Mpg issues

[Zzp98gs]

Hello all, my name is Bryan and I'm new to this forum. I have a 98 regal GS and I think I'm having mpg issues. When I reset my mpg on the dic it says 28.5 hwy and 15-16 city which seems right however when I look to see how many miles I've gone when I put gas in, it's always not alot. I fill up at 1/4 tank usually and go 225 miles with the majority of miles as highway. The mpg looks right but the miles to tank so to speak doesn't. I've talked to people clearing 300 or more to a quarter tank.

My long term fuel trims are 3-5 highway cruising. City they are a little elevated to 7-9. Sitting at 10 at idle. When I go wot I hit 11.7. So I'm leaning out at wot and I'm lean around town. Now my fuel pressure is sitting around 41 psi at idle which is kind of low. Could that be why I'm leaning out under heavy acceleration? I'm kind of stumped at this point. If anyone can shed some light that'd be awesome. Any questions let me know, thank you in advance!

[Fivefingerdeathpunch]

More info about the car.

Recent tunes up?

What are the mods to the car?

What gas do you use?

You might have a small vacuum leak if you LTFT keep creeping about around and past +10.

[Zzp98gs]

It was tuned up back in 2010 with autolite 104's.i pulled them the other day and they looked good still surprisingly. I compared it to a new one and it looked almost identical.

Mods are headers intake 3.4 pulley zzp pcm that's about it

Always use 93 octane

Me and a buddy have sprayed down every vacuum line under the hood and saw now changes in fuel trims unless we spray directly at the intake filter. Maf was just replaced today

[Fivefingerdeathpunch]

3 years is far far too long to run a single set of copper spark plugs and they are too cold for your mods.

It won't make a huge difference but maybe some, but swap those plugs for some AL 605's and only run them for around 15,000 miles. The waste spark ignition system we have tends to make the plugs live a short life. At only like 12 bucks for 6 new plugs its super cheap anyway.

Another thing that will affect it mpgs is your speed. Which I figured you'd already know, do 65-70 instead of 75 on the freeway and you'll gain a mpg or 2 easy.

[Zzp98gs]

Ok I'll swap them out next chance I get. I was under the impression to go with 103 or 104 with those mods. And I do 65 with a light foot on the highway. Maybe 70 on occasion but I knock it down when I realize it.

[88formula]

I get 28mpg with the mods in my sig, doing a consistent 80 on the expressway.

[Zzp98gs]

80 huh? Lol

[L36 Killer]

You might want to start logging exactly how much gas you are using versus the miles driven. fuel gauges aren't the most accurate gauges in the world, so it's better to write down how much gas you put in at fillup, then divide the number by the miles you drove.

[88formula]

80 huh? Lol

Yeah I dont mess around with that 65-70 BS... All last month I was working at a jobsite 85 miles from the house, drove there and back everyday. Gave me a good chance to calc MPG's... Honestly 81-84MPH every morning, and every afternoon. Well the days that I didnt feel like riding my motorcycle that is. Then it was 90+MPH with 40+ MPG lol

[88formula]

And yeah, I dont pay attention to what my display says "Miles to empty" I wrote down my miles between fill ups, and divided by the gallons I put in it. Although my SC tensioner was basically shot during that time, but I dont think that would matter since I was on a flat portion of I75, during clear traffic times of the day, so my cruise control never had to downshift. Basically stayed out of boost the whole time.

[Zzp98gs]

You might want to start logging exactly how much gas you are using versus the miles driven. fuel gauges aren't the most accurate gauges in the world, so it's better to write down how much gas you put in at fillup, then divide the number by the miles you drove.

That's what I do now. I usually average 22-23 mpg with my city and highway driving. I've driven it dry and got 300 miles on the dot, awful lol. But I haven't done that since I put my throttle body screen back in so it may be different now. I haven't gone a tank yet. I got impatient last night and filled up with a 160 miles driven, majority highway, and burned through 7.7 gallons which averaged to 20.7 which is what my dic mpg said. I nearly cried lol

[88formula]

Yeah. I dont like to talk about my city MPG's I am pretty sure it is like 16-18. lol

[Zzp98gs]

Agreed lol. That's about what I'm at.

As far as long term trims leaning out at wot(11.7) what could cause that? Cuz they are normal between 2-4 at normal driving then they spike.

[camopaint0707]

Yeah I dont mess around with that 65-70 BS... All last month I was working at a jobsite 85 miles from the house, drove there and back everyday. Gave me a good chance to calc MPG's... Honestly 81-84MPH every morning, and every afternoon. Well the days that I didnt feel like riding my motorcycle that is. Then it was 90+MPH with 40+ MPG lol

You do not get 40mpg at 90mph. You just don't. Stop trying to BS everyone.

[Fivefingerdeathpunch]

He's talking about his bike.

[camopaint0707]

Was he? I missed that.