Hi Larry, I'm glad to assist you. Yes. It is guaranteed to fit the 1991 Dodge D350 Dually 5.9L. Grab our discounted price and click the link below to buy it today. Happy shopping! https://www.carparts.com/search?q=speed%20sensor&itemperpage=15&vehicle[cylinders]=6&vehicle[liter]=5%2E9&vehicle[submodel]=Base&vehicle[model]=D350&vehicle[make]=Dodge&vehicle[year]=1991&can=cpje1043_CSQA_20230917

Hi Vicki, This item is for the 1990-1991 Dodge D150 transmission speedometer sensor. To check the fitment please use our Vehicle Selector — which can be found towards the top of the homepage as well as all categories, search results, and product pages — can help verify if a part will fit your specific application. To enter your application, simply select your vehicle’s year, make, model, sub-model, and engine type and click “Go.” Once your application is selected, you’ll see a “Guaranteed to Fit” badge if the part will fit, and we recommend that you also double-check Your Vehicle Fit Details as well as the Vehicle Fitment Information table on the product page.

Hi Keith, Yes. The output speed sensor measures how fast a vehicle moves and sends that information to other vehicle systems. The transmission control unit uses the speed to determine when to change gears, adjust the torque converter, and display the vehicle's speed in the speedometer.