Disappointing MPG.

[rscam]

I have a 69 GMC stepper. It has a 355 with roughly 300hp with an Edelbrock 600 carb. I have an NV3500 with 3.08 rear gears and 28 inch tires. 60 mph is a cool 1600 rpms and it doesn't struggle to maintain that speed. I just got my speedo working so I checked my mileage and got a disappointing 12.25 MPG. I drove the truck very easy and didn't get the truck over 2200 rpm when shifting. I was hoping for 14-15. Can I tune the Edelbrock for better mileage? I just put it on straight from the box and really only adjusted the air mixture and idle.

[kgt]

ditch the carb and install a rochester q-jet, the smaller primarys will help.

[71swb4x4]

Was that 12.5 city or highway? How many times have you checked the mileage? Variouis everyday driving conditions will affect your mpg's. Don't get discouraged until you have checked that rating for a few tanks.

[70cst]

Be glad you don't have a BB...10mpg regardless of what you do.

[jamesi20]

My dear old dad (rest his soul) always told me the most honest people in the world will lie about 2 things...gas mileage and how long it takes them to get somewhere LOL I've owned a lot of chevy trucks and vans,with 2 barrels ,4barrels and 12 -14 m.p.g. is about all I could ever get out of them.I never owned one new enough to have fuel injection,but I imagine they do a bit better.

[rscam]

It was the first tank. It was country road driving with some small town mixed in. If that makes sense. No expressway and no stop and go traffic either. I would probably rank it as 40% city and 60% hwy. I am going to do some reading about edelbrocks and check the tire pressure. Its not my everyday driver. I have 2 other rice burners for MPG. Its more of a challenge for me now.

[Longhorn Man]

try driving it in 4th and see if the MPG picks up. your RPMs are below optimum MPG range.
Low RPMs don't always mean better MPG. The engine doesn't have to actually struggle to maintian speed, to be under too much a load.
Poor aerodynamics, and an engine that was built with no regards to MPG (by todays standards) will make for poor MPG. To be honest, i am amazed you got as much as you did. These engines simply were not built/designed to cruise at that low an RPM with that kind of load.

[Sinister]

I'd be happy with 12.25, I get 10 MPG with a bone stock 350.

[big_al_71]

If my truck ever gets 10 mpg I would be very happy.

[rscam]

I just recently changed my 3.73 geared rear end for a 3.08 unit. The NV3500 has a 4.0something first gear and with the 3.73 It was a stump puller. I could easily start in 2nd gear. My reasoning was to change the rear axle to the 3.08 so I could use first gear and then have a highway gear. I will try running in 4th gear as you said Longhorn man. That will probably duplicate fairly close the 5th gear 3.73 combo I had. I am also going to check my timing and set the carb a little better that just out of the box.

[cdowns]

the speedometer works?? but is it correct???

[Camper Power]

that doesnt sound that bad, i probably get 10 with a 350, 27" tires and 4.11 rear end

[rscam]

I calibrated the speedo. It stays right on with the mile markers on the road. Thats as close as I can get it.

[clay68c10]

You can try leaning out your primaries and see if it gives you any more mileage. Jet and rod swaps are SUPER easy on those carbs. Just follow the chart in the back of the book that came with it.
And get the calibration kit, it's only like $40 and you get all the stuff you'll ever use.

[BIGJAKE97]

can i ask, how did you get the speedo working? the nv3500 has a electric speedo? thanks, jake

[Indyuke]

I'd agree with longhorn man. You need to be turning at least 2000rpm at 60mph. At 2000-2500rpm the engine is more free reving and in it's proper cruising range with a higher manifold vac. As said before, a q-jet will take better advantage of this as well and give you better fuel economy.

I have a stock 350 with a q-jet, 700r4 and 3.73 gearing to keep the rpms about 2100 at 65, and I've never gotten below 14mpg. Not great, but about on par with my dad's new F250 with a 5.4L

[Bus Ted Knuckle]

you need to install 3:73 gears, your rpm's are too low for good fuel mileage.

[rscam]

I have an electronic speedometer. I just got it installed and working last week. I posted another thread with a picture of it. Its a VDO unit.

[68gmsee]

The first and only time I checked MPG on my worn, old 69 chevy was on a 250 mile trip I took some years ago. I got 14 MPG highway.

It has a 350 engine, 350 transmission, 3:08 rear end with a quadrajet.

[69RAT]

Which vacuum port did you hook up on your carb? I changed from manifold to port side and went from 8-9 to 13+ highway (60-65) in a hilly area on my BB. I'm running 28" tires with a 410 and a TH400. I've got a 750 Edelbrock with a Performer intake. Changing this week to 3.42's hoping for 15 +/-.