Gear Ratio Chart

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The gear ratio most interesting for truck owners is the ratio between the driveshaft and axels. 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 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.

Another formula

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

This formula is for 4x4's with manual transmission. It should also be fine for an automatic with 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 Gear Ratio Table

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

Tire	Gear Ratio
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
44	1643	1698	1762	1851	1941	2040	2119	2263	2422	2545	2626	2670	2834	3063	3559