Hello all,

I have an 88 IROC and drive it everyday in good weather. The car is stock, has 145k on it and I get about 22 MPG highway. Just wondering what can be done to increase the mileage on these cars. I see some of the guys here with combinations meeting CA emissions and still putting down excellent HP numbers, far beyond what people thought was possible years ago. I think the same is true with MPG numbers as well. I was going to start with a set of headers, good exhaust, open up the intake tract a bit and maybe a set of heads. Any input on how to get 30 MPG highway? Might seem like a long shot but someone has to have some ideas.

 

Could put your 2.77 back in to get some MPG. Then add a Posi for traction if that is important to you. The 2.77 will help get you closer to a 30MPG goal, but I'm not sure I've ever seen a TPI 350 break even the 25mpg mark.

Narrower tires for less rolling resistance, although that would be so minimal, hardly worth the effort.

Synthetic fluids for less rotational resistance.

Free flowing intake & exhaust.

Tuning would help, but not the easiest because of PROM burning & swapping. Could do an EPROM & run off the laptop. Or do the new LS1 ECM conversion. The HP/FT LB results look very promising but I've seen no report on MPG increase, so I don't know how it would cost in MPG gain vs the conversion cost.

 

Increase tire pressure (helps handeling to a point too) and get headlight covers. Those buckets scoop a lot of air. Lowering the car can help a little as well as light weight wheels and driveshaft. Weight reduction would be something to look into as well.

 

I get at least 24mpg highway with 3.23's and the car lowered 2 inches, BUT I did swap in a 5 speed. and about 16-18 city when I drive normal.

other things mentioned by LB9 GTA
fiberglass hood
factory a/c delete
aluminum driveshaft
light weight flywheel
no smog

i'm sure the weight reduction helps

 

most i ever got was on a trip to AR, I nailed 27mpg in my 89 formy 350.
auto and 3.27 gear
It had the full slp kit at the time, minus the nasty chip they have..
3 girls myself and AC rolln!!

 

Thanks for the responses guys I appreciate it!

The 27 MPG figure is what I was thinking these cars should be close to stock after they were broken in. My old 92 B4C was able to get 27 MPG on a trip from Delaware to Florida one year. It was between stops off/on interstate but I still saw the number. Just me and no passenger but I did have some weight in luggage. That was a stock tpi 350, auto with 323 gears, dual converter option and a new O2 sensor. The only performance enhancements were an air foil, modified airbox and a flowmaster muffler. That was 2006 so might have been pre ethanol gas too.

Also been toying with shutting down cylinders through the injectors after it's up to speed. Not sure how I would do that, but it is a thought.

 

For years I drove my 89 350 TPI iroc with the 9 bolt with 2.77 gears and it got about 27 to 28. The stock injectors went out and I added lucas disc injectors (black with green stripe) and a 180 stat. Other than that it was stock with edelbrock shorty headers and I took out the baffles in the air cleaner and added K&Ns to it. I forgot that I changed the 02 sensor when I bought it also. I broke the original while installing the headers. I am sure that helped.

 

Thanks for the response! How long ago and was it a dual converter car?

 

You can use http://www.fuelly.com to track yours & see what others are getting.

 

That was from 90-98. On April 1st 1998 I bought my boss's 1990 ZR-1 Corvette (LT-5) and started driving it mostly along with my 89 Z71 truck. I am a true Arkansas *******. I still have all of these vehicles and they all run which makes me a high functioning *******. The ZR-1 would get well into the 30s gas mileage wise with that special engine and ZF six speed. It has a function that tells you the gas mileage you are getting and sometimes it would say 70MPG. The ZR-1 made 430HP at the rear wheels and the 89 Iroc made 405. The truck even got over 22 MPG. I still have two 81 Z28s I have owned since the mid 80s but they don't run but are still complete and not in bad shape at all. I did install a ZZ4 cam in the iroc in 1993 or so and when we tore the intake off the gaskets were not lined up very well so we trimmed the new ones to fit properly and advanced the timing some and that really woke the motor up. This was YEARS AGO. I had the dual cats and I beat a stock 1995 LT-1 Z28 and a 383 86 Trans am at the drag strip in the stock class. My times were better than corvettes and with the 2.77 gears I could go through the 1000ft in 2nd gear. Those were the days. I had 20 people wanting to buy my car back then because it was the best looking car in town (black) and the fastest stock car. I had a warranty paint job on it in 92. It only cost a case of coors to the shop foreman whom I went to high school with. I drove 15 or 20 Irocs back then before I bought one and my black one was easily the fastest so I bought it. I wanted a convertible but I couldn't find one so I bought the 350 car that would spin the tires much farther than the other cars I drove. Everyone noticed how fast that car was. If it didn't have 138000 miles in 2001 and the fact that I had a ford 9 rear, fully ported Brodix track1 and HSR, solid roller fully forged motor ready to go I would have never touched that car. I never lost a race in that car in the stock class. I have had as much fun in that car as any vehicle I have ever owned.

 

The trick to getting good mpg is running the proper engine temp. This means running a stock t-stat.
Low restriction exhaust & cold air intakes can make worse mpg then stock parts, b/c they move more air which requires more fuel. Those trick might be beneficial if you have much taller gear.
Hot air ingested in the engine diring cold days can really boost winter mpg.