My suggestion would be to store that return value in a reactiveValue where it gets defined whenever your input value is changed. Then use that reactiveValue anywhere you want. Something like this:
reac_val <- reactiveValue(data = NULL)
observeEvent(input$fuel_eff_with_load, {
reac_val$data <- input$fuel_eff_with_load + 0.5
})
output$Myfueleffwithoutload <- renderText({
paste("Fuel efficiency without load of the vehicle will be Km/Ltr", reac_val$data)
})