why does my mileage suck

[patgizz]

hmmmm im scratchin my head here

 

on my way home from dc last month i ran 17mpg.  this ride is all but 5 miles of highway - non stop and go(cruise all the way) (those that know - i270 to 70 to breezewood, hop on the pa pike, take 76 almost to the end and i live right there)

 

little background - 89 silvy, 350/300 crate motor, new 700r4(lockup works right), 3.73 locker.

 

on that last trip i was running my old tires, in back were 255/60 radial ta's, approximately 27.8" tall(1.2" shorter than stock) so i was spinning higher than stock rpm and my speedo was off 5mph @60.

 

now last week i swapped over to 30x9.5 tires(the truck looked way stupid with those little tires) and 1" suspension lift

 

nothing engine wise changed

 

i calculated my mileage on the return trip(got home yesterday), same route, to be 15.8mpg

 

whats up wiht that 15.8mpg all highway???  when i stepped up the tire size(roughly same width tread when looked at side by side old tires and new) i knocked down my highway rpm's considerably - why would my mileage go down too?

[ryr8828]

If you're checking mpg from the odometer, could it be, The shorter tires show you going more miles than you actually did. Or show you going more miles than the taller tires would show.(The shorter tires have to rotate more times to go the same distance)  Just a thought, I think I just confused myself!

[Shaners]

ryr is right,

 

With the taller tires you actually traveled further than you recorded.  With the smaller tires you travelled less than you recorded on your odometer...

 

The 1" lift (believe it or not) will create more wind drag too...

[patgizz]

DUH!!!!!!

 

i didnt even think of that the little tires made it read more miles than it should have the the big tires made it read less than it should have.