Any secrets for a bumping MPG on 1998 305?

94K3500PROJECT

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Remove as many restrictions from the exhaust system as you can. High flow cats, a free flowing muffler, ECT. big tires burn more fuel. A well tuned engine that runs properly will also burn less fuel. Then there is gear ratio. The faster the RPMs the more fuel you will burn. So 410 gears will burn more fuel than 308s will. Add to all of this driving habits and road conditions, all play a part in MPGs. I went from 342s to 373s in my truck, and yes, I am burning more fuel. If you want good fuel mileage, pit some 273 gears in the rear end, but it will be a real dog from a dead stop.
There will be a sweet spot in engine vacuum at any given speed that produces the best MPG.
Throttle position will dictate vacuum.

Going to a numerically lower gear won’t automatically save fuel.
In fact the opposite can be true-and that’s why lifted trucks with larger tires see a huge gain in MPG by going to numerically higher gears.
 

barry_g

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I drive a 96.. 305, 5 speed.
it is right by the window sticker for 25 years..
20 in the summer
really cold starts drops it down.
very impressed. one of my favorites after 23 vehicles.
going onto 8th year.

I just redid spider assembly (my fault), egr and ignition
gained a half point in vacuum at idle (there was bad intake gasket after all - but not enough to bother)
I also have a low CPI cat
no ECM codes.

still the same mileage as when I bought it.
its about 15-18 overall..

check live data for trims.. if they are approching 7+ at idle, the injection is getting sloppy.
I'd peronally opinionate the automatic trans.
I actually gained a little putting synthetic in the diff.
no dramatic miracles.
 

scottydl

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His mileage is around town is pretty much to be expected IMO… low teens for stop-n-go city driving and lots of idling in hot months. Fast acceleration and fast braking ruin mpgs more than anything else, so lengthen those processes as much as possible. The more $$ you spend on “upgrades” to improve mileage, the less chance you’ll ever make that money back in saved fuel unless you drive a TON of miles. Which would require mostly highway, and then your mileage would be better anyway!
 

L31MaxExpress

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There will be a sweet spot in engine vacuum at any given speed that produces the best MPG.
Throttle position will dictate vacuum.

Going to a numerically lower gear won’t automatically save fuel.
In fact the opposite can be true-and that’s why lifted trucks with larger tires see a huge gain in MPG by going to numerically higher gears.
When I went from 3.73s to 5.13s when I had the 9.5" 14-bolt, I gained mpg around town and lost less than 1 mpg highway. Gained alot of pulling power from a stop towing trailers as well. When I put 4.56s in my Hemi Ram I got better MPG than with the factory 3.55s and it also towed much better. I had two Titans as well and both had the towing rear gear rather than the highway gear and both did extremely well on fuel. These manufacturers undergear trucks that actually work.
 

94K3500PROJECT

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When I went from 3.73s to 5.13s when I had the 9.5" 14-bolt, I gained mpg around town and lost less than 1 mpg highway. Gained alot of pulling power from a stop towing trailers as well. When I put 4.56s in my Hemi Ram I got better MPG than with the factory 3.55s and it also towed much better. I had two Titans as well and both had the towing rear gear rather than the highway gear and both did extremely well on fuel. These manufacturers undergear trucks that actually work.
I agree 100%
3.42 wasn’t enough gear on a 4x4 with 265’s
Even 3.73 wasn’t enough if you do much towing IMO.
 

454cid

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Years ago, when I added the valance under the front bumper, I picked a tiny bit of mileage on the highway. I don't recall how uch, but I wasnt to say it was like 1/2 to 1 mpg... not a lot, but I was driving 100 miles a day, and calculated mileage for every tank, for awhile. That was with my 1-ton with 454, and 4.10 gears.
 

Ksanete

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Smaller wheels get you going easier, thus less gas used. noticed on my dad's 96 tahoe and his '80 k10 (all with the same engine) that mine (89 c1500) got TWICE the mpg (14 city with A/C on) compared to his. My truck has the stock 15s and the tahoe had i think 17s, k10 on 16s but with fat offroad tires.
 
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I've read through all these responses, and would like to note my findings:
Don't drive out of tune (keep it running on all eight with a clean airfilter)
A free flowing muffler (non-chambered) is preferred to the stock ****
If most of your driving is in town you should run a 3.73 or 4.11 axle ratio
Computer tuning can improve mileage, but you must know what you're doing, else you risk making mileage worse
Don't floor the accelerator pedal
If you disagree with any of the above and cannot stomach sub-15-ish-mpg, buy a commuter vehicle - you'll get a multi-vehicle insurance discount and if chosen wisely 30+ mpg and none of the above point matter
Now build the next gmt400 every gearhead robs ideas from for their buildup
 
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Nah, seriously though. These trucks ride smoothly and are relatively quiet inside. They're light years ahead of the square bodies and don't have all the electrical gremlins of the gmt800. Find the best example of a gmt400 in your area and tweak it for your purpose. They're great vehicles.
 

L31MaxExpress

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I've read through all these responses, and would like to note my findings:
Don't drive out of tune (keep it running on all eight with a clean airfilter)
A free flowing muffler (non-chambered) is preferred to the stock ****
If most of your driving is in town you should run a 3.73 or 4.11 axle ratio
Computer tuning can improve mileage, but you must know what you're doing, else you risk making mileage worse
Don't floor the accelerator pedal
If you disagree with any of the above and cannot stomach sub-15-ish-mpg, buy a commuter vehicle - you'll get a multi-vehicle insurance discount and if chosen wisely 30+ mpg and none of the above point matter
Now build the next gmt400 every gearhead robs ideas from for their buildup

One other thing, with an automatic you want to accelerate hard enough to get the torque converter to couple well in the lower gears. I accelerate hard enough to get the engine up around 2,000-2,500 rpm before it shifts into the next gear. Get it into 3rd or 4th gear as soon as possible and going fast enough it locks the converter up. Driving around at low rpm slipping the converter only builds heat in the transmission and waste fuel.

One thing I do is enable the shift lock option to keep the converter locked during the upshifts. I shift 1-2, lock the converter a couple mph above the 1-2 shift speed and then keep it locked on subsequent upshifts. Worth 1-2 mpg around town and gives the direct coupled feel of a manual trans. Because customers would complain of shift feel they leave the converter unlocked until 4th gear and even worse incorperated PWM and partial lockup into the strategy. I disable PWM and use the converter as unlocked and locked. My grandmothers last car was an 03 Alero with a 3.4. I changed the shift and lockup strategy, was able to lean out the PE a little as well as bring in a little more timing and the car got 3-4 mpg better around town. That Alero had a severe case of unlocked converter mpg waste. It would shift 3-4 as early as 25 mph and sit at 2,000+ rpm until it locked the converter around 50 mph. I honestly waited a week to even ask her if she had noticed any difference and she said no other than she was getting another day of driving out of the tank. Since my grandmother drove with a relatively light foot and the 3.4L had good power in that car, I really leaned into the early light throttle upshifts and quick lockup after the 1-2 shift. It almost stack shifted 1 to 2 to 3 at 15% throttle. Like cross a 4 lane wide intersection and it was in 3rd gear at 25 mph. Anything 1/2 throttle and beyond though and that car would get with it, squal the tires and go! My younger brother and I took my grandmother to lunch one time though and he was kind of lolly gaging in the right lane at 10-15 mph under the speed limit. Road was empty but my grandmother must have been hungry. She told my brother, step on the foot feed some more, my car will go alot faster than this!

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