Choosing the right gears
Posted: Mon Jan 15, 2007 7:25 pm
It is true, the sport of four-wheeling starts out innocently enough with a basic rig and some basic trail – that is, until you get the itch for some more ground clearance to tackle those tougher trails (like me now). 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 Jeep'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:
[New tire diameter (inches)/ Old tire diameter (Inches) X current axle ratio (:1)]
For towing or performance, you would want the next available gear ratio lower (numerically higher) than the stock equivalent.
\so if your 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, it is usually recommended going with a performance-minded gear ratio for any tire size above 35 inches (if you can pass a 35 inches with Tasjeel). 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 (If you have them in your Jeep). The formula to determine the degree of uncorrected speedo error is:
New tire size (in)/ Old tire size (in) X indicated speed
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 an easy as using handheld programmer or a splice-in-speedo calibration box.
In Wrangler's correcting the speedo is easy by replacing the speedo gear with a new one which cost around 200 Dirhams (Just got the info. today from Emil as he is doing my car)
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.
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:
[New tire diameter (inches)/ Old tire diameter (Inches) X current axle ratio (:1)]
For towing or performance, you would want the next available gear ratio lower (numerically higher) than the stock equivalent.
\so if your 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, it is usually recommended going with a performance-minded gear ratio for any tire size above 35 inches (if you can pass a 35 inches with Tasjeel). 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 (If you have them in your Jeep). The formula to determine the degree of uncorrected speedo error is:
New tire size (in)/ Old tire size (in) X indicated speed
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 an easy as using handheld programmer or a splice-in-speedo calibration box.
In Wrangler's correcting the speedo is easy by replacing the speedo gear with a new one which cost around 200 Dirhams (Just got the info. today from Emil as he is doing my car)
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.
