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Where did my gas mileage go?

I800C0LLECT

I800C0LLECT

Audio Moderator
I've been trying to figure out where to start with this whole thing...

My gas mileage went from 30mpg highway to 21.8mpg. Here's the changes to my car in the last year...

1) 18x8 tires and new rims. The MSR 045's are 4 pounds heavier than my Comp G rims.

2) LIM replacement done by dealership JUST before I had to move from Idaho to Virginia. Never noticed any issues with the engine running odd. Maybe I'm an idiot? Extremely poor timing right before my move too.

3) I noticed my a/c compressor is going out. Perhaps it's adding some drag to the serpentine? I don't see how it could unless the clutch is engaged. When I run my a/c my mileage drops more.

4) Change in elevation from 3000' to -1' due to move

5) I had to start using 10% ethanol since my move to VA. In Idaho I was using 100% gas.

*scratches head* I don't know where to start looking :/
 
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If your overall tire height is not the same as stock, that would be where you would be loosing fuel mileage. For every mile that your car thinks it going, in reality you've only gone per say 7/8 of a mile. The car figures the trip off that so it skews it.
 
Well here's the ratios...

I see what you're saying. I don't know if they'd account for that big of a jump? Would they?

tiresize.jpg
 
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Hope my math is at least close

You used to get 23,334 revolutions/gallon.
Discounting the wheel weight, and caulculating with only the size change at 30mpg, you should get 23,388 revolutions/gallon now.
At 21.8mpg, now you get 16,995.28 revolutions/gallon.

You used to travel 23334*84= 1960056" = 30.9352 mi.
You "should" get 23388*83.81= 1959914.4" = 30.9329 mi.
You currently get 16995.28*83.81 = 1424374.4168" = 22.4806 mi.

Think wheel weight is causing an 8.5mpg drop? My opinion is that you probably shouldn't have lost that much just from weight and friction.

You're not using something heavier than 5W30, are you? Um. Could be bearings in supercharger going, making more parasitic drag? Sticky tensioner pulleys? Out-of-gap/old spark plugs? Dirty intake and fuel filters? Old trans fluid? A sticky brake (happened to me once)?
 
I switched to 5w-20 Mobile One.

I've cleaned out the MAF and intake thoroughly.

Transmission fluid was changed at 91k. Now I"m at 108k.

Brake pads/rotors were changed ~103k.



I had thought that a pulley bearing or something had gone out...but I'm not sure how they should feel once I have the serpentine off. I haven't changed the fuel filter. I guess I could do that this weekend?
 
I decided that I'm going to run seafoam through the fuel system and check out the pulley's tonight after work. I'll follow up with a compression test later. Anything else I should add to the list? I don't have a scanner available yet :( Working on that...
 


Honestly, I'd switch back to 5W30... (unless you're monitoring oil pressure and it's good). It may not be a huge difference, but that could be the problem.
 
I doubt its any of that stuff..
I imagine its related to the style of driving..

If you were getting 30mpg, that was definitely all highway.. If you've even started to do a little city, you would start to see losses.
 
The other thing to consider is not just wheel weight but also it's distribution. See "mass moment of inertia." not saying that's solely your reason but could be a factor
 
Hrmmm. My highway mileage has ranged from 33 driving across the country vs. a second trip with TONS of weight averaging out at 28mpg. Same route and same 1500 miles worth of driving too.

My city mileage has been ~17.1 since this problem started. Before the issue it was ~23

Anyways...update!!!

I put in seafoam during lunch. I had about 2 gallosn of gas in the tank and put in a pint of sea foam. Then I drove it up the road to the gas station. Added 8.7 gallons of gas. After hitting every light and getting stuck at the security gate I still AVERAGED 24.1mpg over 5 miles!!!!

Hopefully this trend continues :) Going to drive a couple hours on the interstate tonight!
 


It's official!!! MY GAS MILEAGE IS BACK!!!! 27.5 combined after (mostly)highway and some city driving :D

SEAFOAM FTW!!
 
Don't go by your DIC. Its innacurate as hell. Do an actual fill up and then run it for a week, then fill up again.
 
Well even so...17.2 & 22mpg vs. 27.5 is a HUGE difference. I doubt the DIC is that far off...

I'm in the Newport News/Hampton area. Travel up to Richmond a lot too
 
i know where your at,
not to too far how far is it to richmond for you?, it takes me about 3 hours going north on 95
 


I800C0LLECT said:
Well even so...17.2 & 22mpg vs. 27.5 is a HUGE difference. I doubt the DIC is that far off...

I'm in the Newport News/Hampton area. Travel up to Richmond a lot too
Click to expand...
You're basing your 27.5 mpg off of less than a day of driving. The day I fill up I can be showing 27 mpg on my DIC when I get home. ( I reset it everytime I fill up) By the end of the week I am down to 22.5 MPG and really only got 21.5 MPG.

The DIC is innacurate, and the only way to get a true MPG is to fill it, drive a week or whatever, then fill again and do the math.

My point is, your mileage may not even been as low as 22mpg, and it might not be as good as 27 mpg.
 
99PontiacGTP said:
My DIC was off by 2mpg last time checked.

Lost about 2mpg due to the temp drop also...
Click to expand...

thanks ;)

Real world math is about as close as you can get! The few times I checked my accuracy was ~1mpg through out a cross country trip, checking at every fill up. I was satisfied


EDIT: FYI Stealthee. This is an issue that has been noticed and watched closely since this past June. I just recently posted. In that time frame my DIC has never approached anything above 23.5mpg. Speaking of DICs...kinda feel awkward...think I'll go now...
 
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