Worse than expected MPG

[Akabot98]

The truck is an 87 GMC Suburban 1/2 Ton with a 4 inch suspension lift, 35 inch bfg at ko2s and 3.73 rear end. 350tbi engine and 700r4 transmission 4WD. Right now I’m averaging 8-9 MPG on about 80% highway driving, usually cruising around 55-60. I don’t have any crazy weight in the truck either. I’m not expecting 15MPG by any means, but based off my research and experience with other trucks I feel the truck should be getting between 10 and 12 MPG. Thinking going in and putting new rotors and bearings in the front, and switching to a manual locking hub setup. Anywhere else I should look to gain mileage? I’m sure the TBI could use a little tuning but I don’t know the first thing about the EFI. Any input is appreciated.

 

[4WDKC]

smaller tires and remove the lift are going to be the biggest gains. smaller gains will be from headers, tbi mods supposedly, free flowing exhaust..etc.

 

[1987 GMC Jimmy]

Yeah, the tire and lift combo will throw a wet blanket on anything you try to do. I think you have a good gear ratio, but you could do 3.42, and it would still be good. 80% highway driving doesn’t sound like lift warranting driving conditions, but if you have to have it, maybe try a 2” lift on 33s? As far as anything else goes, a tune up would be a good idea. New Delco ignition parts, a fuel filter, a new CTS, and make sure your brakes aren’t dragging. Also, doublecheck that the injectors aren’t dripping or spraying improperly.

 

[yevgenievich]

10-11mpg on my stock 91V2500 suburban with wider stock height tire. Was 13mpg on highway stock tires

The truck is an 87 GMC Suburban 1/2 Ton with a 4 inch suspension lift, 35 inch bfg at ko2s and 3.73 rear end. 350tbi engine and 700r4 transmission 4WD. Right now I’m averaging 8-9 MPG on about 80% highway driving, usually cruising around 55-60. I don’t have any crazy weight in the truck either. I’m not expecting 15MPG by any means, but based off my research and experience with other trucks I feel the truck should be getting between 10 and 12 MPG. Thinking going in and putting new rotors and bearings in the front, and switching to a manual locking hub setup. Anywhere else I should look to gain mileage? I’m sure the TBI could use a little tuning but I don’t know the first thing about the EFI. Any input is appreciated.

 

[bucket]

If you have working auto hubs, they are not hurting your mpg.

Does it run well? Engine light on? Have you set the base timing?

 

[rt66paul]

What state are you in? Different fuel requirements, altitude, pollution control device tunes, etc, make a huge difference.

That being said, squarebodies are not known for great fuel milage. A stock height 1500 suburban with OD (or a short bed PU)might get 13 mpg, but that is it. I remember a 75 camper special 350 my BIL had, it got 9 mph with a giant Dreamer camper on it, towing a trailer full of motorcycles or it got 10 MPG empty. Of course, the fuel was better than now.
Once you have a 2500 or a 3500, huge tires, dually wheels, lifted for more air resistance, your milage may vary. Putting a cold air intake, headers, intake manifold, cam change, etc. usually will make performance better, but doesn't help much with gas milage.

 

[shiftpro]

Your gear ration is living in the wrong time zone. 35" tires and overdrive you need 488 minimum.

 

[shiftpro]

Yeah, the tire and lift combo will throw a wet blanket on anything you try to do. I think you have a good gear ratio, but you could do 3.42, and it would still be good. 80% highway driving doesn’t sound like lift warranting driving conditions, but if you have to have it, maybe try a 2” lift on 33s? As far as anything else goes, a tune up would be a good idea. New Delco ignition parts, a fuel filter, a new CTS, and make sure your brakes aren’t dragging. Also, doublecheck that the injectors aren’t dripping or spraying improperly.

His gear ratio is way off with 35s and overdrive. It's like driving with the brakes on.

 

[4WDKC]

His gear ratio is way off with 35s and overdrive. It's like driving with the brakes on.

Agreed, I had 342s and 33s the truck was a dog off the line and any slight hill on the interstate I ha to give it more gas only to watch the speed fall till it down shifted. I imagine its worse with 35s. Try driving in 3rd/D for a tank an see if it gets better mpg also check this site http://southwestpowersports.com/GEARCHANGE.HTML

 

[AzGeo]

IF the motor/EFI is ALL STOCK, I would do this before spending any real money .

First off, does the "locking converter" actually LOCK UP on the highway ? The tires may be such a load on the motor that the "vacuum switch" never allows the converter to lock up .

I would install a manual toggle switch to LOCK and UN-LOCK the converter . Doing so will REQUIRE you to turn the switch on and off every time you drive the vehicle .

Along with that I would install a "restrictor disc" in the EGR passage . You may need to buy a new aftermarket EGR valve to get these "washers with little holes in them" . (use the smallest hole)

Restricting the amount of EGR flow will raise the vacuum signal, and create more low end torque . (which you have little now)

If these two things (together) don't do what you want, change the cam, timing chain and valve springs, to new aftermarket parts .

A cam with .460"/.480" lift, 220/230 degrees @ .050", on 114 lobe centers, installed at 108 degrees BTDC (+6), will improve the TORQUE more than any other change, and still be compatible with the EFI .

 

[malibuman402]

try a 383 rotating assy. lots more torque.

 

[CSFJ]

time to dust this off again...

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[Honky Kong jr]

time to dust this off again...

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Sonic is better for hearing your girlfriend lol

 

[CSFJ]

Sonic is better for hearing your girlfriend lol

Better than electric? i doubt that very much, lol. Besides, I'm hearing the Sonics aren't as great of a mileage monster as people would think.

 

[Honky Kong jr]

Better than electric? i doubt that very much, lol. Besides, I'm hearing the Sonics aren't as great of a mileage monster as people would think.

But a Priss is heavy. Lol