Mopar Register: Tire size and RPM calculator

Tire Size Calculator

Current Tire Size:

New Tire Size:

Your overall gear ratio is important for your towing capabilities as well as acceleration from a stop. If you decide to get taller tires for your vehicle your overall gearing ratio will go down, lowering your ability to tow and slowing acceleration. This can be dangerous when towing loads that are close to your vehicles maximum tow rating. Likewise, if you numerically increase your overall gear ratio (either by switching to shorter tires or by swapping out your gears) towing will become easier and your acceleration will improve. For the 2006 Dodge Ram switching to 3.92 gears from 3.55 gears raises the towing capacity by 1000 pounds. NEVER exceed your vehicles weight ratings. Doing so can be dangerous.

Tire Information:

Speed Rating: The speed rating denotes the speed at which a tire is designed to be driven for extended periods of time. For example, on a tire labeled 265/70/17 89T, the last number (89) and the letter (T) denote the tires load capacity and speed rating. The tire in our example has a speed rating of T (118 mph). The ratings range from 99 miles per hour (mph) to 186 mph. These ratings are listed below. This information may not be present on all tires because it is not required by law. The values used here are found at tirerack.com. Go there for all of this information as well as other tire information and see what different tires are rated by drivers.

Letter	Speed	Letter	Speed	Letter	Speed
Q	99 mph	R	106 mph	S	112 mph
T	118 mph	U	124 mph	H	130 mph
V	149 mph	W	168* mph	Y	186* mph

* Tires with a maximum speed capability over 149 mph are sometimes labeled with the letters ZR. Tires with a maximum speed capability over 186 mph are always labelled with letters ZR.

Load Index: The load rating of the tire is the last number in the description followed by the letter that denotes the speed rating, in our example the load index is 89 (1,279 lbs.). The higher the number for the load index the more weight that tire is capable of carrying.

Load Index	Pounds	Load Index	Pounds	Load Index	Pounds	Load Index	Pounds
71	761	81	1019	91	1356	101	1819
72	783	82	1047	92	1389	102	1874
73	805	83	1074	93	1433	103	1929
74	827	84	1102	94	1477	104	1984
75	853	85	1135	95	1521	105	2039
76	882	86	1168	96	1565	106	2094
77	908	87	1201	97	1609	107	2149
78	937	88	1235	98	1653	108	2205
79	963	89	1279	99	1709	109	2271
80	992	90	1323	100	1764	110	2337

Tread: The tread on your tires is possibly the most important safety feature on your vehicle. Your four tires are the only thing keeping your vehicle on the pavement. The tread provides a grip to the road when driving, this is especially important when the road is wet or icy.

Type: There are different types of tread you can get on your tires. The type of tread is typically indicated in the name of the tire (all terrain, off road...). You will want to pick your tread according to the driving conditions that you will encounter. If you go off roading you will want to get an off road tire, if you drive through different seasons you will want to get an all season tire. The types of tires are:

Passenger Tires		Light Truck/SUV Tires
Summer Passenger		Summer Light Truck/SUV
All-Season Passenger		All-Season Light Truck/SUV
Winter Passenger		On-/Off-Road Light Truck/SUV

For more information about different types of tires go to TireRack.com

Tread Life:

The tread life of the tire should be known before the purchase of the tire. Whoever is selling you the tire (tiresplus, walmart, tirerack.com) has this information and should give it to you freely, its a selling point for them. Most tires are rated for 20 thousand miles, 40 thousand miles etc. The number of miles you actually get out of the tread on your tire can vary depending on conditions in which the tires are used. If you tow or haul heavy loads often the life of your tires would be reduced. If you like to leave black patches of rubber on the ground the life of the tire is reduced. The best way to get the full mileage out of the tire is to drive with a soft foot and rotate the tires every 5 thousand miles. The way you can test how much tread is left on the tire is what is called the penny test. You place a penny into many of the grooves on the tire tread with the top of lincolns head closest to the tire. If the top of lincolns head is covered you have more than 2/32 of an inch left on the tread. 2/32 is the nationally recommended depth at which you are supposed to replace the tire.

TireRack recently did a study on the depth of the tread on your tires and how it relates to stopping distance. They used two different test vehilces and tried stopping them on wet pavement. The tried two different tread depths, 2/32 (recommended replacement depth) and 4/32. 4/32 can be measured by placing a quarter with the top of washingtons head down towards the wheel. If the tread covers up part of washingtons head then you have at least 4/32 of tread left. They found that on tires with 2/32 of tread remaining it can take twice as long to stop versus the 4/32 depth.

This happens because the tires with 2/32 of tread dont have enough depth to adequatly push the water on the road out of the way so that they grip the road to stop. The tires with 4/32 depth are deep enough to move the water out of the way and stop the vehicle in a safe distance. These tests show that replacing the tires using the quarter test is safer.

For more about this study go to TireRack.com