Oh boy couldnt tell ya... I am going to do a fuel system cleaner to hopefully help a little. I dont think I am used to driving a V8 yet so the gas part is had to get used to lol
Check your tire pressures, too. Adding just 2 psi over the recommended pressure can help, without causing other issues.
Are your air intake and exhaust in good condition? The engine needs to breathe properly to be most efficient. Also, try to be gentle (I know!) with the throttle in normal driving.
Are your air intake and exhaust in good condition? The engine needs to breathe properly to be most efficient. Also, try to be gentle (I know!) with the throttle in normal driving.
Don't waste your money on any snake oil products. Let's begin at the beginning.
Tell us what method you are going to use to get your initial calculation for fuel mileage so you can share that with us.
IIGood
Founding Member
For reference, my 2003 GT (which has the same 4.6 V8 as your '99 GT) with manual transmission gets about 19-20 MPG in mixed driving. My tire pressures are at 35 psi and the engine is all stock. Air & fuel filters are changed regularly...well, the fuel filter is a little overdue at the moment, but still.
There was one time...just one time...that I ran a whole tank on all highway driving and pulled 30 MPG out of that. I couldn't believe it myself and did the math a couple times...it all checked out. But that was just once. I really don't do much of any all-highway driving, it's all mixed now...it's mostly just my commuter back and forth to work on nice days now that I have a second vehicle.
But yes, as others have requested, let us know what your routine is now. How will you calculate your MPG? Is it a manual or automatic? What size wheels are on the car (if they aren't stock)? What are your tire pressures like? How do filters look? Are there any modifications to the car?
Oh, and is it a coupe or a convertible?
There was one time...just one time...that I ran a whole tank on all highway driving and pulled 30 MPG out of that. I couldn't believe it myself and did the math a couple times...it all checked out. But that was just once. I really don't do much of any all-highway driving, it's all mixed now...it's mostly just my commuter back and forth to work on nice days now that I have a second vehicle.
But yes, as others have requested, let us know what your routine is now. How will you calculate your MPG? Is it a manual or automatic? What size wheels are on the car (if they aren't stock)? What are your tire pressures like? How do filters look? Are there any modifications to the car?
Oh, and is it a coupe or a convertible?
IIGood
Founding Member
Okay, now that we have the car specs established, what is your current gas mileage and how are you calculating It?
An automatic will most likely get slightly lower gas mileage than a manual so there's one impact.
IIGood
Founding Member
17 doesn't sound too far off from what it should be. Check the tire pressures and be sure they are set to at least 35 PSI when cold. And then do this to calculate the average MPG:
- Completely fill the tank
- Before pulling away from the pump, either reset the Trip Odometer to zero, or write down the overall mileage showing on the odometer
- Drive the car as you normally would until the tank runs out
- Completely fill the tank again and this time, note how much fuel went into the tank...right down to the last thousandth of a gallon (e.g., 12.875 gallons)
- Look at either the Trip Odometer and see how many miles it registered since the previous fill, or subtract the last fill's overall odometer reading from this fill's overall odometer reading
- Divide the miles you drove by the amount of fuel you put in the tank on the second fill; example: 250 miles / 12.875 gallons. This will produce your average MPG for that tank.
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IIGood
Founding Member
No, leave them at 35. Overinflation like that can cause uneven wear and worse, like loss of control. You can add an extra pound or two to help.
It really just sounds like 17 is normal.
When I upgraded the tires/wheels on my other car from 18's to 20's, I knew the door sticker at 35 psi was not valid any more. That is only for the stock size wheels and tires. I started at 35 psi (ride felt "mushy" and steering took more effort), and slowly worked my way up to 45 psi (ride was bouncy and harsh, steering felt very light, car rode "skittish" on the highway), I eventually got it narrowed down to 40 psi. That worked very well, tread wear was good, too. 
Another thing is tire size matching with the speedometer, gear ratio, speedometer gear in transmission all have to work together to give accurate mpg readings, see how accurate you odometer is by doing 60 mph and match the mile markers on the highway or stop watch, 60 mph is one mile a minute so check you numbers.
Go by the door sticker which takes into account tire contact patch surface area and weight on each tire for recommended tire pressure. Tires just list a MAX pressure they can safely be inflated to, but since they can be mounted on many different vehicles, the recommended pressure will vary.
Of course this goes out the window when you change tires or tire sizes. If you go with wider tires with bigger contact patches, you can typically reduce pressure.
Anyway, sounds like your MPG is really average depending on your driving style. I remember my 2003GT used to average 25-27MPG highway, but in town with shifting 17-20 was typical. You really need to drive the car like you have an egg between the gas pedal and your foot to get good mileage. If you want better gas mileage, time to buy a different car.
Since I did my cams I only average 16 mpg in mixed driving and I don't even care. Still gets mid 20's on the highway though. I can deal with it to have a V8.