Are those Imperial gallons? If U.S., it should be getting 28 mpg on the highway. How many miles on it? How long since the plugs were changed?
Check the temperature sensors (there are two, one for the computer & one for the temp gauge). If the one feeding the computer has gone bad, the engine computer will think the engine is always cold & mileage will drop dramatically. Likewise, if the thermostat has gone bad and is not closing, the engine will never actually warm up. Do you get heat from the heater after the same amount of driving as in the past?
99' Maxima automatic 3000 6cyl.Only getting 18 miles per gallon hwy. Checked plugs and air filter.All fine. Should get 23 to 24. Car runs great. Plenty of power and torque. Anybody got any ideas
My 99 SE auto gets a little better than 25 in pure highway driving, provided I keep it around or under 2500 rpm. I get about 18.5 in winter in mixed driving (lettting it warm up in the morning) and around 20 mpg overall in the rest of the year.
I still have a '96 and a '97 with the same engine & auto trans configuration as the '99 noted above. The original EPA sticker was 21 city, 28 highway,
23 average on the '96, '97 and '99.
Here's the actual mileage records for the '96, which I bought significantly used & beat up. This is over over 47,000 miles (from 80,000 when I got it to currently at 127,000): lowest tank: 17.4, Highest tank: 31.7, overall average: 23.7.
The '97 is an i30t that I bought new. It will get 30 on the highway if you set the cruise at 80 (MPH) and drive out the tank, ~28 if you stop a couple of times. Actual calculated mileage over the first 80,000 miles (my daughter started driving it then & doesn't keep detailed enough records anymore):
Lowest: 16.9 Highest: 30.1 Overall Ave: 23.3
With a decade of experience with these cars, I have observed that tire pressure makes the biggest difference. They seem to do the best, both gas mileage and tire longevity, with 33 psi in the front and 31 psi in the back, similar to what most were recommending at the time on Maxima.org for the Maximas of that same generation.
The original EPA sticker on the '00 i30 was 20 city, 28 highway, 23 combination, a little less than the previous body style (heavier?). There were also changes to the engine in '00.
I averaged 28.7 mpg over a 4300-mile road trip last summer in a '00 Infiniti I30, which is basically a Maxima with 100 lb of extra sound insulation; around town I generally get 20-22 mpg. (Currently: 104,600 miles.)
Apparently the Mass Air Flow sensors from 99 up are prone to falling out of spec early in there lifetime. They just wont last. As they age the calibrated idle air adjustment that the ECM is programed with at the factory changes. This in turn makes the ECM change the fuel mixture and you find the car getting less gas mileage. I ask the dealer and he said if you have no other simptoms he can calibrate the MAF sensor again. All they do is have the ECM re zero current idle air calibration. Hope this helps.
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