I tested my all interstate gas mileage last night and I was surprised and pleased. I got 27mpg last night. How does that compare with other modded 5.7 TPI cars? I am running a T56 so that helps a lot too.

 

My 5.7 Stock (i think) except hi-flow cat, full cat back, airfoil (on the car when i bought it) & K&N filter. Odometer read 225,000 miles, but unknown if ever replaced/rebuilt/or what. 700R4 tranny & 3.27 posi rear.....

A pure highway trip netted me 21 mpg & my general mixed in town/highway nets me 16/17mpg.

The double overdrive of a T56 is where a lot of mpg increase can come from. 35mpg on a 2.77 + T56 car?

 

My exhaust modded TPI L98 went 24-25mpg and about 16-17 daily. My 383 HSR was about 15 daily and I think it was low 20's highway but never really confirmed it.

 

Damn that's awesome gas mileage that's near daily driver mpg for me I would sell my civic if I got thatwhen is done

 

12-14 city and 16-18 hwy ratio is in my signature,
I have a feeling my mileage is bad from a bad tune... the former owner of the car didn't do anything right....

I also have small tube headders going into a Y for 2.5" all the way back to a flowmaster 40 (poorly welded and bent)

 

Your T56 is the only reason for that

My 91 GTA bone stock L98/4L60 got 23mpg strictly highway

 

LOL either way you slice it still a thrid gen with a v8 and a tpi that can prolly get down to the 12's which is darn good

 

My '87 IROC with 305TPI/700R4 got 26mpg on the highway, consistently. I always thought that was pretty impressive for a V8 that wasn't....... well, "technology-ified".

 

357 mild cam, Trick Flow heads Eldenbrock Intake base SPL runner Melrose Headers, 24# injectors tune, 307 rearend 700R4 Trans.

27-28 Hwy / City 12-14 if you foot not in it foot in it forget it gallons per mile LOL.

 

With basically a ZZ4 cam L98, 3.27 gears and a 700r4, I got 22 mpg on a 5 hour drive to Atlanta for one of the gatherings.

 

The extra overdrive must make all the difference. I never got better than around 23 when my car still had the T5/305 combo. Its always interesting to learn that the extra gear matters more than the engine size.

 

I get about 20mpg mixed driving with full-exhaust and a tune. I tend to alternate between driving like Grandpa and driving like Satan, so it's a good representation.

 

Not sure on the actual MPG (way too much math for me with all the conversions) but with a cat-back exhaust and sudo-CAI done to my otherwise stock 305tpi and 700r4 I went from Toronto to Ottawa with a little more than 1/8 of the tank left. Thats a steady 4h 15min drive at 68mph with the t-tops off, luggage for 2 people for a 3 day stay and the car running a little rich (confermed by 5 gas analizer). No one thought I could do the trip without stopping at least once.

 

I drive a mix of highway and city miles using %100 gas (no ethanol). I get around 17mpg. I got the bigger gas tank, so its all good.

 

With a 305 TPI, T5, 3.23 gears mild home port work, LT1 cam headers, Hooker cat-back and no cat, I managed 21 combination driving, and 26ish highway. In comparison My 97 LT1 gets 17-19 combo, and 28 highway. These are my economy cars.

 

i can get 18mpg but thats like...granny style all day

 

My 100% stock 21,000 mile L98 with 2.77s gets about 21 MPG in mixed, spirited driving. I've never had it on a long highway trip...hell, I've only put 650 miles on it since I bought it in June.

My Trans Am with a 305 and 2.77s gets pretty much the same.

 

350 crate with TPI with mods and I get the following MPG.
27/28 hi-speed hyw with A/C
14/15 hyw/city with A/C
12/13 city with A/C
your guess.....

 

Stock 5.7, 3.27, 700r4 with lockup, single turbo gave me 21.5 with AC on a 70 mile interstate run at 75 mph + another 75 miles of back roads 50-65 mph. I haven't dialed in a lean cruise for that mph so I feel like 23-24 would be possible. Also I had a hard time staying out of boost when passing someone so maybe another 1 mpg there!

