NowhereFast

Car specs are in my profile. I calculated the mileage of my car today, almost completely highway. I wound up with 20mpg. I was cruising at 55-65mph the whole way for about 50mi, tach hovering at about 1500rpm. Running 89 octane on completely untouched motor.

That can't be right. I've heard of L98's getting 24-25. I thought with the small motor, peanut cam, and 2.73 rear, I'd get at least 25mpg. The car runs perfect too. What's up with that?

rusty vango

actually, that's pretty good for a TPI car.mines only got 2480 miles since the rebuild. and I get 20MPG s in the city and 24MPGs on the freeway

konspiracy

I think that some people on here are using custom tunes to help lean them out.

akanitro

iTrader: (2)

20mpg is about average for a 5.7 L98 engine from my own calculations for all highway. The 700R4 transmission will cause less mileage than a 5spd. I think if you are all stock with 89 octane fuel then you should be good.

Ransford2987

If that's a 305 (according to your profile, it is), then I would expect more than that. A friend of mine has an 86 with the 305 and he averaged around 24 highway stock tune. I would look into getting a tune done; you can enable highway mode and adjust the VE tables to make sure you aren't running richer/leaner than need be. FWIW, I can get ~28-30 mpg highway on my L98 with tuning, so it's worth it

keykey

I've done two highway-only-trips in my car so far and on the first I drove mostly around 70 MPH and had the occasional lead foot, and got 20 mpg.

On the second trip I drove about 55 MPH since I was following a friend in a 2nd gen Trans Am who didn't want to drive faster. That trip I got my best mileage so far 23.5 mpg (exactly 10L/100km).

All stock 350 TPI (except emptied cat, 3" muffler and South Bay Bosch III injectors) TH700R4, 3.27 rear end.

Worst mileage so far was spirited city driving with a heavy right-foot, gave me 12 mpg

Ransford2987

And that's with the 350 and larger diff gear ratio. I would expect more from that 305 cruising on the highway.

IROZINCO

iTrader: (1)

i had an 86 305 TBI that would get 30mpg all day long at 75mph. ALL of my l98's (i've had 4) have gotten an average of 24mpg at 75mph. They had 700-R4, and 3.42 in the rear. So I would say that you need to do some tuning on your car. Could be a plugged cat, bad tires, plugs, wires, the list is endless. But yes, you should be doing better than 20mpg.

NowhereFast

Cat is hollowed out, tires are brand new, plugs and wires were replaced just a year ago.

kx3g

Wow! I'd love to see 20 mpg. Had my L98 rebuilt about a year ago, bored .40 over with a mild cam. Got a 3.70 rear end. I'm lucky if I get 11 mpg with very conservative driving.

IROZINCO

iTrader: (1)

my l98 TPI is built with zz4 cam/heads - bored 10 over - 700-r4 tranny - 3.42 posi rear - dyno at 220hp/320tq AT the rear wheels, at 6000ft altitude, on regular unleaded gas, and will get 23mpg at 75mph all day long. drops to 20 mpg with A/C on. Yes the computer is tuned, but NOT for fuel economy, its tuned for performance, and the parts we have in the car.

kx3g

I've got a 700-r4 as well. My builder estimates it to produce approx 310 +/- hp. Doubt it is at the rear wheels. I have no clue how to tune the engine. Any suggestions as to what I could try to get better mileage?

Thanks,

IROZINCO

iTrader: (1)

For mileage and performance you have to go back to the basics. The engine has to be running correctly. It can 'feel' fine, but not be. Start with:

spark plugs (these cars PREFER A/C delco, and gap must be right)
plug wires
distributor
base timing
vacuum leaks (hoses and intake)
O2 sensor
plugged fuel filter
plugged air filter
fuel pressure/pump
bad cat/plugged exhaust (doesn't take much blockage to cause issues)
computer codes
sensors (EGR/MAF/CTS/O2 are common culprits)
wiring issues
wrong computer (cars are 20+ years old, could have been switched)
dragging brake calipers (esp on rear drum)
TPI shift cable adjusted wrong
Thermostat. thermostat. THERMOSTAT! Too many people put in a 180 to try to get the cars to run cooler, i've seen as low as 160 with a chip. Fix the cooling issue, and then put in a 195 - these cars run best at the stock temp, i've seen as much as 20% mileage gains JUST from changing the thermostat. Most t-stats these days are designed to stick 'open' if they fail, so even if you have a 195, it can be stuck open and running too cold.

Those are the basics for a stock motor (i'm sure i've missed some, its late) If you have a modified engine, well, skies the limit. Parts must be MATCHED for the engine to run efficiently. There are people on here way more qualified than I to tell you how your engine combo should do. But ANY car will benefit from having all the basics working right.

kx3g

Thanks Iroz. That's quiet a list. I guess I have a lot to check.

