I need know the best/fastest way to get from Heathrow to Stansted for a RyanAir connection to Venice. Also, how long does it take to get through immigration at Heathrow? Our flight gets into Heathrow at 7:50am and I need to make a 11:25am flight from Stansted. Can I make it? Thanks for your help.

No. Book a later flight from Stansted.

3h35 - I wouldn't rate my chances of doing that & I'm British and know the system.

Note 1) you do not have 3h35m to make the trip, you have 2h55m as Ryanair checkin closes PRECISELY 40 minutes before departure - ie 10:45

Note 2) The most likely direct National Express coach you could take leaves LHR at 09:15 which arrives at 10:35 (note I'm assuming weekdays), therfore you have precisely 10 minutes to gte checked in. You might get an earlier coach but it only gets to STN 10 minutes earlier

Your best option would be to take the 11:30 BMI flight from Heathrow to Venice. Prices are comparable to Ryanair. See www.flybmi.com.

We looked into this recently. You will not have enough time. Minimum time should be 4 hours and that is cutting it close.

Assuming you are arriving from the US, Term 3 & 4 (where US flights come in) will be very crowded as several flights arrive. The immigrations lines will be monstrously. Allow for at least 1 hour to get thru.

If you are arriving on a weekday, the trip to Stansted by bus will take AT LEAST 90 minutes, more likely 110 minutes.
Train is more expensive and you must go thru London. The fastest way would be to take HEX to the Tube to Liverpool station where you would get the Stansted Express train. This train leaves every 30 minutes and takes 45 minutes to get to Stansted. This whole trip will most likely take at least 1.5 hrs, possibly 2 hrs if you have to wait for the trains. Then add time to get to the departurre desk for check-in. You will be standing in line there.
Most convenient is to take a taxi/private transfer. They will be waiting for you so no down time. Make sure you pick a reputable, licensed taxi service. This will cost between 75- 90 pounds per car depending on how many are traveling and how much luggage you have. However, on a weekday, it will also take them 90-110 minutes to get to Stansted.

Remember Ryan Air is use it or lose it. You must meet their CHECK-IN deadline (45 minutes before flight). It does not matter that you are standing in line at the deadline, they will not move you up like the legacies will. If you are late, you will NOT be allowed on the plane and have to buy a new ticket. The previous ticket cost is completely forfeit.

So given all this, you will realistically have only 110 minutes travel from Heathrow to Stansted AND be checked in. You will NOT make it. Switch to a later flight or go to London the night before if possible.

Thanks Fodorites for coming through as usual. I had suspected that I would not be able to make this connection. I am looking at later flights and also the flybmi site. You all are wonderful!