Alright so I have been turboed for almost 2 months now and gas is killing me.So I started averaging my MPG I was getting.City and back road driving I was getting 20mpg boosting every chance I got basically.But the next fill up i was getting 17mpg driving the same.Well I got tired of my horrible gas mileage and I figured if I change my driving habits my gas mileage would increase.Boy was I wrong I've drove like a grandma the past 110 miles and averaged my gas mileage and I was getting 16.5mpg. I have no CEL's besides an emissions code.Could be a fuel leak? I don't know I've been to busy to check it out.Also my gas hand is all screwy.I burnt 6.5 gallons of gas and i was on 3/4 of a tank, that doesn't sound right.And sometimes I will stay on full for 100 miles then it will drop like crazy.Anyone got any ideas?

My best guesses are

Fuel leak or somthing faulty in the fuel system
HOT and HUMID days 96 degrees today 10000000% humidty
With 18" rims and 245/40/18s my odometer is off.(although it should make that big of a difference)
Thats it.

TIA

Boostin'

Are you running with your AC on...I mean your saying it's hot and 96 there.

Even if you run it on both days you will have different mpg.

Depending on how much it takes to cool down the cabin, is how much extra fuel you will use..

I know it sounds stupid..but it's the only way I can think your mgp will drop on the same driving habits.

good luck !!


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It was a mix of having my a/c on and off since i was driving at night and during the day.But even with the A/C on and not boosting I think my gas mileage would be better than 16.5mpg.

Boostin'

first off, your tires are about an inch too big. use this Tire Rack tire size formula to figure out the right size. it'll keep you from driving too fast and using too much fuel.

what are you using for fuel tuning with the turbo? you might be running rich as hell. also check your O2 sensor, if it's fouled you'll run pig rich.

Erik P.
Official 2GNT thread Hijacker
Some say that he's driven over more Covenant than he's shot, and that his grenades aren't where you'd expect them to be...
All we know is, He's not The Stig... But he is The Stig's Spartan cousin!

Well I know when my speedo says I am going 60 I am going 62 according to
http://www.miata.net/garage/tirecalc.html Tire calculator.So for every 60 miles I would add 2 miles to my odometer right?Or for every mile since it takes 60 seconds to go 1 mile at 60mph?I am guessing its the 2 miles for every 60.TIA

Boostin'

UH, it doesn't quite work like that, since tire size affects the drive ratio, it changes exponentially, I don't have the formula but it's something like add another 2mph every 10-15 mph after 60. so if your speedo says you're doing 85 you're closer to 90+

Erik P.
Official 2GNT thread Hijacker
Some say that he's driven over more Covenant than he's shot, and that his grenades aren't where you'd expect them to be...
All we know is, He's not The Stig... But he is The Stig's Spartan cousin!

I dont know even if that was it I dont think my gas milage would be that bad because i was getting like 26mpg when i was N/a around here and i got 33mpg on the interstate.Im gonna check for fuel leaks and check my 02 sensor.

Boostin'

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Yeah i need to check for fuel leaks and my 02 sensor also gonna test for boost leaks, after i make my boost leak tester.I'm running the basic setup, Walbro 255lph HP, stock injectors, and a Vortech 12:1 FMU.I know I have a boost leak or something cause i can only boost 4 or 5 psi and my MBC was set to 8 before it started or whatever happened.Yeah it is ridiculous how the gas hand works on these cars.

Thanks for everything guys id like to figure this out cause i start college next month.Driving 80 miles a day getting 17mpg is going to suck!

Boostin'

Alright guys so I made my boost leak tester and my boost leak was coming from that little valve thats on the 1st prong of the 3 prong port in the back of the manifold.Im not sure what it goes too but its a service valve looks like its for emissions.The rubber cap popped off so i put a worm clamp on it and tested it again, I have a tiny hole not even the size of a needle in my BOV flange where its welded.Also I got a TINY amount of air to come out of the crankcase breather.Thats all I found for leaks and I replaced a hose clamp drove her and she boosts to 8psi no more hesitating.I'm guessing that could be my gross EVAP emissions system leak CEL too??I read boost leaks affect gas mileage also, and it definitely affected performance.So we'll see how it does now.Anyone else had that check valve boot pop off and cause a large boost leak? Thanks guys Ill check for fuel leaks and my 02 tomorrow when it isn't dark!

Boostin'

Alright guys so I made my boost leak tester and my boost leak was coming from that little valve thats on the 1st prong of the 3 prong port in the back of the manifold.Im not sure what it goes too but its a service valve looks like its for emissions.The rubber cap popped off so i put a worm clamp on it and tested it again, I have a tiny hole not even the size of a needle in my BOV flange where its welded.Also I got a TINY amount of air to come out of the crankcase breather.Thats all I found for leaks and I replaced a hose clamp drove her and she boosts to 8psi no more hesitating.I'm guessing that could be my gross EVAP emissions system leak CEL too??I read boost leaks affect gas mileage also, and it definitely affected performance.So we'll see how it does now.Anyone else had that check valve boot pop off and cause a large boost leak? Thanks guys Ill check for fuel leaks and my 02 tomorrow when it isn't dark!

Boostin'

My tire pressure is correct i checked it yesterday.Amps,subs shouldnt affect anything because they have been in there over the course of the cars life N/A and boosted.A/C doesnt use much more gas than both windows down as that creates drag.I'm talking about city/country driving so i cant draft off semis down the interstate.Although you wreak the benefits from draftings off of semis,you'll regret it when a Re-cap comes off and lands on the front of your car.But if you insist on never using your A/C then remove it, theres some weight reduction and power freed up.

Boostin'