I know it's not the following:
Thermostat - got a new 180 that seems to be ok
Cat - don't have one
Air filter - gotta nice clean K&N

Thanks again

Dartht33bagger

Better than I've ever gotten. My car gets on average 12 MPG city and 15 MPG highway. The highest I've ever gotten was 16 MPG and that has happened only a few times in four years of owning the car.

lonestar7

iTrader: (1)

I get 14-15mpg city, 18-20 highway

Ghettobird52

Before tuning my 350 TBI i would get 28-29MPG on freeway all day! Now tuned i get about 22mpg, city is about 10mpg lol 'spirited' driving!

Reid Fleming

iTrader: (2)

The TBI cars always get better mileage than the TPI cars.....TBI had "highway mode" turned on in the computer chip from the factory. TPI cars did not.

The original (unrealistic) EPA fuel economy measurements said 17/25 mpg. The newer standard changed that to 15/23mpg. But the question is what car are they testing? An IROC with 2.73 gears should get better mileage than a heavy GTA with 3.23 gears.

I drove my car across the country in 2007 and monitored fuel mileage (I filled the tank 11 times in 2.5 days of driving, basically about every 4 hours on the highway). I got 19.5 to 21 mpg on average. I tried constant 60 mph. Constant 75 mph. Cruise control on. Cruise control off. A/C on, A/C off. The A/C seems to use about 1 mpg by my calculations. Low enough that it's worth it to turn it on for comfort.

I've been trying just about everything to get better mileage lately.

spark plugs (AC Delco)
plug wires (MSD)
coil (AC Delco)
Cap & Rotor (MSD)
base timing (currently have it at 8-9°, going to try 6°)
fuel pressure (set at 50 psi WOT, going to try 44 psi soon)
Replaced injectors with Bosch III 22 lb stock size
vacuum leaks (haven't checked this)
O2 sensor (AC Delco, swings back and forth rapidly as verified by WinALDL)
plugged fuel filter (brand new)
plugged air filter (change it about once a year)
bad cat/plugged exhaust (cat is 7 years old)
computer codes (none)
cleaned the IAC and throttle body passages (always a good idea)
dragging brake calipers (all new brakes a year ago)
TPI shift cable adjusted wrong (set right)

My next plan of attack is try the fuel pressure at stock first. Then try the timing at stock 6° base timing. Only other thing I can think of is a vacuum leak. Doesn't feel like one, but who knows.

I do run a 180° thermostat. But June in Texas, the car is going to run at 195-220° anyways. I did ask a fried of mine with a TPI C4 Corvette to check the instant fuel economy on the digital dash at 160, 180, 195°F a couple years ago. He said there was a 1.5 mpg difference between 160 and 180°. And that there was a 0.5 mpg difference between 180 and 195° Higher temp equaling better fuel economy of course.

But overall, fuel economy is a mystery with these cars. I've been getting 14-15 mpg with mixed driving to/from work (mostly highway, about 10 miles of highway and 2 miles of city). The last month I've done plugs, replaced the original wires, coil, cap, rotor, cleaned IAC passages, and replaced the fuel filter.....The car feels a zillion times better. But the fuel economy has only gained maybe 1 mpg.

Reid Fleming

iTrader: (2)

Just to give you an example of how unlikely the "old" EPA measurements were, the highway test used a top speed of 48 mph. Cars accelerated so slowly that it gave a 0-60 mph time of 18 seconds. No air conditioning was used.

The new testing spec uses 60-70 mph with some A/C use.

For cars like us, it doesn't make that much difference. For some of the newer hybrid cars, the old standard gave readings of 60 mpg. But the real world "new" tests only give them about 45 mpg. You can see why hybrid owners were miffed after buying cars that claimed to get 60 mpg, but only get 45.

86Z

iTrader: (4)

built my car for fun, MPG doesn't come in to play, i have other cars for that

FirebirdUSX

Haven't tried and Im not an expert in this but how about ethanol?Here in Europe people(or at least some) put that in their cars...will that work with TPI engines? A gallon of fuel in europe is @8 US $ ,ethanol/alcohol/moonshine is alot cheaper.

3.8TransAM

91 GTA 350 was 26mpg on good gas. High 24 on ethanol gas. Bought it in Salt Lake City with 19000 miles and drive it 1100 miles home.

91 Formula w/ 350 I averaged 18-20 around town and easy 24 on the highway.

Other friends cars were similar.

If you are not getting something around this mileage, you need to get your car running right before you try making further modifications.

FirebirdUSX

How much is ethanol in US?
Will the car work without any modifications?I see the difference in mpg is not that great .

