* Vehicle Speed: This is the primary determinant. Once the engine speed and vehicle speed reach a pre-programmed threshold, the transmission's computer (or the hydraulic system in older models without a computer) will shift up to the next gear.
* Engine Load: If you're accelerating hard (heavy load), the transmission will hold a lower gear longer to provide more power. Going uphill will also cause this. Conversely, if you're coasting or lightly accelerating, it will shift up sooner.
* Throttle Position: How far down you push the accelerator pedal affects the shift points. A gentle acceleration will lead to earlier upshifts than aggressive acceleration.
* Transmission Fluid Temperature: In some cases, especially if the transmission fluid is very cold, the shift points might be slightly altered to protect the transmission.
* Transmission Condition: If your transmission is experiencing wear or has low fluid, the shift points might be erratic or the transmission might slip between gears. This is a potential problem that needs professional attention.
* Computer Calibration (if applicable): Your E250's transmission may have a computer controlling the shift points. While unlikely to spontaneously change, some potential issues with the computer's programming or sensors could impact shift points.
In short: The shift from 2nd to 3rd and overdrive is a normal function of your transmission's operation, designed to balance fuel economy and performance. If the shifting seems unusually harsh, delayed, or erratic, it's a sign that you should have your transmission checked by a mechanic to rule out any problems.
