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Old 01-27-2008, 07:56 AM   #1
chevymotocross
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Better fuel mileage from 87 305 TBI?

I have an 87 silverado 2wd swb with the 305 tbi and 700r4 transmission. It also has p265/75/15 tires. True dual 2.25'' exhaust from manifolds back, with high flow cats and dynomax mufflers. There is a new EGR valve installed. I only average 13mpg with a mix of highway/city sometimes pulling an open 5x8 trailer with 2 dirtbikes. If I'm on the highway, I try to keep it under 75. In the city driving, I drive with a light foot. Engine, although has high mileage, is in good tune. Any tips on getting better fuel mileage with the 305 tbi? I was hoping to get an average of 16?

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Old 01-27-2008, 10:20 AM   #2
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Re: Better fuel mileage from 87 305 TBI?

I have an '87 305 TBI also and am getting about the same mileage, although I currently have a 3-speed auto tranny. I am going to get the injectors cleaned by these people: http://www.injectorservices.com/, and rebuild the throttle body, make the fuel pressure regulator adjustable and crank it up a bit. I also plan to replace the oxygen sensors, plugs, wires, cap, rotor. Hopefully this will pick up a few mpg. Or I could just wait for you to do it and see how it turns out! I think 16 mpg is very attainable and I've heard of people getting 20 mpg with your combo.
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Old 01-27-2008, 11:05 AM   #3
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Re: Better fuel mileage from 87 305 TBI?

What gears do you have?
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Old 01-27-2008, 07:18 PM   #4
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Re: Better fuel mileage from 87 305 TBI?

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What gears do you have?
Not sure on the gear setup, but I will say it has to be some radical highway gear, atleast 3.00. One thing I don't like, and this is why, even with only 2 dirt bikes and a little 5x8 trailer, I keep it out of OD. If I keep it in OD with such a small load, it hunts in and out of OD. Even with no load, it's hard to keep it at 65, since with the slightest uphill, it loses speed quickly and downshifts. I eventually plan on lowering the truck 2.5/4 and swapping the big 265/75/15 tires with 255/70/15 tires.
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Old 01-27-2008, 08:06 PM   #5
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Re: Better fuel mileage from 87 305 TBI?

Slow down. 68 vs 75 will give 1-2mpg. Cruise rather than footing it is good for 1/2mpg (keeps yer foot out of it). Tonneau or low cap is good for 1-2mpg.
I made my 4.3 700R4 3.08 P235-75R15 truck get 23mpg. 16 should be easy on the 305.
Slow down to 65 towing, will reduce the hunting and aero rsistance. Wind resistance is a square, double speed, 4x resistance. Best mpg is at 54-65mph, where the factory set the truck up for.
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Old 01-27-2008, 08:53 PM   #6
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Re: Better fuel mileage from 87 305 TBI?

16 is within reach but that 305 will have to be running in what I call the sweet spot as far as rpm is concerned. Around 2,000 rpm at the usuall highway cruise speed should net you the best mileage. You can use gear ratio calculators to figure what it's turning now and see how to get it to where it needs to be. www.richmondgear.com has a couple of calculators you can play with. It is a big help.
You say at 65 the tranny hunts for a gear. That's a real good sign the truck is geared too high. My 88 with 305, OD trans and 2.73 rear did the same thing. At 70 it was turning way under 2,000 rpm. Dropped it out of OD and rpm picked up and it pulled much easier. My guess is your's has a 3.08 gear and would benefit from a change to at least a 3.43 if not a 3.73 if you are going to the 255/70/15 tires. Take a look at that richmond gear calc and play around with the numbers and see if you can't better your combo.
Also pm piecesparts about his combo. I think he is reporting in the neighborhood of 20 with his 305.
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Old 01-27-2008, 09:17 PM   #7
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Re: Better fuel mileage from 87 305 TBI?

Gentelmen. This may be a case where more=less. I have an 84 K20 with a 350M engine fed by a quadrajet carb and rear axel ratio of 3.08. Trans is a 4 speed "rock crusher" Tires are 235-85R 16. Most of the time vehicle is empty (7100 LBS) but I do pull a 8 X 16 utility trailer sometimes. I admit I am a light footed driver and run 60 to 65 on the interstate. When I'm empty I get between 17 & 18 MPG. When I pull the trailer (7000 LBS GVW) my mileage drops 13 to 14 MPG. Total combination weight loaded is 15,600 LBS. The 305 doesen't have much torque so it's working harder and using more fuel as a result. One has only to look at a semi to see what torque can do. 80,000 LBS and around 10 mpg.
Don't push it so hard. Like d13 says. Also take into account the terain. Hills, even little ones will kill ya. Is the TCC working correctly? If the CE light doesen't come on the engine is working correctly as are all the related systems. jim
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Old 01-29-2008, 03:17 AM   #8
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Re: Better fuel mileage from 87 305 TBI?

I had an 87 k10 a couple years back that cam with 3.08. I broke those axles and swapped in a set of axles with 3.42 and my mileage went up to about 16-17 from 13-14. My motor was lugging and the 3.42 help it out a little. Also check the tire sizes, I think factory tires were 235/75/15, get a lighter tire for less rolling resistance. Grease all u joints, change fluids and gear oils, if you have a bug shield take it off and you will get close to 1 mpg gain.
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Old 01-29-2008, 04:21 AM   #9
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Re: Better fuel mileage from 87 305 TBI?

