I'm looking for ways to boost the fuel mileage for my '99 Mercury Mystique. I'm only doing about 20 mpg city / 25 mpg highway. My friend has a '99 Ford Escort ZX2 with the same engine and tranny but it's doing 28 mpg city / 35 mpg highway. The Escort is only 5 inchs shorter and about 150 lbs lighter.

This is what I've done so far:
Replaced fuel filter, air filter, spark plugs, tranny flush, O2 sensor, thermostat, proper tire pressure (Note: it is using slightly bigger tires. It is supposed to have 205/60/15 and it has 215/60/15. These were the tires that were on it when I bought it.)

There are no CEL and the car runs and drives perfect. You guys have any ideas?

 

Those tires are throwing it off a little bit, but I can't say offhand how much. Have you pulled codes anyway? I'm not too familiar with computerized engines yet, but supposedly it'll store codes without necessarily tripping the engine light.

Also, is there a brake dragging? Maybe a wheel bearing going south or not packed properly? How's the alignment? And of course, how hard are you driving it? The last one oftentimes is the biggest determinant to your MPG.

 

All those bearings are probably permanently sealed. So unless they're noisy or there's some play to them, there's not much that can be done there. The slightly larger diameter tire will make the speedometer/odometer read slightly low, so you may in fact be getting slightly better mileage than what you're calculating. Just do a search online for a gear/speed calculator and compare the differences in speed when you change the tire sizes. Also, inflate your tires to within a couple of PSI of the rating on the tire, not the sticker on the car. The sticker on the car takes some allowances for ride quality and advertised weight capacity and may not be the optimal tire pressure for fuel economy. Just keep an eye on your tire wear though.

What are your driving habits? If you regularly drive 75-80 mph all the time then you're going to have a difficult time getting good fuel economy. Slowing by 10 mph or so on the highway could easily net you an extra 2-3 mpg. Also try coasting a bit more when coming to a stop (especially when approaching a red light) and just try to keep moving. If you're in the habit of nearly flooring it from a stop, try letting off the throttle a bit. Most ECUs command open loop (no O2 feedback) at about 75-80 percent throttle or if the mass air reading gets above a certain value. Usually the open loop tables run significantly richer than stoichometric in normal part throttle closed loop operation.

 

Small and permanent adjustments to that thing connected to your ankle will make the most significate improvement to your fuel mileage. Also, with a small engine, smaller and properly inflated tires will make a significate difference.

 

The tire difference puts my speedo off 2%. I don't speed by much. The way home has a speed limit of 80 km/h (50 mph) and I set my cruise at 90 km/h (55 mph). Everyone else cruises past me at 100-120 km/h (60-75 mph). I hold a steady acceleration upto speed. I guess I can get the codes read, but there is only one guy in town that can do it but I don't know how much it'll cost. The bearings are permanetly sealed and to replace them you have changed the whole hub. Any other ideas? I had posted a while ago on Contour.org but the thread went offtopic.