OVERSIZED TIRES

When a vehicle is manufactured, it is designed to perform at all around peak performance. This performance can only be achieved when the RPM’s of the engine and the Gear Ratio of the drivetrain are able to run in a specified perimeter.

Large, Oversized Tires will change the running RPM’s and Gear Ratio on your truck, an increase to the Gear Ratio is going to (increase stress on the transmission). A Larger Oversized Tire has a larger circumference, and now one revolution of the driveshaft is going to make your car move further than it did with the smaller more efficient tires. Larger Oversized Tires can cost you thousands in corrective modifications and damage.

The engine of the truck now needs to deliver (more horsepower and torque) to move the truck a farther distance because of the Larger Oversized Tires. It requires (more work) to turn the entire driveline one revolution with larger tires than it does with the smaller more efficient tires. The larger tires are now going to move the truck further in distance. The more horsepower expended from the engine the greater the (force) that is put on the transmission and driveline. Because of Larger Oversized Tires on a truck, (more horsepower and torque) is now needed and sent down to the transmission. It takes (more force) to turn the driveshaft and spin the axle which puts (extreme stress) on the transmission in order to turn those Large, Oversized Tires down the road.

In a lot of cases after the Larger Oversized Tires are mounted on the truck there is a loss of low end power due to the Gear Ratio change made by the Oversized Tires. Accelerating from a stop is now sluggish and it is necessary to step into the throttle more, thereby putting (more force and stress) on the transmission to overcome the power loss. The RPM’s are now not where they were at highway speeds and the speedometer is no longer accurate, again due to the Gear Ratio change made by the Oversized Tires. The next modification is to put a tuner on the truck to (increase the horsepower and torque) to overcome the low end power issue and correct the speedometer. This is only a band aid as it only covers up the problem and now puts (even greater force and stress) on the transmission.

In extreme cases, if your tires are large enough it can adversely affect the RPM’s, preventing the torque converter from locking up completely. This will cause slipping, overheating and damage to the torque converter and transmission.

If larger oversized tires are something you must have on your truck, it is an extremely good idea to get the differential or differentials on a 4X4 regeared. This will bring the engines horsepower, rpm’s, speedometer, and gear ratio closer in line to what the manufacture intended. The formula for a regear is:
Height of the new tire size Times the original differential Gear Ratio Divided by the original tire height 7” X 3.73 = 138.01 : 32” = New gear ratio 4.31
To protect your transmission and driveline It is best not to exceed a

30 inch tire height on a 3 speed automatic transmission

32 inch tire height on a 4 to 5 speed automatic transmission.

33 inch tire height on a 6 to 8 speed automatic transmission.

We are Lifetime Transmissions Inc. We have seen more transmission damage caused by Oversized Tires in the past 4 years than ever before. One customer came to us after going through 2 transmissions and was told his diesel engine was worn out. We installed an upgraded transmission and re geared his differential. He now has his power back and the transmission shifts great. Another customer had 6 transmissions put in his gas truck. We performed the same service and had the same outcome, as well as many others If you need help, we are here. We have the answers, they may not always be the ones you want to here but they will be honest ones.

Written by Lifetime Transmissions

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