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As a rule of thumb, an automobile or truck engine is most efficient at 60 to 80 percent of the RPM at which peak horse power is developed. The accompanying chart shows engine RPM at 55 mph for various combinations of final gear ration and tire diameter. If you want to determine RPM at a speed other than 55 mph, or for a combination of ratio and tire size not indicated here, the formula is:

If you don‘t know your vehicle‘s final gear ratio but it‘s a recent model, look on the inside of the glove compartment door. There, you‘ll find a specification panel which should include the proper figures.

If it‘s an older model, you may have to crawl underneath and look for the ratio on a tag bolted to the differential. On a four-wheel drive, it may be on the front differential instead of the rear one.

About This Chart

Overdrive Transmission Owners may select a gear ratio lower on the chart for more power.

NOTE: the information given above will help you read the chart, which shows both tire size and gear ratios. Find the tire size you wish to use listed on the top row of the chart then look down the row for the numbers in the red background, then reading across the row to the left find the gear ratio that is best suited to your vehicle.

THIS CHART SHOWS ENGINE RPM @ 55 MPH

Gear
Ratio
Tire Diameter

24" 25" 26" 27" 28" 29" 30" 31" 32" 33" 34" 35" 36" 38" 40" 42" 44"

2.56 1972 1892 1820 1752 1690 1631 1577 1526 1478 1434 1391 1352 1314 1245 1183 1126 1075

2.73 2102 2018 1940 1869 1802 1740 1682 1627 1577 1529 1484 1441 1401 1328 1261 1201 1147

2.94 2264 2173 2090 2012 1942 1874 1811 1753 1698 1646 1599 1552 1509 1429 1358 1294 1235

3.08 2372 2277 2189 2108 2033 1963 1897 1836 1779 1725 1674 1626 1581 1498 1423 1355 1294

3.21 2472 2373 2282 2197 2119 2046 1977 1914 1854 1798 1745 1695 1648 1561 1483 1412 1348

3.31 2549 2447 2353 2266 2185 2110 2039 1973 1912 1854 1799 1748 1699 1610 1529 1456 1390

3.42 2633 2528 2303 2341 2257 2179 2107 2039 1975 1915 1859 1806 1756 1663 1580 1505 1436

3.54 2726 2617 2516 2423 2336 2256 2181 2110 2044 1982 1924 1896 1817 1772 1635 1558 1487
3.73 2872 2757 2651 2553 2462 2377 2298 2224 2154 2089 2027 1969 1915 1814 1723 1641 1567

3.90 3003 2883 2772 2669 2574 2485 2402 2325 2252 2184 2120 2059 2002 1897 1802 1716 1638

4.10 3157 3031 2914 2806 2706 2613 2526 2444 2368 2296 2228 2165 2105 1994 1894 1804 1722

4.27 3288 3156 3035 2923 2818 2721 2630 2545 2466 2391 2321 2255 2192 2077 1973 1879 1793

4.56 3511 3371 3291 3121 3010 2906 2809 2718 2633 2554 2478 2408 2341 2218 2107 2006 1915

4.88 3758 3607 3469 3340 3221 3110 3006 2909 2818 2733 2652 2577 2505 2373 2255 2147 2050

5.29 4073 3910 3760 3621 3491 3371 3259 3154 3055 2962 2875 2793 2716 2573 2444 2328 2222

5.38 4143 3977 3824 3682 3551 3428 3314 3207 3107 3013 2924 2841 2762 2616 2468 2367 2260

5.71 4397 4221 4058 3908 3769 3639 3517 3404 3298 3198 3104 3015 2931 2777 2638 2512 2398

6.17 4751 4561 4385 4223 4072 3932 3801 3678 3563 3455 3354 3258 3167 3001 2851 2715 2591

7.17 5521 5300 5096 4907 4732 4569 4417 4274 4141 4015 3897 3786 3681 3487 3313 3155 3011

WHAT GEAR RATIO IS BEST?

About This Chart