Whenever you change tire size, there will be a direct effect on power and driveability. The chart below will assist you in finding your new gear ratio needed to give you optimal performance. This chart is based on a drivetrain running at a 1:1 gear ratio at 65 MPH using a manual transmission, meaning the transmission and transfer case are passing power through from the engine at a 1:1 ratio without any overdrive or reduction before it reaches the differential. Automatic transmissions will run at a slightly higher RPM and applications with an overdrive transmission will run with slightly lower engine RPM.       
First locate your current gear ratio and tire diameter and then find your approximate RPM at 65 MPH, then reference your new tire diameter with your desired RPM to find the new gear ratio needed.      
                                           
  Most Power, Best for Towing, Reduced Fuel Economy                                  
  Daily Driver, Best Overall Performance                                  
  Highway Driving, Good Fuel Economy                                
                                           
Gear Ratio        
  2.73 2.94 3.07 3.21 3.31 3.42 3.55 3.73 3.91 4.11 4.27 4.56 4.88 5.13 5.29 5.38 5.71 6.17      
25" 2385 2568 2682 2804 2892 2988 3101 3259 3416 3590 3730 3984 4263 4482 4621 4700 4988 5390      
26" 2293 2470 2579 2696 2780 2873 2982 3133 3284 3452 3587 3830 4099 4309 4444 4519 4796 5183      
27" 2208 2378 2483 2597 2677 2766 2872 3017 3163 3325 3454 3689 3947 4150 4279 4352 4619 4991      
28" 2129 2293 2395 2504 2582 2668 2769 2909 3050 3206 3331 3557 3806 4001 4126 4196 4454 4813      
29" 2056 2214 2312 2417 2493 2576 2674 2809 2945 3095 3216 3434 3675 3863 3984 4052 4300 4647      
30" 1987 2140 2235 2337 2410 2490 2584 2715 2846 2992 3109 3320 3553 3735 3851 3917 4157 4492      
31" 1923 2071 2163 2261 2332 2409 2501 2628 2755 2896 3008 3213 3438 3614 3727 3790 4023 4347      
32" 1863 2007 2095 2191 2259 2334 2423 2546 2669 2805 2914 3112 3331 3501 3610 3672 3897 4211      
33" 1807 1946 2032 2124 2191 2263 2349 2469 2588 2720 2826 3018 3230 3395 3501 3561 3779 4083      
34" 1754 1889 1972 2062 2126 2197 2280 2396 2512 2640 2743 2929 3135 3295 3398 3456 3668 3963      
35" 1704 1835 1916 2003 2065 2134 2215 2328 2440 2565 2664 2845 3045 3201 3301 3357 3563 3850      
36" 1656 1784 1862 1947 2008 2075 2154 2263 2372 2493 2590 2766 2961 3112 3209 3264 3464 3743      
37" 1611 1735 1812 1895 1954 2019 2095 2202 2308 2426 2520 2692 2881 3028 3123 3176 3370 3642      
38" 1569 1690 1764 1845 1902 1966 2040 2144 2247 2362 2454 2621 2805 2948 3040 3092 3282 3546      
39" 1529 1646 1719 1798 1854 1915 1988 2089 2190 2302 2391 2554 2733 2873 2962 3013 3198 3455      
40" 1491 1605 1676 1753 1807 1867 1938 2037 2135 2244 2331 2490 2664 2801 2888 2937 3118 3369      
41" 1454 1566 1635 1710 1763 1822 1891 1987 2083 2189 2275 2492 2599 2733 2818 2866 3042 3287      
42" 1420 1529 1596 1669 1721 1778 1846 1940 2033 2137 2220 2371 2538 2668 2751 2798 2969 3208      
43" 1387 1493 1559 1630 1681 1737 1803 1894 1986 2087 2169 2316 2479 2606 2687 2733 2900 3134      
44" 1355 1459 1524 1593 1643 1698 1762 1851 1941 2040 2119 2263 2422 2545 2626 2670 2834 3063