1996 Other Toyota Models

i have just changed my toyota premio car from diesel to gasoline engine.when in drive position the car is running in only 1 speed. Having difficulty to start running in drive since it is very slow. as far as for the other two speed they are in good way.one garage told me that it may be a sensor problem , whereas others told me it may be an electrical conection problem. please advise me the right solution so as i know which work to do.

thanks and regards