Tire_Height_Changes

CHANGING TIRE HEIGHT TO LOWER RPM'S

Here's two formulas to help determine your current RPM's and MPH.

You can also check your speedometer for correct calibration by plugging your RPM's for a given speed into the formula.

The formula can then be used to determine what the change in tire size will do to your RPM's.

MPH X DRIVE RATIO X 336
---------------------------------------- = RPM'S
TIRE DIAMETER

If you would like to know your speed at a particular RPM, the formula is:

RPM X TIRE DIAMETER
--------------------------------------- = MPH
DRIVE RATIO X 336

(In both cases, "336" is simply a constant to convert the tire diameter to the linear distance the tire travels)

Determining your Drive Ratio

The drive ratio may be located at the following locations depending on how old of a vehicle it is:

A newer vehicle will often times have it listed on a spec sheet glued to the glove box door
An older vehicle can have a tag located on the differential cover. On 4x4 it could be on the front axle.
Their are also stamping codes located on the passenger axle that can provide differential information.

If nothing is listed for the rear end then you can use the following method to determine the gear ratio:

Raise the rear end and place it on jack stands
place a chalk mark on the tire and the drive shaft
have someone count the revolutions of the drive shaft as you rotate the tire one complete turn
This will give you a close approximation of the gear ratio

If for example, the drive shaft turns just a little over three turns the you probably have a 3:08 gear ratio.

All this may seem like a lot of work but knowing what the effect of a tire change will have on your engines working rpm can save it from excessive wear and tear and maybe even extend its life.

Here's a little quick reference table to show how the rpm's change with tire size

TIRE DIAMETER
DRIVE RATIO	22"	23"	24"	25"	26"	27"	28"	29"	30"	31"	32"
2.73	2502	2392	2293	2201	2117	2038	1966	1898	1835	1775	1720
2.90	2657	2542	2436	2339	2249	2165	2088	2016	1949	1886	1827
3.07	2813	2691	2579	2476	2380	2292	2210	2134	2063	1996	1934
3.21	2942	2814	3696	2588	2489	2397	2311	2232	2157	2088	2022
3.31	3033	2901	2780	2669	2567	2471	2383	2301	2224	2153	2085
3.40	3116	2980	2856	2742	2636	2539	2448	2364	2285	2211	2142
3.54	3244	3103	2974	2855	2745	2643	2549	2461	2379	2302	2230
2.73	3418	3269	3133	3008	2892	2785	2686	2593	2507	2426	2350
3.90	2574	3418	3276	3145	3024	2912	2808	2711	2621	2536	2457
4.09	3748	3545	3436	3298	3171	3054	2945	2843	2748	2660	2577
4.56	4179	3997	3830	3677	3536	3405	3283	3170	3064	2965	2873
4.88	4472	4277	4099	2935	3784	3644	3514	3392	3279	3174	3074
5.12	4792	4488	4300	4129	3970	3823	3686	3559	3441	3330	3226