I have the Sniper Special forces tuner that I have used to tune my 1997 Expedition 5.4 4x4. Recently I upsized my tires from 265/70/17 to 285/70/17. I changed the tire size setting in the Sniper tune to account for the bigger tire and hopefully correct my spedometer. I say hopefully because I am not sure it worked. Can anyone else who has the Sniper tuner vouch for the speedo/computer correction? I am worried my speedo is still incorrect because according to my trip computer my MPG have dropped way off. I was expecting a little drop, but I have gone from over 15 mpg to under 14 (and that is the best I can do taking it REALLY easy). I would probably be in the 12's if I put my foot in it.

Anyone have any ideas? Thanks.

 

big tires reduce mpg in general; feather foot'in is all ya got

 

I haven't changed the size of my tires on mine, but I really don't see how it could be any different than any other programmer.

I guess one way you could maybe check is to go on the interstate, and see if you travel on mile between mile markers (assuming those things are accurate).

But yea, bigger tires will usually drop your milage a bit.

 

Thanks for the responses...you guys are probably right. I was just hoping to only have a drop off of a 1/2 mpg or so...of well...I like the way the new tires look. I think next time I am on the interstate I will check out the mile markers.

 

If you want a better tune, send me a pm. I know a guy that uses the delta force, not special force, software to create truley awsome tunes. I am thrilled with the results of mine. I know that for the tuner software, the tire size goes by revolutions per mile, not neccisarily tire size.

 

If it's Matt who you are talking about, yea, he does great tuning for the Expeditions.

As for the tire size thing, you could do it either way. Find the circumference of the tire and divide that into who many every inches there are in a mile. I think that should give you revolutions per mile.