I often get questions like:
If you are planning on 35" or 37" or even bigger tires for your Jeep or 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 of 1:1 means that the driven gear rotates at the same speed as the driving gear and is usually true for the ratio between the engine and the transmission when it is in 4th gear in a manual transmission. Going higher and you are in overdrive where the output of the transmission is revolving faster than the engine's output.
The gear ratio most interesting to 4x4 truck owners is the ratio between the driveshaft and axles - the differential gear ratio or axle gear ratio. It is controlled by the number of teeth on 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.
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.
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.
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.
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.
Tire Size Calculator helps in your decision making while planning modifications for your newest project.
Lifting your 4x4 truck is what you need when fitting the big tires and wheels. Custom modified and lifted 4x4 trucks are less subject to damage in situations like river crossing and rocky trails.
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