 

Some of these MPGs seem pretty optimistic, but then again my Formula is rarely on the highway, so that is probably why...

 

Drove to Chicago for Thirdgen fest last year and averaged 22mpg on the highway. It was pretty hot, so the t-tops were off the whole way.

 

With my '86 IROC TPI 305 700R4, I am getting between 28 and 29 miles to the gallon on the highway. This is doing close to the speed limit (+5). I have removed the useless smog pump. I have headers and a 3 in. exhaust. I also have a high flow cat on her. I have dropped her a little over an inch and a half, and she is breathing through K&N air filters. When I get back from Iraq, I am going to put an adjustable computer chip into her. This will supposedly let me have a economy setting and a little bit more power setting. The manufacture claims a 2-3 mpg increase with this chip.

 

That sounds pretty interesting. I would think if the chip slammed the timing, and ran as lean as safely possible with the car sitting lower as well you will see the maximum effect.
My car is lowered a couple of inches and I think that is helping me as well.

 

i have the exact same problem i think, i get the same mpg (actually a lil worse i think) and the last owner had the top of the engin redone and it dosent seem like great work at all... the guy had the tps at 3.4!

 

Stock 5.0 w/ 700R4...I average around 21 mpg, mix of highway and around town. On the highway the best I ever got was about 24. I thought it should do better, but, oh well.....

 

my tps wasn't that bad~.64 but my fan turns on before its in closed loop ~170 and it had a 180 thermostat...

 

Oh i just realized that most of the people here their "city" is a mix of city highway wich is like 15-17, my city is real city, like i pretty much never get on the highway (highschool student in a small town) so thats why my milage is so low! Haha i figured it out! I get about 12-14 city sometimes below if i step on it alot and dont shut my car off when waiting for awhile.

 

Yep there is a huge difference in the city/Highway. Skipping gears, slower pull aways, not "running" up to the next light can help city mileage a lot. I love the massive torque I am getting so I don't currently follow my own city driving advise though. still the 6 speed TPI obviously can be quite fuel efficient.

 

I just found mine to be 14 -15 on my edelbrock tpi, headers, full exhaust, tune, afpr, etc. half highway and half town, with frequent spirited flybys.

 

i get 20-21 with a constant mix of city/hwy. I don't baby it either. good headers(1 5/8prim. 3in collector), chip, k&ns, msd wires, 3inch single exhaust.. 3.42's and a t5 305 all on 275s. I pulled 26 all highway the other day.

 

When I drove 2400 miles across the country, I stopped for gas 11 times (in 3 days) and kept track of miles per gallon. It ranged from 19.5 to 20.5 mpg.

Speed (60 mph constant vs 75 mph constant) had no effect. Having the A/C on or off was what made the 1 mpg difference.

So I'm happy that the A/C doesn't draw much fuel. On the other hand, 20 mpg isn't all that great (especially when you have the smaller 12 gallon tank). But hey, it's a 3.27 geared heavy GTA. And I'm over 200 lbs myself.

I could see 2.77 geared cars getting 23-24 mpg. Above that? Not without a T56 and probably a good tune.

 

Stock L98 220 H/P 330 TQ 13 City 24 Hwy.

1st new motor 355 Mild built 360 H/P 390 TQ 13 City 26 Hwy.

2nd new motor 383 Lt4 hot cam 435 H/P 450 Tq I dont know yet I should have it on the road here soon.

 

As an update, my newest purchase (88 TA LB9 M5 with 26k miles) gets the best mileage of all my TPI cars...23.5 with the last tank, and I was having a ball running it hard on open roads...

 

When I bought my Formula 350 Convertible in North Carolina and drove it to Michigan, I calculated about 26Mpg highway, that was with the top Up, and driving with cruise on I-75 doing the speed limit 65Mph & 70Mph depending on the state.

