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Choosing The Right Ratio

Posted in How To: Wheels Tires on April 1, 2006
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It is true, the sport of four-wheeling starts out innocently enough with a basic rig and some basic trails-that is, until you get the itch for some more ground clearance to tackle those tougher trails. To achieve more ground clearance, you need to upsize your tire size, but bigger tires will limit the amount of power from your engine that gets to the ground by effectively changing your truck's gear ratio (which, lets face it, probably was pretty conservative from the factory anyway).

With a bigger tire, the truck will feel like it is geared higher (numerically lower), which is great for highway cruising, but not for low-end grunt, off-the-line acceleration, or passing power. To bring your rig back to stock performance after a tire size upgrade, it is important to regear the truck accordingly. A simple calculation will tell you what gear ratio would get you back to your stock equivalent. The calculation is your new tire diameter, divided by your old tire diameter, multiplied by your old axle ratio, will equal your new axle ratio:

New tire diameter (in.)
x current axle ratio (:1) = new axle ratio (:1)
Old tire diameter (in.)

For towing or performance, you would want the next available gear ratio lower (numerically higher) than the stock equivalent.

So if our truck had 30-inch tires stock, and a 3.73:1 axle ratio, that calculation would tell us we need a 4.103:1 (which rounds off to the readily available 4.10:1) gear ratio to get us back to stock with 33-inch tires. Depending on the axle, the next lowest ratio from a 4.10:1 is most likely the 4.56:1, which should be selected for towing and performance. Because of the extra weight associated with off-road tires, we usually recommend going with a performance-minded gear ratio for any tire size above 35 inches. As a general rule, four-wheel drives need both the front and rear differentials regeared to the same ratio to prevent severe driveline bind.

The second problem you will run in to is in regards to speedometer error. If you are regearing for stock performance, your speedo will be close, but if you are regearing for economy or performance, you will still need to address your speedo's accuracy. Bigger tires cause the speedometer and odometer to read slower than the vehicle's actual speed, altering transmission shift points, traction control, and ABS functionality.

The formula to determine the degree of uncorrected speedo error is:

New tire size (in.)
x indicated speed (mph) = actual speed (mph)
Old tire size (in.)

So, if you've just swapped out your stock 31x10.50s for a suspension lift and a set of 38s, your uncorrected speedometer may only read 60 mph on the highway when in fact you're going 73 mph. Not good if you want to keep a clean driving record.

The solution on older trucks is to change the speedo gear at the end of the speedometer cable. Located either in the transmission or transfer case, this gear is simple for anyone who is mechanically inclined to replace. Newer vehicles sporting the latest in electronics often have a Vehicle Speed Sensor (VSS) that uses electronic sensors to tell the truck's computer how fast the truck is moving. Speedometer calibration on a vehicle equipped with a VSS is as easy as using a handheld programmer or a splice-in speedo calibration box, such as Superlift's TruSpeed

With new gears and a calibrated speedo, you'll be able to get the most enjoyment and performance from your new tires and newfound capability.

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