Hey there.I just got a swell 2000 Mercury Grand Marquis and it is immaculate.Garage kept and rust checked.Intiror is mint.The exhaust was replaced with an aftermarket one and the gas mileage I am getting id crazy .Good that is.The best I ever got from my 95 Marquis was 34 mpg with mostly highway use,synthetic oil and a K&N air filter.This 2000 though is getting an average of 36 mpg,6.5 litres per 100 km.{I'm in Canada].It has duals and not stock.Normal air filter.I'm guessing a chip may have been put in,but I'm confused.Verry happy,yet confused. Any and all feedback would be cool,and appreciated.

 

Hello, Ron, and welcome to the Mercury Forums -- That 2000 Marquis sounds like a fine ride -- good luck with it !!
You are correct-- your gas mileage (as reported) IS crazy -- your claim of 36 mpg as an average is, frankly, impossible !!
I believe that the method of determining your fuel mileage is faulty.
Here is the CORRECT method for determining your EXACT fuel mileage:
1.Fill the fuel tank completely.
2. Record the odometer reading.
3. Drive the car to run the gas down to about one-quarter tank.
4. Fill the tank completely, and record the amount of fuel used to two (2) decimal places.
5. Record the odometer reading.
6. Subtract the previous odometer reading from the new odometer reading to obtain miles (kilos) traveled.
7. Divide the miles (kilos) traveled by the number of gallons (litres) used. -- Use 2 decimal places for accuracy.

I keep a Fuel Log on my '95 Marquis Restoration -- it shows:
1. Odometer reading.
2. Gallons purchased.
3. Cost-per-gallon.
4. Amount paid.
5. Date of purchase.
6. Time of purchase.
7. Location (city).
8. Brand of fuel.

I have kept a fuel log since 1972, showing travels in all 50 US states, 3 Mexican estados, and 3 Canadian Provinces.

 

I am aware of what to do.i do appreciate your reply.A change of 2 mpg is not impossible at all.I got 32-34 on highway with my 95 with synthetic oil and a k&n air filter.An aftermarket dual pipe sytem and a tuner chip has to be the answer as my math done the way you stated,as I have always done,is correct.I do wonder though,how to find out if it has a chip in it.This is not an area that i am familiar with.

 

Review:
In your first post, you stated that "the best I ever got was 34 mpg with mostly highway use, synthetic oil, and a K&N air filter." That was on your '95 Marquis. It was your "BEST".
That's believable, but only because the "BEST" my '95 MERC ever got on a tank was 33.2 mpg -- ALL freeway, ALL downhill (loss of 2,200 ft. elevation), ALL in 4th gear (overdrive), with Mobil 1 5-W-30 synthetic oil, dual exhaust (2-inch glasspacks), and a K&N air filter. Chevron 87 octane fuel -- With Driver, One Passenger, and luggage. ACTRON #CP9145 Scan Tool hooked up and running.
Those were absolutely ideal conditions for high fuel mileage, and were never encountered again.
95 MERC's Trip mileage averages @ 24 mpg, with occasional tanks yielding up to 28 mpg. Your '95 Marquis should be no different.

The 2000 Marquis:
You say: ."This 2000 is getting an average of 36 mpg." I call your attention to the word "average", with a reminder that there is NO WAY that science, math, or Jesus can make a 4,000-lb vehicle with a 4.6 Litre V-8 and automatic transmission get 36 mpg as an AVERAGE. The only possible answer to both the '95 and the '00 getting those fuel mileage numbers is that the method of calculation is incorrect.
But hey, I don't come here to argue -- I come here to read, learn, and help others to solve problems with which I have knowledge and/or experience.
About those "chips" -- There are 2 kinds of chips. There's the "Tuner Chip", which costs hundreds and must be set up by a pro engine tuner, and there is the "Performance Chip", which sells for around 70 U.S. dollars and can be installed by anyone with sight.
I bought, installed, and tested one of the 70-dollar so-called chips -- and tested it for 2,000 miles with an ACTRON scan-tool hooked up and running.
After the test:
NO measurable performance increase.
NO reduced 1/4th-mile strip times.
NO fuel mileage increase.
After such poor results, I cut the "chip" housing open -- only to find that there is NO CHIP of any kind inside the housing !!
There is a small resistor inside, whose function it is to fool the engine's computer into thinking that the inlet air is colder than it actually is, thus adding a richer mixture and theoretically gaining more power.
It's a scam !! The factory Oxygen Sensors detect the rich mixture instantly, and automatically compensate to bring the mixture back to the ideal of 14.7-to-1.
The result is a constant battle between the so-called "chip" and the on-board computer. You lose every time !!
If your car has one of those "cheater-chips", you can find it and un-install it easily.
There are 2 wires attached to the "chip" -- and one wire goes to the M.A.F. Sensor. Disconnect both of the wires and remove the "performance chip". Say a prayer for the 70 dollars that was wasted.!!
The "tuner chip" is a lot of expense to invest in an 18-year-old, 2-ton, 4-door sedan that is no longer manufactured. I doubt if a tuner chip has been installed, but don't know of any way to be sure.
Tuner chips for the 4.6 Litre V-8 are most often run on light-weight Mustangs that run the 1/4th mile. Geezers in Grand Marquis have no need for speed, but may be concerned with their fuel mileage.
I try for a combination of both. 1/4th-mile @ 16.8 sec. - 86.2 mph.
Fuel mileage averages:
16.4 mpg City
24.6 mpg Highway

 

Trying to be somewhat polite, here, but your claim of 32-36 mpgs on a 2-ton car is simply NOT POSSIBLE to average for an entire tank of fuel (250- 350 miles).
There is NO CHIP and/or NO DEVICE that can be installed on a Grand Marquis that would yield such little fuel usage.
Yes, you may get 1.71 gallons to last for 62.1 miles, but you won't get the same result after driving until the tank needs a fill-up.
It's a matter of physics -- moving so much weight into the equation -- makes it impossible.
Your method of fuel mileage calculation must be at fault -- there is no other explanation.

95 MERC Engine Bay