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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.
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?
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)
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
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.
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.
Definitely interesting results! It is also interesting to see how the march of technology has helped power and mpg. You can have much more powerful engines getting better mileage in heavier vehicles now days.
I actually kept a log on Fuelly.com for my 2012 Titan when I owned it. Logged more than 15,000 miles worth of driving. That included everything from towing heavy trailers, often loaded to near payload capacity, to driving into the mountains, to daily stop and go traffic. My highway speeds are higher than most as well, ranging from 75-90+ mph.
I have not kept a log on my current car, but the car has its own log and I have stayed at an ECM calculated 19.6 MPG for the past 24K miles, since I reset the total accumulative. The ECM was accurate enough that I have not kept a log outside of what the car shows because it was accurate within +/-0.2 mpg the times I checked. 2011 M56S weighing 4,400 lbs empty making close to 500 HP at the crank. With myself and 2-3 passengers I am often rolling down the road at over 5,000 lbs.
When my white 91RS was a 305 TBI 700r4 w/2.73 gears 30+ was easy on the highway. Car was in ideal state of tune, good tires properly inflated, good alignment, all maintenance meticulously done. Friend of mine didn't believe it was possible to get 30 mpg in the car. He was with me when I filled the gas tank at the same pump and the whole 440 mile round trip. It was almost all interstate. He was with me when I refilled the gas tank with ~14.4x gallons of gas. The RS odometer was within 1 mile of GPS on mileage covered.
Made a believer out of him that my white RS could get 30+ mpg.
The white RS could get 32-33 mpg consistently. Some of it is driving style. Easy on the gas
My 91 RS Convertible can only hit 28.5 mpg, same maintenance etc. I suspect the transmission in the convertible slips just a hair.
89 L03 TBI. 15psi spring kit. 160Thermostat. 3.73 Posi. 700R4 w/ Shift kit+corvette servo. S10 Torque Converter (maybe 2300RPM Stall) Open Element. Open Cowl Formula Hood. 3in flowmaster cat-delete exhaust system. Custom Chip. 91xxxmi. Over 25MPG have done closer to 27 on highway trips but I don't daily drive it most trips are on highways going to shows.
My old ZZ5.0 with ZZ4 heads, cam and an RPM intake with 3:42 gears and a 3200 stall got 26-28mpgs on the freeway consistently. It got better mileage than my 2.8 lol
I purchased my 91 rs from dad in Florida it has a 305 with 5spd I drove it back to Virginia almost 1000 miles at 70 to 80mph and averaged 26mpg it also has 250000 miles on it
I'm sure some inflate numbers too. 