Also why they ask people for gas guzler taxes (they exist both in europe and US under different names) if your car is 100% eco friendly while using ethanol?

8Mike9

18-20 around town?...You must not have much traffic

My '89 in pure highway driving was 24.5, but stop and go (city/commute) in the 11-12 range...this was when it was all "as factory", as I bought it new.

No idea where it's at now though, haven't checked it in years.

Reid Fleming

iTrader: (2)

All gasoline in the US is either E10 (10% ethanol) or E85 (85% ethanol).....Only new cars that are dual fuel can run both E10 or E85.

Everything else runs E10. It doesn't matter if if you buy regular, midgrade, or premium, you end up with E10. There are very few gas stations that sell gas with no ethanol in it.

Reid Fleming

iTrader: (2)

Unless you have a 6 speed manual or a tricked out computer chip, I can't see 26 mpg out of a GTA L98. I'd have to be shutting the car off on the highway and hypermiling to get 26.

Is there anything you could add to the list of things to tune up for better mileage?

Kennerz

My 88" L98 Iroc I've owned since 90" with the stock L98 never got over 16mpg Highway/mixed. With the vortec setup and a tune (not dyno tuned) it gets a steady 15c/17hwy I may be able to see a little more on a trip maybe 18mpg. I have highway mode enabled in the chip. I chased down everything over the years, Runs and starts very good. Can't find that "secret" to get the mileage everyone is claiming.

My car IS modded, & I think to see highway mode working correctly the computer has to see the perfect storm of the righ mph/tps%/IAT/EGR/maybe tach rpm/ etc...(whatever, I'm speculating) but how to debug the car to make the ecm get the right inputs to get good mileage is beyond me. I've tried everything (Been on this board since before 2000, even though my sig says later) so over the years I've tried almost every possible thing I've ever heard on this board that sounded like a reasonable cause of poor mileage. I've seen small improvements, but never gained back like 5mpg (like I said, the best I've seen is the 17 I get now) You tell me???

Another factor is california, crappy E10. The gas here is not so hot. You lose mileage right off the bat in an older car with E10 I've heard as high as 10%. so for me- good gas could probably net another 2mpg.

In my experience though, if your getting much under 15, os so something is wrong with the tune up of the car. And ANY of the above items listed in this post can cause the mileage to suffer significantly. The problem with TPI's is all this junk has to work in concert for the car to even run right. You ever notice that on the new cars most of the emmissions/sensors/things that go bad on a thirdgen are missing?(No Egr,Air injection/ charcoal canister/distributor/maf relays/etc..)

johndinu

89 350 tpi 700r4 mostly stock. 24mpg best ever on hwy, 15 city veeery relaxed driving.
In Canada u can get premium at Shell without any ethanol. pure gasoline I've heard has more energy. there is also a winter mix which returns even lower mileage, but who drives these cars in winter anyways.

Ghettobird52

WHAT!!!!!!!!!!!!! lucky guns they are. Yeah its true but to so much. I guess our Ethanol of 10% rating here in California drops gasoline power by about 6%. So that means we are getting 94% of energy/power from our blended mix compared to 100% gasoline. 6% isnt that big, but could add up quickly.

RamIt

Don't you guys use imperial gallons? 1 imperial gallon = 1.2 us gallons.

johndinu

We use liters and kilometers and express fuel consumption as liters/100km. I converted using 1gal = 3.785liters and 1mile = 1.6km.

VincentZ28

iTrader: (2)

In my sig with the Miniram, I was getting 16/24 with 3.42 gears. You can tune the car in the chip for performance and drivability with the initial timing set at 6*. This was at cruising speed of 75mph. Also with 30lbs injectors.

IROZINCO

iTrader: (1)

Colorado has ethanol added in also. All year round. I never saw a difference, and i had the car in Colorado, California and washington state, and all points in between. 18/24. Consistently. Now granted, I didn't race around town, but ALL those cities have plenty of traffic, and thats still what I got. Now that I've modded the car, the mileage is down a bit, but not too much. about 16/23 with the above mentioned HP/TQ numbers. To restate one more time, you have to have ALL the parts in the car working optimally for the mileage to be 'good'. And no, this isn't my DD, but when you go on a 1000 mile road trip, mileage does count. Esp out here where you have a 15 gallon gas tank, and it can EASILY be 50-100 miles between gas stations! I sure with that there was a (reasonable) way to put a bigger fuel tank in these cars!

3.8TransAM

I have to laugh at the mileage some of you folks are reporting.

U either simply cannot drive to get mileage or your car has issues that need to be addressed.

Folks who literally start the car and pull out to sit in traffic are excused, the rest do not have one.