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305 TBI gas mileage?

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Old 10-09-2017, 06:38 PM   #1
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305 TBI gas mileage?

So Iím hearing a lot of people say theyíre getting mid to high 20s mpg, in their 305 tbi cars. Mines a 1991 firebird with a 305 tbi, it has 145k miles on it and has brand new injectors, fuel filter, completely rebuilt tbi, spark plugs, ignition wires, ICM, distributor cap & rotor. Mine gets consistently 14-16 mpg. Iíve never gotten more than 16 and never got less than 14, so how are people getting these higher numbers? The majority of my driving is high way driving too.
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Old 10-10-2017, 06:46 AM   #2
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Re: 305 TBI gas mileage?

Got 2.73s with overdrive doing 55 on highway? I don't know what my car got stock, when it got to empty I filled it I'm sure some inflate numbers too.
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Old 10-10-2017, 09:10 AM   #3
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Re: 305 TBI gas mileage?

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Originally Posted by Mattkovitch View Post
So I’m hearing a lot of people say they’re getting mid to high 20s mpg, in their 305 tbi cars. Mines a 1991 firebird with a 305 tbi, it has 145k miles on it and has brand new injectors, fuel filter, completely rebuilt tbi, spark plugs, ignition wires, ICM, distributor cap & rotor. Mine gets consistently 14-16 mpg. I’ve never gotten more than 16 and never got less than 14, so how are people getting these higher numbers? The majority of my driving is high way driving too.
That's not far off from what I get with my 350 K1500. I'd expect better from an F-body. The F is lighter, smaller engine, and more aerodynamic.

1. What is the cranking compression of all eight cylinders?
2. Restricted exhaust (Plugged catalyst, crushed pipes, damaged inner pipe wall, etc.)
3. Brakes dragging? (Stuck caliper pistons, seized emergency brake parts, etc?)
4. Ignition timing set properly?
5. Hilly terrain? Hills will kill fuel economy.
6. Cruise control working properly?
7. Tires inflated?
8. Transmission has four gears + converter lock-up? Or five manual gears?
9. O2 sensor failed?
10. What does the scan tool show for short- and long-term fuel trim?
11. Knock sensor retarding timing?
12. Coolant temp sensor failed?
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Old 10-10-2017, 09:43 AM   #4
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Re: 305 TBI gas mileage?

Even with 3.73 gears I'm getting over 25mpg out of mine. Is the car running sound?
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Old 10-10-2017, 11:58 AM   #5
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Re: 305 TBI gas mileage?

Everyone is going to have different values, relating to your driving conditions. If i purely drove freeway, my 350 with EBL can get 28mpg cruising at 60mph. Yet if i drove at 85mph same car, it would dip VERY fast.



If im WOT every street light i get 6mpg...

My Average for 90% city 10% freeway i get 11-12mpg. (350 TBI that is)
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Old 10-14-2017, 03:47 PM   #6
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Re: 305 TBI gas mileage?

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Originally Posted by Ghettobird52 View Post
Everyone is going to have different values, relating to your driving conditions. If i purely drove freeway, my 350 with EBL can get 28mpg cruising at 60mph. Yet if i drove at 85mph same car, it would dip VERY fast.



If im WOT every street light i get 6mpg...

My Average for 90% city 10% freeway i get 11-12mpg. (350 TBI that is)
I could put my 5,300# G20 van to the floor out of every stop sign and traffic light and on every ramp when it still had a mostly stock 305 and still average close to 14 mpg. Running 75 mph down the highway it would get 17-18 mpg consistently. That was with a 1983 carb 305, 601 heads, 204/214 @ .050 cam with TBI conversion, doug Thorley tri-y headers, exhaust, and some chip tuning. I later rebuilt the same engine, changed the intake manifold, changed the camshaft to a stock 350 camshaft, and it would get 22-23 mpg running 70-75 mph with lean cruise enabled at 16:1 a/fr

Last edited by Fast355; 10-14-2017 at 03:57 PM.
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Old Yesterday, 09:16 PM   #7
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Re: 305 TBI gas mileage?

Interesting subject, and some interesting results.

I'm one of those nuts that keep track of every gallon of gas he buys. No, I don't always fill up at exactly the same pump...there are some MINOR variable. This is mostly in town, a few highway miles in there. And it's almost always driving a little faster, and accelerating a little harder from every stop than I ought to. There are NO economy runs on this page...no all highway, cruise control set, getting the best mileage I can runs.



So of the lowest averages are from a trip to the drag strip, or the Woodward Dream Cruise (4 hours of idling through traffic), or short fill-ups, and the next one is unusually high. Let me save you the trouble...that's an average of 18.1 for all of those.
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