg. All highway, back in the 55 saves lives days and we were "flying" at 70mph 'cause I spent the day checkin for Smokies with the truckers on the CB! (Never had a truck with 2 tanks before that, never had a CB and frankly, never had a truck roadworthy enough to make a trip like that.)

And I remember asking why we couldn't take the truck up to WI to our cabin in the summer because it was so much more awesome than moms car. And I remember dad saying, "cause that thing only gets 10mpg, can't afford to drive it all that way!"

Or short story, trade the big block in that truck for the 4wd in yours and I bet it's very similar.

 

[Grit dog]

And yes, the 4.10s are killers of mpg. Our truck is 3000rpms +/- at 70mph (gps. Speedo says 65mph), on bigger tires.