Quote:
Originally Posted by James McClure View Post
Gentelmen. This may be a case where more=less. I have an 84 K20 with a 350M engine fed by a quadrajet carb and rear axel ratio of 3.08. Trans is a 4 speed "rock crusher" Tires are 235-85R 16. Most of the time vehicle is empty (7100 LBS) but I do pull a 8 X 16 utility trailer sometimes. I admit I am a light footed driver and run 60 to 65 on the interstate. When I'm empty I get between 17 & 18 MPG. When I pull the trailer (7000 LBS GVW) my mileage drops 13 to 14 MPG. Total combination weight loaded is 15,600 LBS. The 305 doesen't have much torque so it's working harder and using more fuel as a result. One has only to look at a semi to see what torque can do. 80,000 LBS and around 10 mpg.
Don't push it so hard. Like d13 says. Also take into account the terain. Hills, even little ones will kill ya. Is the TCC working correctly? If the CE light doesen't come on the engine is working correctly as are all the related systems. jim

Jim is getting there. You need more torque in the usable rpm range you normally drive in. You need the throttle open much farther with the 305 than you would with a 350 or 383, because they are making more torque. You want to increase torque and you can do that with the 305, but it will take a bit of effort and some new parts.

A quick and effective way to bump the grunt of a 305 is a head and cam swap. A set of Vortec heads drilled so you can use your stock manifold, and a cam that makes its power from idle to 4500/5000 rpm. A cam like the Comp XE264, nothing radical but capable of getting the grunt out of your tiny engine.

The engines in the 80s werent set up for mileage nor power, they were set up for emissions and everything was compromised and crutched to make them pass. You can improve the efficiency and torque of your engine and have it burn cleaner with the right selection of parts. The 87 454 in my swb gets 8-10mpg because it is neutered from the factory. All it really needs is an intake, cam, and valve spring swap. It should have no problem pulling my car trailer, with my roadbox and tools in the bed, and the car on it once its done.

If I can get 22-25mpg from a 450hp/500ftlb 455 in a 4200lb 70 GTO, with a Qjet and 2004R trans, you should be able to get 18 with the 305 if you get it to grunt. The GTO would loaf along the interstate at 1700rpm, wouldnt need to downshift up any hill or to pass, it ran on 87 octane and had 2.93 gears with 28" tires. make the grunt in the RPM range you drive in and you will get better mileage. FYI the aero on a 70 GTO isnt much better than my swb, and with that setup it would run 13.50s @105mph all day long.

With a truck you want torque, hp is nice but does little for you.
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Old 01-29-2008, 04:36 AM   #10
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Re: Better fuel mileage from 87 305 TBI?

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I had an 87 k10 a couple years back that cam with 3.08. I broke those axles and swapped in a set of axles with 3.42 and my mileage went up to about 16-17 from 13-14. ......
There's your answer right there. Since a stock 305 is low on torque, trying to get it up to speeed with what may be 3.08 gears or higher is killing your mileage. You might even have 2.xx gears which really suck gas until you are going at a steady speed.

I suggest you verify what gear ratio you have, and consider going to 3.42's. They are an awesome gear for moderate low end torque, and a decent cruising rpm.
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Old 01-29-2008, 05:02 AM   #11
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Re: Better fuel mileage from 87 305 TBI?

I havnt figgured out my mileage on paper but im guessing im getting 20mpg with my 87. I can go twice as far on a tank of fuel with it compared to my other truck. Its a FI350, 700R4 and 373 gears.
Seriousely I think you need a bigger motor and lower gears!
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Old 01-29-2008, 07:08 AM   #12
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Re: Better fuel mileage from 87 305 TBI?

Any way to decode the vin to figure out the gear ratio? The sticker in my glovebox is completely faded. I highly doubt it was changed since I bought it from the original owner.
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Old 01-29-2008, 07:33 AM   #13
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Re: Better fuel mileage from 87 305 TBI?

Tune it http://www.cfm-tech.com/GM_tbi_tuning_tips.htm

Oh, did you know the 86 Qjet was rate 1 MPG better than the TBI!
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Old 01-29-2008, 06:51 PM   #14
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Re: Better fuel mileage from 87 305 TBI?

Chevy: Glove box sticker won't tell you what you need to know. you need to roll underneath and look at the top of the right side axel tube 4 to 6" away from the pumpkin. You will see a series of numbers and letters. Record this info and head to your local Chevy store and they will decode it for you. If you can't pull up the numbers or the rear has been changed and\or Parts can't find the numbers, you can figure out the ratio yourself. Pop the pumpkin cover and look for stampings on the edge of the ring gear. Someplace in the number strings you will see something like 15:41. Those are the numbers you want. 15= number of teeth on the pinion gear. 41= number of teeth on the ring gear. Devide 41 by 15 and you have the ratio. In this case you come up with 2:73's. jim
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Old 01-30-2008, 06:48 AM   #15
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Re: Better fuel mileage from 87 305 TBI?

Mine gets 15.5mpg at 80% highway and 20% city. I just checked it the past week. I have gotten 22.5mpg on my last trip to maggie Valley running 65mph. BTW, mine is stock with 3.08 gears.
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