Why do the rpm’s run high on 2500 for 1-2 miles before shifting in 4th on my 2000 Ford Ranger XLT and sometimes inbetween the lower gears also. Other times, it runs ok. Why, also does the OD Light ( reading ‘off’ ) send the rpm’s up to 2500?
Probably a bad thermostat. If so, your problems with the transmission would happen when it’s just cold outside. If on cold days your temperature gauge never gets to the halfway point, the engine is running too cool. That is probably because the thermostat is never closing when it should, and so the engine gets all of the coolant all of the time. Using the heater motor further cools the engine. To counteract this, your transmission will be instructed to stay in a lower gear than the optimum gear.
Now that it’s spring and some days are warmer than others, see if the problem happens less often on warm (say, 60f +) days, and if it goes away entirely when the temperature gets into the 70 degrees f range. If so, replace the thermostat before the end of summer. This is a very inexpensive repair, replace the fluid and thermostat for well under $100.