Goal 25mpg! Can you help me?

[Johnny Blaze]

Okay, I am looking to make this car an effecient daily driver. With that in mind I have set a goal of 25mpg. Now I do a decent amount of highway driving, so that will help.

Car is a 1988 Trans AM. Curently has a 305 TBI ( non running) and a TH700r4. 3.27 rear.

I am thinking of what would be the best way to go about this. FIrst thought was LS swap. Probably the easyiest but a little costly at first.

Would it be possible to reach this goal with a Gen 1 small block?

I would have no problem going to a different efi set up. I was thinking about swaping to a 6speed as well.

Of course lighting the car will help, and I plan to do as much as I can, but I can only lose so much.

So, what should I do? I have a feelign everyone will say LS1. IF so, I would be better with the 6 speed right? IF you think I can do it with the gen 1, give me your build ideas!

[TPI Monte SS]

Well, if it's your daily driver, why sink a ton of $$ into it doing an LSx swap? Honestly, that car should turn 25mpg highway easily just the way it's set up. I had a cammed TPI 305 with a 200-4R and 3.73 rear in my Monte and that got a best of 22mpg highway. With a 3.27 rear and a similar OD ratio, 25mpg should be cake. There are a lot of guys on www.thirdgen.org that report near 30mpg after chip tuning; enabling the lean cruise on the highway works wonders. Why isn't the current 305 running? If it's blown up, just toss another cheap 305 in, burn a chip, clean the TB and lines out, replace the fuel filter, add in a good set of plugs/wires, and you should be there. Good luck!

[Paul_J]

Is your goal to average 25mpg city/hwy or just hwy?

[Nine Ball]

4th gen f-bodies with the LS1/T56 will typically average 27-28 mpg on the highway stock. When we install offroad (no cats) longtube headers and Y-pipe, with a good 3" single catback, they hit 30 mpg on the highway. Automatics (4L60e) tend to get 3-4 mpg less.

[BADNBLK]

My 91- TPI camaro with 3.42 gear and the 5 speed, was pulling 26-27 when she WAS stock. From what my friends with TBI auto trans cars have mentioned about MPG, you should easy be able to pull that out of that combonation.

I would recommend get her running with new tune parts, then get your baseline. If your still not at your majic number, put some 3.08 gears in the back and 6 speed swap.

[Fuelie Fan]

I typically don't trust claims of old cars getting 30 mpg, at least not on any consistent basis.

My commute is mostly highway, but even so the 5-10 minutes getting to and from the freeway eats my economy in 2004 gto LS1/T56. I will get anywhere from 18-22 combined, and I usually baby it and short shift. Which, of course, is the main factor: the driver. I did hit 29 on one trip, but that was averaging probably 65 mph and was pure highway. In the other direction, averaging more like 75, i was down around 24-25, and that was still by doing things like putting it in neutral on downhills and so forth.

That being said, I've done a number of TBI setups and was never impressed with the mileage, so I would definitely consider a TPI swap. My 70 nova with a TBI 350, 700r4, 3.42 rear, 26" tires scraped at 20 mpg, but was mostly 17-19 hwy. I checked mileage after every single fillup; my girlfriend hated waiting for me to calculate my mileage! If i were running 2.73's to get the highway rpms down that would've helped. Consider changing gearing and try to get down around 1700-1800 rpm at cruise.

I continue to get the impression that many people on the various forums do not understand how to report mileage fairly and accurately, so keep that in mind too. There was one guy was talking about all this work he did to improve his aero, and related how he had made a trip in one direction without it and got 25 mpg, and in the other direction with the mods he got 30. What he forgot to mention initially was that his trip included a 4000 ft change in elevation, and of course he had his aero package on the downhill side. So, while numerous people will tell you how easy it is, I wouldn't expect the big 3-0 out of your car on any sort of consistent basis.

[Johnny Blaze]

FUelie Fan. I agree with you. Our Grand Cherokee with the 4L is avaraging 18.3. SO with those same driving habits and commutes, I am thinking the bird would take some work to get it to 25.

[Nine Ball]

Fuelie, the GTO is approx 500 lbs heavier than an f-body though. Our 2006 GTO averaged 25 mpg during power tour in 2007. Bone stock back then, 6-spd.

[Jim Nilsen]

If you are looking for more mileage than performance you can find 2:90 and 2:73 rear ends that will get you there with an overdrive and a good wide rpm band cam and you should get consistent 19 to 21 mpg city and nudge the 27 to 30 depending upon conditions.

There are so many other ways to get there and your budget will dictate as always ,but the correct gearing and torque range you choose and how you use it will be the path to follow.You might even go with a V6 and cost a lot less?

A budget of 2k can get you there if done with used and borrowed parts and it can go to 6k or more to do something that looks good and will last many miles without troubles.

Goodluck, you can do it easily if you just stay conservative.

