I'd thought I'd bring this topic up while we all are probably paying more for gas now than we were a few weeks ago. I've been driving my 1989 Iroc Z L98 with around 135,000 miles back and forth from school for a total of 66 miles. 62 of those miles are on the highway and 4 of them are in city. I've gotten an average of 23 mpg when I drive 60-65 on the highway and do the speed limit in town. Before prices got higher, I drove 70-75 on the highway and got an average of about 20 mpg. I think I am getting terrible highway gas mileage from my L98. With my 1989 Iroc with the LB9, I'd get over 27 mpg on the highway. I was wondering what can I do to improve mileage on my Camaro with the L98? . I've noticed a few odd things, but I don't know if they would affect mileage.
- I get a loud noise from the fuel pump when I am down to 4 gallons (clogged fuel filter?)
-I don't have my AIR hooked up
-my cooling fans turn on shortly after driving even when it's below 30 degrees outside.

Also, I was wondering if other third gen Camaro owners can post their own average city/highway mpg to help me get an idea of what I should be getting.

 

Winter gas blends give poorer mpg.
Winter air temps give poorer mpg. While its better for power, the colder air is denser, so it requires more fuel.
Cold temps make the engine slower to warm up which effects mpg. Try a fresh t stat. 195*.
Check the fuel pressure & leak down rate.

 

The fan coming on is a sign of a real problem.
You need to hook up a datalogger, and at the very least pull your error codes to look for clues.

Until you find the problem you can pump you tires up to 45 PSI and drive a little slower, - closer to 55 MPH and get a bit better mileage.

 

I have to look into it when I get time. I think it might be a thermostat problem. The coolant temp barely moves off the first line. I'll probably put some air in the tires today because I have them all set at 38psi.

 

If you have your HVAC controls all set to OFF (no defrost), then your fan should not come on with the engine cold.
A cold thermostat will not make your fan come on.

The ECM controls the fan based on engine temp as read from the CTS (not the sender for your temp gauge), so you have to check that sensor separately.

The fan can also be triggered by an over-ride type coolant temp switch that is often installed in the passenger-side head.
Having the A/C clutch engage and pressurize the system (such as happens when you use the defrost/defog setting) will also cause the fan to run.

Otherwise the fan can run when in ALDL diagnostic test mode, or when certain error conditions are set in the ECM.

 

i try telling my friends this everyone looks at me like im crazy. i vouche the LB9 motor, in my case gets 28mpg on highway over 20-22 around town. 40miles on a secluded road with 3 traffic lights, doing 60ish mph. 20 miles on parkway going 70-75mph. a few miles thru city. Giving me 125 miles round trip. Be easy on gas pedal but use the torque.

I also got 24mpg highway miles at another jobsite, with about 10-15 mins of dead stop highway traffic, creeping along. So its not my math, but proven.

its why i cant find it in me to sell my car she realy isnt that bad on gas and it has all the torque i need for daily driving! its been a month since ive driven her, im love sick :P

 

On my 1989 GTA, I just built my L98 engine balanced, blueprinted, with flat top pistons(9.96:1 compression), mild roller cam, roller rocker arms, SLP headers and exhaust, SLP intake system and mild intake base porting to gasket match with the
cylinder heads. I am averaging about 18mpg driving highway at 85-90mph and around 14-15 in town driving like a normal adult. I don't know how this compares to others on the board, but I am satisfied using 23 year old technology.

 

are you running the windshield defroster/defogger it would kick the fans on since it uses the ac

 

20 MPG is about right, i think. according to the window sticker from my T/A
it's rated for 18-24MPG i'd have to dig it out again for the specifics but i hope that helps.

 

im currently driving a 89 iroc 305tpi auto getting about 220 miles per tank thats about 15 miles per gallon i find that terrible
my summer car has is a 91 rs and it gets about 300-320 per tank and it has a zz3
thats around 22 mpg

 

"- I get a loud noise from the fuel pump when I am down to 4 gallons (clogged fuel filter?)"

You should never run fuel down to low on these cars as the fuel pump is cool by the fuel. It shortens their life tremendously and the are a PITA to replace. Also the more fuel deadens the sound of the pump.