Car had 35,000 miles on it for the drive, now has 37,000 miles

John

 

I'm getting 24-25 city and 32-33 highway with my stock 305(except for the headers) 92 TBI RS. The 87 IROC got about 25-26 on the highway...305 TPI. Until we cut some holes in it inside the engine compartment, right where the fog lights were installed. Then it got 33-34 on the highway, same 25 or 26 in the city. Take the fog lights out and cut those holes, more air goes into the engine compartment. Also the A/C was disinstalled in each and an A/C delete box installed.

 

My old (stock 350s) got around 15mpg mixed , best was around 25mpg highway.
Just city was around 10mpg.

Now with my 383 its just worse
About 13-14mpg highway , and ~10mpg mixed.

 

Wait, seriously?? This fog light cutting thing sounds interesting..but i don't really get it. Would you care to elaborate?

 

Sorry it took so long to get back here... There is plastic ductwork that comes from where the foglights are usually mounted. If you pop the hood and look under your two square air filters are, you can see it ..and in mine there was a little hole in each one. You cut that ductwork to divert more air to the air filters. They almost act like mini air dams. I would say that you could just take the connecting piece of ductwork out, but then you'd have to worry about picking up trash on your radiator. I'll post a picture in the next couple of days, because it's kind of hard to describe.

I regularly make a run from Atlanta to East Texas, and yes, my average gas mileage is 33 mpg. I'm not sure how much affect the type of gas has on it, but I only run Shell gas in my cars. Ran some cheap racetrac gas in it once, and the gas mileage was horrible.

 

I drive mostly highway and I get 23-25 mpg, that's going no more than 5 over unless I'm passing. Mixed driving is around 19 mpg, now if I jump on it it drinks the gas.

 

A jigsaw blade and two cuts on each at the duct overlap rear piece top. Then push that flap down into the duct. When done you can stick your arm through the ports in front and punch the filters out. Takes all of 5 minutes.

 

Sprited recreational driving has netted me 21 mpg average.

 

I need to do a serious tune-up on my GTA...I'm only getting about 12 mpg hwy :'(

Although, my 2.8 T5 Camaro can get 35+ (best was 50) if driving super conservative. Had a 350 TBI Firebird that was 30 mpg hwy.

 

Man how do people do that? My 2.8 T5 gets mpg that'd be expected on a 350. It's not too cool having neither the horsepower or the fuel savings, lol.

 

$10 gets me 50 mi, $20 gets me 100mi, However when I put $50 in, it doesn't get me 250mi....why, I don't know. Gas is about $3.70 here.
(Gas mileage ranges 17-20mpg city) Cleaning out my air intake certainly increased my mileage.
What is our gas tank capacity?...I thought I read 11 Gal somewhere, but I just put in 14 Gal yesterday, and it still had a gallon or two left in it.

 

I get 31 mpg with a T56 and a 383 L31 TPI. Having heads that dont require much timing advance helps alot too. Thats driving like grandma though and keeping it at 70. Drops to 27-28 at 77ish MPH. Thats also with a very efficient chamber, a tight quench, and the 0411.

Thats one of the reasons I chose Vortec heads. The chambers are light years ahead of some of the more common heads used, like the L98 ZZ4 head. They were profiled on top of that.

If I drive in the city and put my foot in it even a little it will drop into the high teens. If I drive like , well I stop alot more often for gas.

 

89 formy 350.
full SLP kit (CAI,runners,headers,TOTLexhaust,cam,shift kit,converter, no cats)
TFS heads, 1.6 rockers, descreened MAF, my own TBody,ported plenum, Stock chip-timming bump-knocksensor unplugged, 50psi fuel 24lb inj.
running 75-80, A/C crankin an 4 peeps n car
She got 27 on highway 15 in town.

 

19 town 28 highway

 

I get 25+ highway on a stock 305 tpi. Only thing aftermarket are K&N filters.

 

My 89 GTA 5.7 Auto was just rebuilt to stock specks along with the transmission (although the emissions pump and associated plumbing was removed but stock exhaust remains for now) and I am getting 24 MPG on the HWY. Haven't clocked around town as of yet. Perhaps a new non cat exhaust might improve things. I also have replaced the fuel injectors with Southbay units and installed a K&N filter.

 

Nobody cares.

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