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Discussion Starter #1
Somewhat general question, I'll give you the background first.

I put new wheels on the ride (86na) a couple of months ago. 17" Motegis with Dunlop SP 255/40 rubber. They're inflated to 40, max psi listed as 50. I noticed the alignment was off a bit around the same time I noticed a drop in mpg (from about 15-16 to 10-11 street, heavy stop-and-go)....can't remember which happened first.

How and why does poor alignment affect my mpg? I'm getting about 20-22mpg on the freeways consistently, but blowing a tank of gas in almost 1.5 weeks because of this problem. CHTS is also out, but will be fixed soon (still stuck in head).
 

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my alignment is a little off and i'm getting 28mpg, so i wouldn't think thats your problem. I am just too lazy to fix it, i'm buying new front tires pretty soon and i'll fix it then. generally over inflating your tires will help mileage a bit, but will cause your tires to wear too fast in the center. is your engine heavily modded? I just can't see how you can only be getting 10-11 mpg, mine used to do about 18 when i was racing around, now that i use acetone it gets about 24 under the same conditions. I'd say your engine probably needs a full tune up the timing checked too.
 

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u must have a leak. dont smoke and drive. change ur air filter, check tire pressure thats two ways to cheaply inprove ur gas mpg
 

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Discussion Starter #7
I'm putting in some UV dye before I leave work and will check tonight to see if I have any leaks.....the lack of mileage is bugging me, especially with all these claims of 18+ in city traffic w/o acetone and me being happy if I score over 10mpg street.

Also, it was my impression that increasing, not decreasing the tire pressure would lead to better mpg at cost of quicker wear on the center rubber. Since my max pressure is 50, 40 in city should be fine. Clarification?
 

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u r right about tire pressure, but it is un safe and costly on tires. if u want get some narrow tires with the largest size that will fit ur car . that should inprove ur miles about 3 percent depending on how big ur tire is. remember that ur speedo will be reg a slower speed
 

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over inflation = wears in center, slightly better mileage, more risk of blowout due to impact (potholes, etc)

under inflation = wears on edges, worse mileage, risk of exposing the steel belt after too long.
 

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holy ****, nobody bothered to post the correct answer? the higher pressure give better milage, cant argue that.

mis-alignment unless severe would not have a measureable effect.

new crapmaster bigger (and by looking at the specs, ridiculously heavy) wheels and tires have a massive effect, especially in stop and go. ****, i thought this was a trick question since he gave the biggest clue in the world.

Is this a test question and I win a prize or was the original post just so darn descriptive that the obvious answer was ignored by everyone?
 

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heavy wheels will definitely impact mileage. no question. But 10-11mpg in city is definitely due to way more than heavy wheels. the stock wheels on my car are pretty **** heavy and i'm getting 28mpg. I think the largest problem is his engine is in need of a serious tune up.
 

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Discussion Starter #13
PHX , I'm going to check the timing and replace plugs/wires this weekend. I checked the car last night and no visible leaks, so I'll check again tonight in case dye hasn't drained down yet.

Clowns, thanks for the answers also. However, the 17" Touges are actually just a bit lighter than those tank stockers that I had. I lose a little performance off the light, but I don't think I'd lose 5mpg city from tires alone. But thanks for giving me another place to look.

I'll keep you updated....maybe this can help someone else, since apparently I'm not the only in the Great Performance, Craptastic Mileage Club (GPCMC to the uninitiated).
 

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"I'm going to check the timing and replace plugs/wires this weekend. I checked the car last night and no visible leaks, so I'll check again tonight in case dye hasn't drained down yet."

make sure you replace distributor cap, rotor, and o2 sensor if you're doing the plugs and wires. no sense in doing half a tune up.

is your engine modded heavily? cause I just don't see how an n/a can get that low of mileage without running like crap...
 

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Discussion Starter #15
cap and rotor replaced last weekend and 02 replaced earlier this year. Engine is a complete rebuild, slightly overbored, but runs just fine.
 

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one other thing to consider then i guess would be the AFM. There was an adjustment screw on it on the earlier z31 AFMs that would righen/lean the mixture. See if that screw is exposed (theres normally a cover over it) and if it is, it may have been tampered with. You'd have to have a air/fuel mixture guage to check it or adjust it back.
 
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