In my experience with the 1989 Chevy 4wd i owned you MPG expectations are close but don`t be surprised if less. Mine usually averaged 14mpg on highway, got 16 once. Around town it averaged 10-12. Of course i have been told i have a heavy foot, but i don`t think it will vary much more. I have the same engine, a 5.7 with the TBI in my 57 and it usually averages between 16-20 mpg.
Terry

 

You will see 15 mpg combined most times maybe little less or more. The 93/94 were the TBI motors. Not power houses but work well.

You can still get tuned chips for them that help, best thing is a tuneup.

Also how many miles are on it?

As for a carb its violation of US federal law to add a carb to an EFI motor in its original vehicle. You my want to check you local or country's laws before doing that, I am sure they have something similar up north.

 

Hmmm...I was just re-reading this, and realized I wrote 5.7 and 454...which is what my BIL told me (he is not a really big car guy...more of a turn key and drive type person) wouldn't the 454 be a 7.4L?

I don't need a power house, but good/reliable engine I'd be very pleased with
I've read about the tuned chips, intake, high flow cats, exhaust, taller tires...and wonder what I should start with to see how/if she improves little by little.

5.7 TBI in a '57 getting between 16-20 mpg sounds decent to me! :tu

 

Hmmm...I was just re-reading this, and realized I wrote 5.7 and 454...which is what my BIL told me (he is not a really big car guy...more of a turn key and drive type person) wouldn't the 454 be a 7.4L?

I don't need a power house, but good/reliable engine I'd be very pleased with
I've read about the tuned chips, intake, high flow cats, exhaust, taller tires...and wonder what I should start with to see how/if she improves little by little.

5.7 TBI in a '57 getting between 16-20 mpg sounds decent to me! :tu

454 is 7.4L
350 is 5.7L

Either one is not a power house the TBI 454 aren't big power houses either so don't expect much out of it.

First thing I would do is a tune up. If not done by the BIL. So plugs,wires,cap, air filter. I would also run some seafoam in the gas tank as well.

Check out this place http://www.tbichips.com/

Not much in the way of intake there are some out there though. Then do exhaust and tune. Taller tires could hurt MPG depending on what gear ratios are. You should beable to ID them look in the glove box for the RPO code.
GT4 was most common in those trucks.


GM Axle Ratio RPO's
RPO Ratio
GM1 2.59:1
GU2 2.73:1
HC2 2.76:1
HE3/G44 3.07:1
GU4 3.08:1
GU5 3.23:1
GU6 3.42:1
GT4 3.73:1
GT5/GT8 4.10:1
HC4 4.56:1

 

Thanks for the feedback...some decent info on the tbichips site

Still on the fence...

 

My 1987 TBI trucks drink gas like I owned an oil well. If I want to save fuel I break out my 72 truck or Blazer with a Q-jet

 

I had a 1992 reg cab shortbox 2wd 700r4 with 3.08 gears with the TBI LO5, yours is 200hp/290or 300 lb.ft torque.
I got about 450kms from a tank in all around driving from the 90 liter tank.

I got the exact match to this truck but it had a Vortec 350 L31 255 hp@4600rpm/330lb/ft torque@2800rpm, sam egear, same trans ratios, but ised Central Sequential Fuel injection (CSFI-Vortec) and I got an extra 100-125km from the same sized tank.

I think 18-19mpg is very optimistic. IMO The TBI 454SS in 1990 made 230hp/385 lb/ft 3.73 gears and th-400 trans had an epa rating of 9mpg city 10 mpg highway.
1991-1993 454SS 255hp/405 lb/ft torque, 4.10 gears with 4l80e O/D trans rated at 10 city/11highway.

I had a 1989 Caprice with the TBI 305, with the 200r4 OD trans and 2.56 gears and I got a measured 25mpg from her highway.

The TBI 305/350 L03/LO5 engines in the trucks use flat tappet hyrdrailic cams, the Caprice/Vette and F-bodies all got the roller cams which add some economy.

peace
Hog