[hoosiergta]

My stock except for a 3 inch flowmaster exhaust pulled 24 highway on the 04 power tour 350 tpi 700r4 3.33-6 gears . No big deal and I was doing 75mpg . I'd drop to a 2.73-3.08 for a little nore advantage. What really kills my mileage is running up thru the gears during the other half of my commute . I average 17 . I have an 89 formula 305tbi 700r with 3.08 gears that I'm trying to get on the road for this same idea. Mine is a leaky T-top car so I've already saved some weight and gutted the rotting interior.

[LS1NOVA]

Junkyard 5.3 with a cheap as dirt LS1 intake/injectors/TB setup. Camaro accessories, 6 speed, keep the 3.27 gear. I bet that will get 30 and not break the bank. A slightly modded 5.3 will get 20hwy with almost a 7000lbs truck.

[tracar91]

I have just finished my 1991 Z28 Ls1/6 speed swap. It put down 366hp at the dyno and averaged 25 mpg from Louisville to Somerset going 70-75 on the backroads most of the way.

(but if you'd like to convert to TPI, I have that setup leftover from the swap too. )

[Fuelie Fan]

Weight isn't a huge discriminator in steady highway driving (depending on how hilly your highways are), but is an often-overlooked way to improve combined mileage, and a huge factor in around town mileage. So, I'm sure it had some effect but the different gearing (1.71 effective final drive versus 1.97 in the GTO) also plays a key role. I toyed with putting my car on a diet, and i decided that rather than trying to shave 400 lbs, I'd buy a vehicle that weighs about that much total (Honda F4i). Are 10 second passes and 45 mpg possible? You betcha!!! ;)

Back on topic...I think 20 mpg city for a tbi car is asking quite a bit. highway mpg is mostly about aero and gearing, so mid twenties should be doable, but 25 combined? I don't want to discourage you, but that's asking a lot out of an electronic carb and a 3500 lb car.

Some perspective: a 2009 malibu v6 still gets sub 20's around town.

Fuelie, the GTO is approx 500 lbs heavier than an f-body though. Our 2006 GTO averaged 25 mpg during power tour in 2007. Bone stock back then, 6-spd.

[twosaturns]

it's not the weight, nor the setup; it's the DRIVER that is the single largest contributor/inhibitor to good mpgs. a friend has a 5.0 mustang 5 speed w/ like 2.73 gears, he gets 28mpg hwy. me, in my DD saturn VUE, I used to get 25mpg hwy commute. then modified my style, shifting by the shift light, slower take offs, and setting cruise at 60mph. my best so far is 32mpg, that's a 7mpg improvement w/ no other changes. driving style improvements work in every car, and it's free.
for the OP: I'd suggest 2.73 gear, a good tune on the 305, a 6 speed swap if you've got the cash, and drive like you have an egg on the gas pedal.

[Fuelie Fan]

it's not the weight, nor the setup; it's the DRIVER that is the single largest contributor/inhibitor to good mpgs. a friend has a 5.0 mustang 5 speed w/ like 2.73 gears, he gets 28mpg hwy. me, in my DD saturn VUE, I used to get 25mpg hwy commute. then modified my style, shifting by the shift light, slower take offs, and setting cruise at 60mph. my best so far is 32mpg, that's a 7mpg improvement w/ no other changes. driving style improvements work in every car, and it's free.
for the OP: I'd suggest 2.73 gear, a good tune on the 305, a 6 speed swap if you've got the cash, and drive like you have an egg on the gas pedal.

Which, of course, is the main factor: the driver.

Agreed...

[motorheadmike]

Pfft... buy my 91' Z28. It gets 22mpg MIXED w/modded 305 TPI, 5spd, and 3.08s...

$3200 come and get it. :D

[MonzaRacer]

dont got to small on tires, free flowing exhaust, tune the ecm(chip, try moates.net), spacer under TBI injector tower(or Edelbrocks larger TBI, had great luck on trucks getting better mileage with them), 305s done right can scream and get mileage, dont change gears, that will kill your take off and to swap rears will be between $150 to $500 (trust me just did three for friends, one rebuild and 2 salvage yard swaps and all cost way more than planned with new seal nad replacing bearings and fluids and such) Dont be afraid to bump the timing(base timing)up and let the knock sensors keep track of it, You can also do some junk yard diving if ity uses a knock module seperate from computer look for older Corvette units(oe GM) as they let it advance quicker(ie lat got of retard faster).
TPI can help but dont get wild.
also make sure the Fuel filter is good and pump is up to task, a lot of these cars got bad rap as they had stopped up filters ,then owner installs new one and old in tank pump is shot and performance falters and owner drives it harder killing mileage.
also bumping the fuel pressure to max spec(most tbi is 9-13) or just above helps in atomization. and build a true cold air package and free flowing ducting.

[jeff s]

My 07 GMC 2500 Duramax 2wd gets 24 mpg highway.
An LS1-LS3 six speed should be over 25mpg in a 3rd gen, maybe 30 if you baby it.