poor mileage

[Deezel4x4]

well I guess a little history on my prob. would help so here it goes. My truck is a 97 with 12v and auto. My first mileage figures stock were 17 in town and 21 highway at 70 unloaded. I added a topper,305/70 16 tires and a # 8 plate. Now I am barely getting 14 mpg in the city. I have slid the plate and the afc forward a 1/8 on an inch and have lost track of where the star wheel started at or haw many click I have moved it. I get pretty good acceleration with a puff of smoke when I nail it from a stop. I am just wondering what to do to get some mileage back... . afc star wheel movement,Slid the afc housing back or slid the plate back?

 

[vc]

Deezel4x4,

If you went from the stock 245/75/16 to the 305's with out changing the gers in your speedo it will read slower than originally. According to a program I found on the net it should be approx 7. 69% less, so you should have to remember to factor that back in. Also, did you ever check your speedo before changing tires at a calibration shop? If not you can still check it and they can tell you what % it's off,or you can get a handheld gps and check it like that on the highway(I-95) for consistency. I would expect with the bigger tires you might loose 1 mpg on your truck. What's your rear ratio - you don't have a signature.

 

[Bigramguy]

The bigger tires will hurt the economy a little,as well as throw your mileage figures off as stated above. I have found with the auto,sliding the AFC all the way forward is a little too much. If you are getting smoke on accel,slide the AFC back a bit,and then tune it with the star wheel. You should still be able to get good low end without the smoke,which will help a bit on the MPG,especially in the city.