Even if you did the change at 0 miles. By the time you reached 70kmi you only saved 3500miles as an extension at 5%

 

If you use 265 50 20 tires your speedo will barely be off, according to my gps. If youre gonna lower your truck those tires look sick, otherwise they leave ya with some gap to fill. look at my gallery to see what Im talking about. My truck came with 17s as well.

 

Your calculation is correct and I will gain maybe 2000 miles, but I am greatful for small gifts. I got lucky with my power steering leak, got it to the dealer with 35,840 on the clock. I know the dealer would have given me a hard time if it was 36,160.

By the way, I get excited on each 5 cent reduction in the price of gas. It's only about a dollar per fill up, but it at least is going in the right direction. $2.329 last Thursday for Amoco was good and the nickels add up. Thats $20.00 less per tankful from $3.33 I paid last September.