Mileage drop

I did a search and couldn't find quite what I was looking for. My gas mileage is getting terrible. I used to get 18-21 highway, now I get 14-15. I have 30" tires as opposed to the 235s I had before. But I continued to get 17-18 after installing the 30s, up until about the past month. I was told my filter was clogged (its a K&N), so I bought a new one (so I could switch between them and not have any down time due to cleaning). No change. The tires are all properly inflated. I even washed all the mud off the outside and that didn't even help . We're getting ready to take a road trip to AZ to visit family and meet up with fergie for some 4-wheelin so I don't want to be getting 15 mpg for the 400 mile trip. Any suggestions? I'm thinking of having the fuel system serviced. Would spark plugs help at all? TIA

Well the 30's you've got on there now are a little over an inch larger in diameter than the 235s.
If the speedometer gear is the same, you're actually covering a little more ground than what is being shown on your speedometer.

What did you inflate the 235s to?
What are you inflating the 30s to?

I'd say inflate the tires a little higher. On the highway it might get you better MPG. Just make sure it's not too high. (check the max pressure shown on the sidewall)

Personally I got rid of my K&N. I noticed that my oil was getting a lot more dirty when using it so I switched back to paper filters. The K&N does get better airflow but this also allows more dust to get by.

MAP

Pulled codes recently? Bad O2 sensors or a plugged/bad cat will cause mileage loss. The tires will make a difference in your mpg as well unless those figures are corrected for the different tire diameter.

o2 sensor is usually the first thing to look at with poor mileage. it can be tested with a multimeter. im at work so i dont have the specs.

also, a tune-up couldnt hurt. plugs, cap and rotor. good throttle body cleaning and injector cleaner.

with increase in tire size, the speedo and odometer are probably not accurate.

I don't think the tires are it cuz when I mounted them up I saw a drop to 17-18 highway and 15 rather than the consistent 15-16 city. I'll check the codes though, see if there's anything I can find there.

O2 sensors, thats what was why my mustang got bad gas mileage. After I changed them..............right back to normal.

automatic? My milage just took a dump (18 down to 15) and I think
the problem is the torque converter has stopped locking up.
Its pretty subtle but I think its running a few 100rpm too high..

yeah, auto, AW4. It does seem to be slipping a bit more than normal.

Finally had a tune up done and of course it was nice and simple...spark plugs appeared to be original 1996 vintage, 123,000 mile old Champions, ditributor cap and rotor were replaced and new spark plug wires. Mileage went back up to 17 on the way from Ft Mohave down to Tucson AZ and that was with a number of full-throttle passing attempts on 2 lane highways.

Well, my '96 has new everything-mentioned-here and my mileage STILL dropped to 15 this winter. I've always had 30" tires too, but got 17-18 even so.

I'm hoping it's the winter gas additives and my frequent use of 4WD Hi because of the incredibly snowy winter. The only other possibilities I can come up with are (1) that my rear drums weren't adjusted properly when I had a complete brake job done last fall and are dragging a bit, (2) that I got a bad new O2 sensor. Is that last possible?

To insure best performance, reset computer when changing/replacing EFI components.