Gear Ratio / Tire Size
Good Information To Have Around

Good "rule of thumb" calculation is to multiply .12 by your tire diameter. (.12 X 38" = 4.56) - see more formulas at bottom of page.

The below table can be used to get a rough idea on gear ratios. The colors represent ideal RPM's at highway speeds (65). For highway cruising and best fuel economy stay towards the yellow (2600 rpm), around town daily driving is color coded green (2800 rpm), and for better towing power or just more 4-low power use the ratios near the red (3100 rpm). These calculations are assuming a manual transmission with a 1:1 ratio. If you drive an automatic your RPMs will be higher, and the opposite is true if you have overdrive (your RPMs will be lower).

Gear Ratio
T
I
r
e


D
I
a
m
e
t
e
r
    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 7.17
27"   2677 2766 2872 3017 3163 3325 3454 3689 3947 4150 4279 4352 4619 4991 5800
28"   2582 2668 2769 2909 3050 3206 3331 3557 3806 4001 4126 4196 4454 4813 5593
29"   2493 2576 2674 2809 2945 3095 3216 3434 3675 3863 3984 4052 4300 4647 5400
30"   2410 2490 2584 2715 2846 2992 3109 3320 3553 3735 3851 3917 4157 4492 5220
31"   2332 2409 2501 2628 2755 2896 3008 3213 3838 3614 3727 3790 4023 4347 5051
32"   2259 2334 2423 2546 2696 2805 2914 3112 3331 3501 3610 3672 3897 4211 4894
33"   2191 2263 2349 2469 2588 2720 2826 3018 3230 3395 3501 3561 3779 4093 4745
34"   2126 2197 2280 2396 2512 2640 2743 2929 3135 3295 3398 3456 3668 3963 4606
35"   2065 2134 2215 2328 2440 2565 2664 2845 3045 3201 3301 3357 3563 3850 4474
36"   2008 2075 2154 2263 2372 2493 2590 2766 2961 3112 3209 3264 3464 3743 4350
37"   1954 2019 2095 2203 2308 2426 2520 2692 2881 3028 3123 3176 3370 3642 4243
38"   1902 1966 2040 2144 2247 2362 2454 2621 2805 2948 3040 3092 3282 3546 4121
39"   1854 1915 1988 2089 2190 2302 2391 2554 2733 2873 2962 3013 3198 3455 4015
40"   1807 1867 1938 2037 2135 2244 2331 2490 2664 2801 2888 2937 3118 3369 3915
41"   1763 1822 1891 1987 2083 2189 2275 2429 2599 2733 2818 2866 3042 3287 3819
42"   1721 1778 1846 1940 2033 2137 2220 2371 2538 2668 2751 2798 2969 3208 3728
43"   1681 1737 1803 1894 1986 2087 2169 2316 2479 2606 2687 2733 2900 3134 3642
44"   1643 1698 1762 1851 1941 2040 2119 2263 2422 2546 2626 2670 2834 3063 3559

 
Here are some common gear formulas: