Plane bearings are used as rotating bearings for gear wheels of vehicles transmissions. The design of these units provides for the operation of bearings in two main loading modes: sliding mode (without load transfer) and rolling mode with slipping motion (with load transfer). The mode of sliding bearings operation in the rolling and slipping mode is not sufficiently studied, which makes it impossible to quantify at the design stage the influence of the operating conditions of the vehicle on wear processes, and, therefore, on their reliability. It is necessary to take into account the fact that the condition of the bearing coupling affects the conditions of the gearing. The results of statistical studies of wear and accelerated wearing tests of sliding bearings of gears of a wheeled tractor show the low reliability values of both gear wheels and sliding bearings. Results of bench tests that show the effect of various operational and design parameters on stress distribution in the bearing material along its perimeter are presented. It has been experimentally determinated that the change in the temperature of the supplied oil and the rolling speed of the assembly does not significantly affect on the change in pressure distribution along the perimeter of the bearing for two equal loading conditions using a lubricant of the same viscosity (SAE 20). In addition, the use of different viscosities oils also does not significantly affect the value of the maximum stresses in high-loaded bearings. The results of studies of the gap on intensity distribution in the bearing material demonstrate an increase in maximum stresses in the bearing by an average of 50% with an increase in the relative gap in the bearing by 45%, and a further increase in the gap of 3.75 times led to an increase in maximum stresses by 2.5 times. An improved methodology for estimating the probability of failure-free operation of bearing assemblies is presented, based on the definition of the quantile of the law of normal distribution. The methodology is based on the tribological characteristics of the contacting materials, the loading conditions of the bearing being examined, and the maximum stresses in the bearing material. The key point of the presented technique is the consideration of the maximum stress in the bearing material, which is determined through the value of the effective radial force in the gearing, the angle of contact of the stud and bearing, and the average value of the operational gap. The results of calculating the probability of failure-free operation for three variants of plain bearings with different mounting gaps are presented, which makes it possible to evaluate the reliability of the bearing assembly during the design phase. In general, the presented research results demonstrate the ability to manage reliability indicators of high-loaded transmissions of vehicle transmissions without significantly changing the design of the bearing assembly taking into account the conditions of its operation.