I did my search and didn't find the answer. I'm not sure if this is a problem or not but it certainly does worry me. I just changed my spark plugs past Thursday and installed DENSO Iridium Spark Plugs. I filled up my tank friday afternoon. Most of my drivings are highway milage and I've always able to hit about 300 miles before I fill up. usually at 150-175 miles i'm at half tank. But Friday night after I've ran nearly 60 miles at about 80 mph average, to my surprise, my gas meter has not dropped under the F. It seemed as if I had just filled up. Usually I'd be at quarter tank by then. I haven't felt any power loss or wierd noise of any sort. if anything, the spark plugs gave me better response and the car felt more powerful. so this morning I got really worried when I hit 75 miles and decided to just fill up again to check. It fill up with 1.8 gallons... WTF? is there something wrong? did i install the plugs wrong? anyone ever heard of this?

 

So your fuel gauge is reading lower than normal or you're loosing gas somewhere. Do you smell gas when you step out of your car?

 

you drove 75 miles on 1.8 gallons?
and before. it used 3/4 of a tank to do the same thing? Is that what you are saying?

First you have to consider if you stopped at the first shutoff click on the nozzle or you kept topping it off. There can be 2 gallons burn off before the needle drops when you fill the tank to max.
Then high speed makes for hot exhaust which expands the gas in the tank.

Was this this first plug change- if so, on a 99- I would expect amazing new feel and increase in fuel mileage.

 

mmm... no, I'm actually getting better gas milage to the point where it's scary. I've gone 75 miles and the gas meter has not dropped from full tank.

 

this was the first plug change for my 99. usually it will use 1/4 tank for about 75 miles. when i fill up, i always click two more times after the first. This is just a huge difference in gas mileage so I'm just making sure. even still. it's a little absurd to see my car do nearly 35 miles a gallon.

 

I think you've got to fill her up and reset the trip and drive it to a few clicks before E and then write down the mileage and fill her up again and then divide the mileage by the # of gallons you just put in and test it the right way. That's the only sure fire way to see what your MPG is.

 

stop overfilling the tank for a start!!- the gauge sendor is made for a certain range- and you are exceeding that- plus you will never get an accurate mileage calculation unless you do a standard first shut-off and stop filling.
They have it set to shut off for a reason- including fuel expansion and protecting the
cars evap cannister from fuel entering it.
You should get 300+ miles on a normal tank from full to the fuel warning light

on that 99 do you still have plug wires and cap/rotor- change those too
then read the DIY in gen2 titled Seafoam

 

werd man.. it even says "do not top off"

 

Some of us even touch the bare metal of the pump before touching the fuel nozzle, especially if you have ever gotten a shock getting out of the car!

Dont forget- minimum 3 clicks on the GAS CAP after fueling- its an airtight system.

 

And this is a problem because......

I wish I had this "problem".

 

Thanks for the tip. I'll update after this tank runs out.

 

I never said over fill it. Just fill her up until the pump clicks off. I believe my method is very accurate.

 

the first click that the pump shuts off at is FULL as far as the car is concerned!
Sure you can OVERFILL it with 2 more click shutoffs, but based on exhaust temp and other factors, the tank could actually suffer expansion deformation-
an extreme case example yes- but, you are an extreme driver.
Going past the first click shutoff will cause the guage to stay on Full for some time.
The 2nd half of the tank always goes faster to- just to make up for it~

 

I believe 01tl4tl was referring to post #5.

 

my bad.