So over the christmas break i had several changes on my truck. I put in a 2.5" leveling kit, a 1.5" add-a-leaf, 275/65-18 BFG all terrains, and my 89 custom performance tune got uploaded. Ive been running this set-up now for about 2 weeks and noticed my MPG has dropped about 4ish MPG... I expected a little bit of a drop, but this seems a little bit steep?

 

If the tires are taller, did you change tire size in the computer? You will lose mpg when raising and with heavier tires but that seems excessive.

 

ya, i used that formula out of the gryphon manual to enter in the new tire size when i uploaded my new tune.

 

Are you checking mileage by math? Miles driven divided by gallons? That is the only accurate method. You might consider returning to stock and running a tank and compare to a tank on the tune, see if it is the tune causing it.

 

i havent done it by hand yet. im just basing it off the message center. i know that usually reads a little high, but it should still give a good general idea.

but i was really wondering if it was the tune as well. that seems like the most likely cause for such a drop to me right now.

 

I think think your pushing more air with the lift and larger rubber in addition to the increased tire mass and resulting gearing issue. You could blame the tuner if you're driving more spirited, but with straight highway miles, the tune shouldn't be coming into play much. It would be a good idea to verify how close you are on the speedometer with a GPS. If the speedometer is off, the odometer is off too. An easy way to rule out the tune is to return it to stock and check your results.

 

Is your speedometer/odometer correct? I'd check your speedo with a GPS. I had to play around with different tire sizes on my Superchips tuner to get my speedo correct.

 

How much did you change your tire size? If you went up significantly, and most of your driving is around town, you will see a significant drop, due to pulling teh taller tires. Like Wandell stated check your speedo/ odo with a GPS, to make sure it is dead on.

 

i went from 265-65/18 to 275-65/18. So not that much different. It is a more aggressive tread though. i will grab a gps as soon as i can to see how spot on my spedo is.