newbie with milage question

[doc606]

Just found this website, and its great. I have a few questions i thought someone might help me with. i have an 87 s-10 2.5l with 4 speed. i have been getting about 27mpg with it, however 2 weeks ago i went from p20570r14 to p20570r15 on the truck. the same weekend i had to change the valve cover gasket, and of course had to remove the egr valve to get to a bolt.I put the egr valve back on with a new gasket. ever since that weekend my milage has gone down to about 22mpg. so my questions are:
will the larger tires affect my mpg that much, or did i possibly mess something up with the egr valve. and how do i check it. i just recently tuned up the truck and it runs ok for the 2.5l. it has always had a "slight miss when idling and i cant seem to find out what causes this. i really would like to get the mpg back up to about 27mpg as i drive it everyday over 120miles roundtrip back and forth to work. any suggestions will be appreciated.
Tks,
Doc

[brooster]

You're mixing apples and oranges I think. By changing tire sizes (14 to 15), you've also changed the accuracy of the odometer. You're probably traveling the same distance, but the odometer doesn't reflect that. But it shows you traveling a shorter distance and using the same amount of fuel. Result is lower mpg when you crunch the numbers.

[Fox_Rocks]

i agree, although at the same time you are pushing bigger tires so you might be losing some there too...just a thought.

[greendayhbt26]

if you do get bigger tires, is there any way in a newer truck the computer can be reset to comensate for this, and is milage affect much by it?

[E_C_M-town]

milage is affected more as the tire size is raised. as for the case of changing the wheel size and keeping the same size sidewall, he increased the overall size by one inch.(correct me if i'm wrong) But as far as the calibrating the speedo, too much expence for the benefit, to me.

[MarkSS]

Changing from a 14" to a 15" rim will not have effect on the mpg unless you change the diameter of the tire. The width of a tire may cause more drag and effect the mpg.