Sorry but they DID have 2.73 gears.
This 25-30 mpg is pure

4 of these cars is a hell of a lot more reliable info than someone
with one claiming they got 28 mpg. Yeah right buddy, you best have your speedo/odometer checked out.

 

25-30 is entirely possible. Maybe not with your right foot, but it is possible. My 305 TBI making 240 RWHP was able to get 23 mpg over 725 miles on the highway in a fullsize van. I see 25-30 entirely possible.

 

What's clear here is four low mileage cars maintained
by myself.... well tuned, two with 2.73 rear gears
got 21 max no matter how I BABIED it. It has nothing to do with my right foot and everything to do with reality. 80% highway also.

Some are not telling the truth, cannot do division or have faulty odometers. My four cars are the best cross section
you will find here period. Believe what you want , I know
the reality of the situation.

 

Do you have any that have had prom tuning with lean cruise enabled?

I have personally witnessed Bosch oxygen sensors knock 2-3 mpg off of a car by running the engine richer than it needs to be. The switching points are different than a Delco sensor.

 

What effect does the heated TB have on performance/economy?

 

86 TransAm, 406 cid, TPI, 700R4, 3.23 gears, and 255/50R16 rear tires. (a/c off, 75 degree outside air temp) 20 mpg with 2200 stahl lockup converter, 24 mpg with 1900 stall lockup. That's all freeway at 75 mph, no stops, not corrected for the larger rear tires, and burning gasahol. Switching to straight unleaded should get 1 more mpg, and correcting for larger tires puts me right at 26 mpg.

 

When I got 26 mpg, I drove along with a buddy of mine in his V6 Cavalier. My car does have slightly oversized tires and I believe my odo was about 15km short on a 430 km trip. So I used his distance. He got 28, I got 26. Both are what you would have seen on the EPA paper when these would've been bought brand new.

Just because your cars cant do it, does not mean it is impossible.

 

I had an 86 305TBI/auto that would get 30mpg all day long at 70mph.
The IROC's and GTA's I've had with the 350TPI/Auto averaged 22-24 highway and 16-18 city depending on how hard you mashed the throttle.

 

I get 26mpg in my bone stock 1991 L98 3.23 geared Fully optioned out GTA loaded to the **** with crap and passengers. I didn't use A/C though cause it was the fall. Did it on 91 Octane, and new filters and synthetic fluids all around. I wish i could get a Highway mode tune from somebody and see how much more i could get from it. Any takers?

 

My 2 cents on fuel pressure.
Fuel pressure that produces an optimal spray pattern for fuel atomirization from idle to WOT is all that is required, anymore will cause fuel injector spray patterns to deteriorate due to nozzle failure from effects similar to sand blasting. Any increase in mileage from an increase in pressure indicates that the fuel atomization wasn't optimal. Just like knock control, the ECM will pull fuel if it's more than the converter can handle.
Air charge is the controlling factor.
Catalytic converter design is what determines how much fuel is used. Engine management systems are designed to minimize emissions not optimize power output, which means it won't use any more fuel than it needs to burn the oxygen in the air charge. Having said that any modification that maximize the power output for a given charge i.e. better intake/exhaust, higher compression, lower air charge temperature, minimal knock, gearing ...etc will have a greater affect than trying to push more fuel through the injector via fuel pressure because the ECM will shorten the injector on time to reduce the fuel to the required A/F ratio over time. Idle control will suffer if the injector pulse can't be properly controlled. In other words that initial seat of the pants feeling from an increase in fuel pressure will disappear over time.
But, if you're not required to control emissions then all bets are off.

 

Hi,
Wow, 26 MPG with L98 and 3.23 rear, Gee I only get 27 with 2.73 rear. Just goes to show that this kind of MPG is not inpossible!!!

 

When I first got my car road-ready I got 26.8mpg on the first time out cruising at 55mph mostly out of town w/o A/C. since then I have changed my thermostat out for a 180 and put on a banks Y-pipe and catback. Last time I was out I was traveling at 70 mph in 45* weather and got 19mpg though I did hit a little bit of rush hour (about 3 miles of it).

Oh, its a 89 350 tpi 2.77

 

wow thats pretty much exactly what my 383 tpi gets... and i have 2.73 rearend gears.

i can get slightly better gas mileage, like around 13 on the highway but if i go flooring it around its all over... 2mpg or worse

 

On the highway, if I can get it up to speed (65-70) and HOLD it there for an extended time, it will get 24, maybe 25. In town, in traffic, stop and go, It's probably single digit mileage, pretty bad.

 

Here is a question that I haven't seen posted and maybe some of you guys can answer it. Which fuel delivery system gives you the better gas mileage the TPI, TBI or carb'd systems?

I'm just curious what everyone's experiences are.

 

Jetmeck,
With my car running like **** and me giving it hell I never get below 20 mpg. I think taking it easy on the interstate for a whole tank, 25 mpg is no problem.

 

Mileage depends on quite a few things, from weather, to driving habits, to the state of your car, the fuel in the tank, the road you're on, etc. I've had widely varying results from seemingly similar cars.

My 89 GTA 350 vert gets about 22 highway, but I've never tried to maximize it or even tune it up for that matter. I havent had it on any real long trips, around 200 or so miles one way. Its all stock except for a Midas style and quality exhaust thats probably worse than stock.

