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Terrible MPG
#458043 March 31st 2011 5:25 pm
Joined: Oct 2009
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My most recent MPG calculations leave me at 7.5 MPG! Yuck! When I bought the van with old everything I was getting 10-12. Now that the engines rebuilt and I have a new carb w/electric choke and a new mechanical fuel pump I'm getting 7.5.

All the things I've done in an attempt to improve mileage has made things worse.

My rear tires are 10" and I have a flowmaster exhaust. I heard these can decrease mpg, but I swapped those forever ago.

Where's my gas going?


78 Vandura
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Re: Terrible MPG
vanburgo #458049 March 31st 2011 6:17 pm
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Need more info. Engine size? Transmission? Rear axle ratio? Modifications to the motor?

I suspect that when you rebuild the motor you added some go-fast parts. You will need to spend the time dialing everything in to maximize performance.

As a general rule of thumb, on a smallblock motor in a van, you want a carb that flows less than 500 CFM. This basically limits you to small Holleys, the Edelbrock performer that flows 500 CFM, a Holley 4360, or a Quadrajet or Thermoquad. Whatever carb you have, make sure the choke is adjusted properly, there are no vacuum leaks, and that the carb is tuned right for your van. Spreadbore carbs can get better MPG than squarebore, but yo need to keep your foot out of it.

You also want to be sure to have a distributor with (a) functioning vacuum advance; (b) functioning mechanical advance (old grease can harden up and keep the weights from moving); and (c) a timing advance curve that is tailored to your van. Distributor recurving is too big a topic to go into in one post, but you should research it. The timing curve can have a big impact on performance and economy. Also, double check that your base timing is set properly.

You don't want an intake manifold that is designed to make high RPM horsepower. You want low RPM torque. This typically means an intake manifold with smaller ports. I recommend Offenhauser dual port intake, Edelbrock SP2P intakes, and Edelbrock Performer intakes. You want to keep the intake manifold heat operational, and the EGR isn't a bad idea if you are working to maximize fuel economy.

Open element air cleaners decrease fuel economy and performance since they suck in the hottest air in the engine compartment. I recommend running a stock closed air cleaner and rigging up a cold air intake system.

Synthetic oil is a proven way to improve MPG. Run 0W-30 or 5W-30 in a freshly rebuilt motor. Anything thicker is a waste and causes unnecessary parasitic drag.

Clutch fans are also a good idea to improve fuel economy.

Keep your tires inflated to the maximum pressure listed on the sidewall. Fat tires add rolling resistance and will decrease economy.

Free flowing exhaust helps with power and fuel economy. Flowmasters are too loud for my tastes.

Driving style has a big impact on fuel economy. Keep it to 55 MPH on the highway and accelerate slowly. Install a vacuum gauge and drive to maximize vacuum.

Install the factory rated coolant thermostat. Running too cold of a thermostat will keep the motor from fully warming up leading to oil being too thick and corrosive compounds building up in the crankcase.

Those are the items I can think of off the top of my head. Adding things like aerodynamic mods or weigh reduction measures also helps. Be sure to check everywhere for vacuum leaks, and make sure nobody is stealing your gas.


Windows- they're what make a van worth owning!
Re: Terrible MPG
vanburgo #458054 March 31st 2011 6:44 pm
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First off, Reed: If I ever see you in public, I'm buying you a beer or six. Thank you for all the info.

I have a 78 G25 shorty with a rebuilt 5.7 350, rv cam, stock intake manifold, pretty much stock everything internally. 600cfm Summit carb (adapter for square to spread bore) properly running electric choke and an open air filter. Aftermarket fuel pump, all new lines and fuel filter. My fan is NOT clutch, it's on a belt directly connected to the engine. Turbo 350 tranny (which I desperately want to swap to a 700r4). No vacuum leaks as far as I can tell.

I'm changing the oil tomorrow (first time since rebuild). And I do need to inspect the vacuum advance.

11mpg and I'd be a happy fella. I'd expect 14+ with the 700r4. I just wish that for once I spent money on the van and the mpg went up instead of down..



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Re: Terrible MPG
vanburgo #458062 March 31st 2011 7:16 pm
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Another obvious one is the rear tire size will throw your mileage off as odometer isn't reading true.Admitted lowpro tires will spin the speedo/odo faster(hense more miles showing)so you could be getting even worse mpg and that would really suck.Good luck.


79 B300 4x4,80 B100,83 B150.If it's not a van I just don't understand it.
Re: Terrible MPG
vanburgo #458064 March 31st 2011 7:17 pm
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Well, I would recommend ditching the 600 CFM carb, even on a 350. If you have a bone stock motor, I recommend a Quadrajet or a Holley 4360. If you are willing to run the spread to squarebore adapter, then the Edelbrock Performer 500 CFM carb is a fine choice. I recommend putting the closed element air cleaner back on and rigging up the cold air induction to get cool air to the carb.

If you are still running the original distributor, take the time to take it apart and clean and lubricate the gets. You want the weights and springs to move freely. Make sure the vacuum advance pod holds a vacuum and is working right. Make sure it is hooked up to the proper type of vacuum source for your motor (I don't know if Chevies use ported or manifold vacuum for the distributor).

You should be able to find a clutch fan off of a Chevy with a 350 in the junkyard. Those things are everywhere.

Was the cam properly degreed when it was installed? I would make sure that the builder degreed it properly, and, if possible, see if you can't advance it a few degrees to improve low RPM torque.


Windows- they're what make a van worth owning!
Re: Terrible MPG
vanburgo #458091 March 31st 2011 10:21 pm
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You should be getting better gas mileage that what you are getting......
unfortunately with a 350,a stock grind cam,and a good tune up the best you will get driving it conservatively is around 9 MPG.
A nice low end torque cam will wake you stock build 350 up and give it noticeably better performance and MPG.Even then depending on the rear end gear ratio you are looking a around a 3 to4 MPG gain in city driving.
Chevy 350's where never know for great gas mileage.The performance and the build ability they offer make up for it in the long run.....

......Wookee


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Re: Terrible MPG
vanburgo #458116 March 31st 2011 11:28 pm
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My buddy gets 14 out of his stock 350,but only if he keeps his foot out of it. shift

Dont remember what his rear end is.


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Re: Terrible MPG
vanburgo #458188 April 01st 2011 7:56 am
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If you can, have the emissions checked also. Even though you replaced a bunch of stuff, you may not have it all adjusted properly, like timing, etc.

I agree with Reed too. Get rid of the open air cleaner, work the distributor and since you just bought the carb new you may need to jet it way down for the stock application.


Ken Schwarz
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Re: Terrible MPG
vanburgo #458219 April 01st 2011 10:01 am
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Ken, I am surprised that you of all people, didn't mention the most OBVIOUS solution. Chop the top, to reduce wind drag! grin van

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