very low miles per gallon in Blazer I6 250ci. Why?

[GUILLERMOARIAST]

Hi, i have a Chevrolet Blazer 1980:
- inline 6 cylinders
- 250 ci
- 4x4

It's comsumption level in city is 7 miles per gallon, but at the highway it is 20 miles per gallon.
Why is it happening?

I already have:
- Changed spark plugs.
- washed carburator.
- bought a new battery.

The mechanic said me to change the size of the carburator jet. Is it right?
i will appreciate any comment or sugestion

[VileZambonie]

wow, check for fuel leaks, check for vacuum leaks. Does it run good?

[zieg85]

You should see 14-15 in the city.  My bet is your carb is in need of a rebuilt.  When a 6 is that bad in the city versus that good on the highway it usually is the carb.  My $.02

[Bitzer!]

could it be a lead foot? !!!   

[Lt.Del]

You may have something like 2.70 rear gears....which means it is slow from the take off--and most city driving...therefore causing more acceleration, constantly.  ON the highway, she sips fuel because the rpm's are so low.

That range you state still seems kind of large though. Check for vacuum leaks. Does she idle smoothly?

[Dave Kay]

Hmmm... wondering if maybe the 4 wheel drive is causing your low mpg... is it full-time 4x4? What are the rear axle numbers? How tall are your tires?

I own a '77 Chevy C20 with a bored-over 292, single barrel Rochester w/stock intake and Clifford header, dual exhaust, plus a Corporate 14 bolt w/4:11 gearing and Detriot Locker... (lotta' drag there) and the best I ever got was 16 mpg highway and 9 city and that's if I stayed out of the throttle... cruising at 70 she revs around 3000rpm but pulls really great uphill.

[DirtyLittleSecret]

They did put in TALL gearing for those.  Coupled with the runty 6cyl, I'd say that you are working the motor to death trying to push air!  More specs would be needed (ie: tyres, how many miles on the motor, tranny, weight of right foot, etc).