Well i used to average about 20 mpg, but as of late ive been hitting 17 every time. I tried changing my driving habits, slower acceleration, shifting at 4.5k rpm and so on and my mileage isnt increasing back to the 20 i used to receive.

This is a 84 -SE with a large street port. Rb header/presilencer, cheap muffler. MSd ignition, strut tower bar, torque brace, lowering springs, removed emissions.

New plugs, wires, cap and rotor as of about 800 miles ago(my first attempt to fix my mileage issue)

My one thought is that maybe the cheap muffler is getting clogged and causing the loss in mileage. Any input would be great.

 

How's your air filter and injectors?

 

Did this change happen when the seasons changed? Gas mixtures are changed and you could be gulping some ethanol in your gas which will kill your mpg......yet another downside to the ethanol myth you won't commonly hear.

 

17mpg is still reasonable for a big street port. They really gulp fuel in the lower gears so has the traffic congestion become marginally worse? Changing at 4500rpm means the engine is always working in urban conditions. If the change in consumption is due to the engine going off tune, you should notice the difference in performance.

I find I get better fuel consumption when my wife is a passenger!

 

seafoaming my engine got me 3mpg more on average per tank, try that

 

With that kind of a gain, I'll have to try it!

 

Took the words right out of my mouth. Just to clarify though, put it through the gas tank, not through a vacuum line or in the oil.

 

air filters pretty dirty, bought a charger kit today, im actually cleaning it as i type this. Injectors have never been cleaned so thats my next thing i will probably try.

 

Since you have an SE, it could also be a faulty temp sensor.

 

which temp sensor?

 

I'm getting 15mpg city and 25mpg highway with my bone stock (except for a header) '84 SE. There is a discolouration on the passenger side of my back bumper, so obviously the car isn't running properly. I know there is something wrong, but I don't know where to begin looking. This thread is useful, so lets keep up the responses

 

just run a search for temp sensor and you will find many threads covering this. Look for posts by Longduck and RX7 doctor...

 

As a quick reference check; 17mpg city isn't too bad really. These cars were not known for their excellent fuel efficiency given the thirsty nature of the engine cycle.

That said, I did a full-on Six Sigma (process analysis) study based on fuel economy of my 84SE in the signature line and came up with City average of 17.5 and Highway average of about 25.5 - since most of us drive in 'mixed' conditions, you should expect something around 18-19 most of the time. Fuel mileage will tend to trend upward in summertime due to hotter outside air being less dense, and thus less fuel is required to get stoiciometric balance of 14.7:1 A/F ratio. Wintertime driving will have lower outside air temps, requiring more fuel to burn to keep the same ratio. Also, Oxygenated fuels (used in some locales) does NOT affect average MPG, as much as you hear anecdotal information to the contrary. The averages for both City and Highway in my multi-year study were NOT affected at all by oxygenated fuels (MTBE Ether or Ethanol).

That said, if you're trying to increase average MPG in city conditions, lay off your right foot - this is the single biggest root cause for poor fuel mileage. Spinning at higher RPM's and higher power levels requires more fuel. Shift at 5k conservatively and you'll gain about 1-2mpg on average. Leave more braking room in front of you so you're driving more smoothly (less braking, less acceleration), and you can gain another 1-2mpg average. Choose a driving route that has longer stretches between lights and stop signs (less acceleration), and you can gain some more.

Getting best MPG performance from your car is the opposite of getting maximum acceleration performance, so any mods you do to improve overall engine HP output will have a resulting loss in MPG - don't let anyone tell you otherwise.

Oh, yeah, and change your fuel filter if you've not done so in a few years. Put on a fresh air filter, change your plugs, cap and rotor, and wires if you need them. On top of that, pump your tires up to 35psi (if rated for this pressure), and get an alignment done. All of these will enhance fuel efficiency, but again, 17mpg City isn't bad for an SE. HTH,

 

See, what did I tell you? LongDuck is the man...