 

Thats a myth, anyways the return line sprays onto the pump.

 

If you say so. But I've seen lots of older cars pumps ruined by running low or out of gas.

 

Tires aren't gonna show SQUAT for any measureable difference. I went from the stock 16lb 16x8 wheels with 245/50s to 2-17x9s with 275/40s & 2-17x11 (the 11s alone without tires are 40lbs each) with 315/35s (never did weigh the full 17 combos but they are much heavier) and my combined average hasn't budged from the 16mpg.

Look on http://www.fuelly.com to see others mpgs. The more people that add themselves, the broader view we can get.

 

thinner taller tires get better mileage changing to a wider taller tires just keep it the same

 

Then explain why my switch from 4 light wheels w/245s to 2 heavier 275s & 2 really heavy wheels w/315s resulting in a zero measurable difference mpg....I can tell you why. Because it is so minute that is isn't worthy doing.

The whole thing is good in theory but it is just that & will NOT give you a measurable difference. Unless you were to go from like 4-335s down to 4-195s, then you might see the rolling resistance difference in your MPG.

 

It's true, the 305 really isn't that bad on gas. I'd drive the 305 iroc from peoria,il to st louis 2 days a week and I would get over 27 mpg. I bought a 350 iroc, its been terrible on gas from the first day I got it, but today I found a vacuum leak, so maybe that's why it's bad on gas.

 

I think the previous owner did something to kick in the fans as soon as the engine starts. I don't use any of those, and the heater sucks really bad when the temperature is under 30 degrees outside.

 

jeeze, you might wanna check out your plugs, wires, and distributor rotor because that is really bad for a 305.

 

plugs "boush" wires "msd" fuel injectors "accel" cap and rotor"msd" k&n air filters and fuel filter and thermo also boush 02 are all new.
about 3k miles on every thing and new edlebrock c coated headers new magnaflow cat flowmaster exhaust 2k miles
33k on the car
no codes iac and tps set correctly as is the tv cable i just put in new trans fluid rear end fluid and oil change as well as coolent flush
i have no clue why it is getting such crappy mileage but atlest it ran a 14.3
my 91 rs has same parts but also has a zz3 and gets way better mileage and it has 3.23s iroc has stock 2.73s

 

i have had 3 TPI 350's. They get 16-18 around town and 24-26 on the highway. The 305TBI i had would get 30mpg on highway at 75mph all day long.

 

Well in my camaro witha 350 and carb It costed around 40 dollars to drive 130 miles.

 

When I was looking at my motor yesterday, I found two of my new msd plug wires melted onto the exhaust manifolds. They were shorting out, causing misfiring everyday I drove on the highway. The only way I found out about this is that one wire completely melted down to the point where it was constantly shorted out, leaving me with a huge loss in power. I couldn't accelerate over over 1700 rpm for two days. I replaced the two bad wires, making the car feel like I just installed a supercharger. It also eliminated the motor rumble at lower rpms and the "exhaust leak" noise. Bad plug wires cost me huge performance and may be the reason why I was getting bad gas mileage. I think I should be getting over 25mpg on an L98 driving at 60mph.

 

I get somewhere around 14mpg highway right now... I have a 2300 stall, 700r4, & L98... Headers, no cats, K&N filters, 3.42 gear... Bout to clean the MAF and idol control switch tomorrow... I hope it improves!

 

How do you get the motor to function correctly without the cats? When I didn't have my cats, I would get terrible gas mileage and the computer would always set off the check engine light. I bought and installed magnaflow dual cats, and have had better gas mileage and it hasn't set off the check engine light. Anyways, with that set up, you should get at least 18 mpg on the highway.

 

It didn't have cats on it when I got the car... On my 98 LS1 car, I started with no cats and put magnaflow high flows on it within 2 weeks of buying the car... Didn't have any issues before I put them on or after so... I really don't have past experiences to go off of

 

unless they are bad or under sized, the cats should have no effect on how the motor runs or fuel mileage.
if installing cats did in fact have an effect, then i would say there was an exhaust leak close enough to the O2 sensor to cause it to read the exhaust gases improperly.