My 86 TA that I've had for quite some time with varying mods and engines and it gets terrible mileage, always has. Best its ever done is 21, its been as low as 17 highway. It just doesnt do well. I've had this car on numerous multi-tank long trips, its a gas guzzler to the extreme. Its getting about 12 city right now, but then again I'm used to it sucking the tank dry so I just try to ignore it.

Then years ago I had an 85TA, 305TPI, 2.77 in the rear, about as stock as you can get. I had little things like better plug wires and hotter coil, but thats about it. No other mods, no exhaust mods, no shift kit, stock air filter, etc. It always did well, rarely under 25 highway. I took it on a trip once 420ish miles zero fuel stops (if you want cities to google or yahoo mile it to verify my obviously correct odo you're more than welcome to) and it did just under 30MPG. No joke. I intended to get as much mileage as I could out of it, and it worked. From the way I drove the trip to having correct tire pressure, clean car, anything that could help I did it.

So, when I see people saying they get 16MPG highway, I'm not surprised. If someone else says they get 30MPG highway, I'm not surprised... well I am but I know its a possibility.

From a non-Fbody but TPI'd related point, there's a car in the family (in the aerodynamic shape of a brick) with a cammed 350 putting down 330 at the wheels and it did 21mpg recently (fully loaded with people and stuff) vs previous typical 18mpg and currently its a bigger cam than before. I'm sure I can get it up to 23-25 with some tuning.

 

I have an 89 trans am and I get 26 mpg on the highway. It has the 305 tpi.

 

Mine gets 19-21 combined and on road trips I've seen 25.4. I always cruise about 10 -15 mph over the posted speed limit. I have not noticed a difference with or without A/C running, seems to not matter as long as the windows are up.

 

ditto,from arizona to fla nonstop when i bought the car bone stock same setup!

 

If U want some Freon I live in Michigan I could get a recharge kit and send it to u

 

I get about 275 to 300 km to my tank thats be4 i did the coolant bypass of the throttle body and now i get pretty much the same, so on average i in miles i get 174miles to a tank, from what i have read that seems poor gas miliage, where should i start, i do idle tho but where can i start to improve this, im getting headers put in soon with a dual exhaust so i donno how that will affect my milage, but where would be a good place to start on improving this, change injectors....? can anyone offer me suggestions i would appriciate the help

 

Since this is such a big discussion, i want to make sure that everyones speedo is perfect, because if it is off, then its going to mess up your milage. The odometer runs off of your speedo (at least in my stock 91).

Stock 91 305 tpi auto
Lowest-14 MPG
Highest-26 MPG

 

Hi, new poster here, just num. 2.

I'd like to weigh in on the mpg thing only because I was quite surprised at what happened to me with my 87Z.
Long story short, bought the car with many needs:
needed tune up, 4 tires, a little bit of interior work, and a l/f fender.
For a tune up; the usual obvious, plugs, wires, cap, rotor, you know the usual secondary stuff.
Then It got interesting; pulled the injectors and sent them to cruzin performance for a clean and balance job. Ported the plenum, matched the runners etc.
Then I installed Ed's TES, (I know, not everyones favorites), and a 3" catback with a hi flo cat.
When I had bought the car it was barely running and the tires were bald for the most part, and I limped home with this project. . .about 8yrs ago.
The car had 175XXX on it so I knew it was not going to be in the best of shape enginewise.
It used oil, and ran pretty good for a hi mile peanut cammed 305 with a 2:73 gear (A4).
I was forced at one time to drive it on a 500 mile round trip because the 'dependable' care broke.
So off I went, anticipating lo mpg but was pleasantly surprised to see that I got 31-32 mpg on the way out. I checked twice, once 80 miles out, and then when I arrived at the destination. Got the same answer two times.
The return was into a headwind, and I thought that would be terrible, but It was only 30-31, Maybe 29-31. Needless to say I was not only pleased, but surprised.
Two other things I remember, de-screened maf, and hi flow air filters.
Still used oil, fuel was 91 & 93 octane. 10* base timing.
Cant remember anything else
Bye

 

I just bought my 91 RS (305tbi,5spd,3.08) in Omaha and drove it back to KC. I got 26mpg at a steady 70mph. I was checking the odometer against the mile markers on the way back and found that the odometer lost 1/10 of a mile in every 10 miles (1% slow). And this car is in bad need of a tune as it has no horsepower what so ever.

For reference my 86 Caprice coupe with a computer controlled Quadrajet 305/200r4/2.73 would get 28mpg with the cruise set at 75mph. I drove it from KC to Dallas without stopping for gas.

 

How much does timing factor into fuel economy?

I'm at wit's end trying to figure out why I'm getting crummy mileage. New plugs, cap/rotor, O2 sensor (even took out my new Bosch for a new AC Delco), new air filter, fuel filter. Everything inside the engine was meticulously cleaned when the new head gaskets went in. Converter does lock up. All emissions equipment is hooked up. New cat. Car smells clean when you pull into the garage. Walk around to the back and there is no gas smell at all. Tried tire pressure from 30-37 psi. Tried fuel pressure from 44 to 50 psi.

I'm getting 15-17 mpg during fill ups. All that much worse when you consider I have the small fuel tank. I'm getting about 250 kilometers to a tank of gas. (around 150 miles)

*throws hands in the air*

I did put a timing gun on it during the weekend and noticed it was at 8° BTDC. Would that extra 2 degrees make that much difference in fuel mileage?

I do have the SuperRam (which supposedly gains mileage) but am still stock heads/stock cam. I've thought about putting a Vigilante 2600 in, but with the low mileage I'm getting now I couldn't bear to watch that gauge go down even faster.