Ford f150 3.31 or 3.55 gear ratio

[FlyFishing]

I am looking to purchase a ford f150 and want to know how many mpgs i will lose if i go with the 3.55 i stead of the 3.31 axle ratio.
Thanks.

[LDB]

It's a 7.25% change in ratio so it might make a 7.25% change in mpg but I have no clue if it's a linear change like that or not. If it is, and if the 3.31 gave 20mpg, then the 3.55 would give 18.55mpg. I'm just going with the straight math. Hopefully someone that knows some of the engineering will come along with an answer.

[Matt715]

Is everything else the same such as tire size, extended cab, drive line, ect.?