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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: dtaylor7897 on February 05, 2017, 10:04:43 am

Title: 79 Suburban MPG
Post by: dtaylor7897 on February 05, 2017, 10:04:43 am

I get between 6 and 7 MPG depending how my foot is feeling. I feel like this is horrible. 79 K20 Suburban Goodwrench 350, TH400, 4.10s. I'm not leaking fuel and i don't think it's running rich. What could this poor mileage be from?

Drew

Title: Re: 79 Suburban MPG
Post by: Irish_Alley on February 05, 2017, 04:37:58 pm

tires will effect that. but you have to make sure the engine is running efficiently as possible and air tires up. in my 79 k20 350/sm465 and 4.10. i was getting 7 or so with 38x16x16 super swampers. i changed out to military take offs which were 37x12.5x16.5 and it went up to 10 if i kept it under 60

Title: Re: 79 Suburban MPG
Post by: VileZambonie on February 06, 2017, 11:38:15 am

A 700R4 will help

Title: Re: 79 Suburban MPG
Post by: Stewart G Griffin on February 10, 2017, 08:40:54 pm

A K-20 suburban likely weighs about 5400 lbs.   So i think the only practical way to improve mileage is to go diesel.

Title: Re: 79 Suburban MPG
Post by: rich weyand on February 10, 2017, 11:24:53 pm

You should probably be getting 1 to 2 mpg more than that.

Make sure the vacuum advance is connected to manifold vacuum, not ported vacuum.  Manifold vacuum will have vacuum at idle, ported vacuum will not.

Advance the base timing to 14* BTDC.  This is a better setting for engines that do not have emissions controls mounted.  You might even be able to go to 16* BTDC without knocking, depending on altitude, gas, etc.

Title: Re: 79 Suburban MPG
Post by: jeffrey.constance on April 11, 2017, 06:20:49 pm

I get 6.8 to to 7.5 mpg in a 86 C/20 c6p (hd chassis) with stock 454 TH 400 and 3.73 gear .  Stock 235/85-16 tires.  My Dad bought it new and I inherited it in 2009 and that has been the ballpark mpg since new.  It has a rebuilt 454 now and all the smog removed( 2 air inj pumps, efe, etc, removed timing advance to 12 deg BTDC ) has factory s/s header like manifolds and factory duals ( still all orig and the q- jet is louder than the tailpipes) .  Weighs 5260 with 1 tank full and approx 1/2 in 2nd according to scale at local landfill..lol
So


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Title: Re: 79 Suburban MPG
Post by: jeffrey.constance on April 11, 2017, 06:28:45 pm

I would think weight for burban would be higher and so that would cancel the bb/ sb difference in engines along with the 4:10 vs 3.73 gears.  I feel a little better about my mpg.  Hope this did the same for you.   ( I know that I keep looking for a puddle of gasoline under it or a gas trail following it down the road...Lol)


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