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Gear Ratio Chart Guide

Choosing the best diff gear ratios for your 4x4 truck

I often get questions like:

  • What is the best gear ratio for 37 inch tires?
  • What gear ratio do I need for 35 inch tires?
  • What gear ratio is best for fuel economy?
  • What is the best rear end gear ratio?
  • What gears should I run with 38 inch tires?

If you are planning on 35" or 37" or even bigger tires for your 4x4 truck this guide will show you the best ratio to choose. In this gear ratio guide you'll find the formulas and a simple chart that shows how you should lower your ring and pinion gear ratio.

The gear ratio most interesting to 4x4 truck owners is the ratio between the driveshaft and axles. It is controlled by the number of teeth in the ring vs. the pinion. A ratio of 4.56 means that as the axle turns one whole circle the driveshaft turns 4.56 circles. The higher the number the lower the ratio becomes.

Chart Gear Ratio

The correct axle gear ratio is important for drivability and economy. When you increase the tire size without changing the gear ratio you increase fuel consumption and you can not drive at the low speeds needed for the off roads.

 

A simple formula

The most simple formula for the correct gear ratio is this:

New Tire Size x Old Gear Ratio
New Gear Ratio = ――――――――――――――――
Old Tire Size

Example:

The original tire size is 30" and the original gear ratio is 3.91. You plan on moving up to 38". Then the formula would be calculated as: (38*3.91)/30 giving you 4.95. Not all gear ratios are available so you have the choice between 4.88 and 5.13, 4.88 being the closest.

Choose a little bit lower gear ratios if you do a lot of off-road driving and a little bit higher if you do more long distance highway trips.

For calculating this formula on-line go to the Tire Size Calculator.

An old axle gear ratio formula

To calculate the rotations pr. minute of the engine you can use this formula.

This formula is for 4x4's with a 1:1 final drive ratio (no overdrive). It should work fine for manual transmission as well as an automatic when they are not in overdrive.

MPH x Gear Ratio x 336
RPM = ――――――――――――
Tire Size

Example:

Adding to the last example. You usually start with 65 MPH, since that is the natural highway speed, multiply by 3.91 and then again with 336, dividing all by 30 gives you the before RPM of 2846. Calculating for the after ratio of 4.88 gears and 38" tires gives the RPM of 2805.

A 4x4 Differential Gear Ratio Chart Table

To sum it all up it can be handy to use this table for both your front and rear axle gear ratio selection. It is based on the above formula. You can use it to quickly estimate your trucks best gear ratio.

Tire
Size 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
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 3438 3614 3727 3790 4023 4347 5051
32 2259 2334 2423 2546 2669 2805 2914 3112 3331 3501 3610 3672 3897 4211 4894
33 2191 2263 2349 2469 2588 2720 2826 3018 3230 3395 3501 3561 3779 4083 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 2202 2308 2426 2520 2692 2881 3028 3123 3176 3370 3642 4232
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
45 1606 1660 1723 1810 1898 1995 2072 2213 2368 2490 2567 2611 2771 2995 3480
46 1572 1624 1685 1771 1856 1951 2027 2165 2317 2436 2512 2554 2711 2929 3404
47 1538 1589 1650 1733 1817 1910 1984 2119 2268 2384 2458 2500 2653 2867 3332
48 1506 1556 1615 1697 1779 1870 1943 2075 2220 2334 2407 2448 2598 2807 3262
49 1475 1524 1582 1663 1743 1832 1903 2032 2175 2287 2358 2398 2545 2750 3196
50 1446 1494 1551 1629 1708 1795 1865 1992 2132 2241 2311 2350 2494 2695 3132
51 1417 1465 1520 1597 1674 1760 1829 1953 2090 2197 2265 2304 2445 2642 3070
52 1390 1436 1491 1567 1642 1726 1793 1915 2050 2155 2222 2260 2398 2591 3011
53 1364 1409 1463 1537 1611 1694 1760 1879 2011 2114 2180 2217 2353 2543 2955
54 1339 1383 1436 1509 1581 1662 1727 1844 1974 2075 2140 2176 2309 2495 2900

Choose lower numbers for better economy and highway usage and higher numbers for towing and better off road capabilities.

For more calculations go to the:

Tire Size Calculator

Tire Size Calculator helps in your decision making while planning modifications for your newest project.

Read more »

Thrandur Arnthorsson

Thrandur is the chief-editor and owner of 4x4 Off Roads and an 4x4 off road enthusiast. Living in Iceland he loves to share the coolest 4x4 trucks and supporting more offroad freedom.
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