Tight turnaround @ FCO from US

If you have checked luggage your prospect of making your FCO-CDG flight are dismal. With just carry on luggage, its slightly better, but WAY to tight for me. Also you have to have 2 prior flights be on time to get you to FCO on time to have any chance of making the FCO-CDG flight.
Personally I would plan a next day flight day to Paris.

Ideally a SFO-CDG flight would be the the thing to do.

Was it really that much cheaper to fly SFO-FCO plus a separate ticket to Paris vs a SFO-CDG routing?

@joe

Luckily we won't have any checked luggage with us, so I just pray that all our connections are on time.

The price difference was >$250 to fly into CDG that day from SFO and we were hoping we could just jump straight over to our friends place in Paris that night. We used points to cover our SFO-FCO flight and that's why we were stuck with that initial flight path into Europe.

We're trying to save money not staying in a hotel over night at the airport hotel and paying more for a Saturday ticket out of FCO which are more expensive than Friday flight out to CDG.

Ideally, of course, we would have flown straight into CDG but I had budget and parameters to work in... Let's hope I didn't blow the budget!

I know it probably makes sense to you but for most of us it does not make any sense to go to Paris via Rome. Your first statement is correct because of two separate tickets. Rome is immigration - passport control - and customs - two min walk through. I have been through Rome several times - usually immigration is roughly an hour. Have done it in 45 minutes and one time it was close to three hours. Is that a direct flight from SFO? That is a little late for most direct flights from the US. Personally I think it is too tight given the options you cannot control. Why are you not flying directly to Paris?

PS -- We were posting nearly the same time so I had not read your above response. You have no margin for error. I think all of your perceived saving goes down the drain if you miss the flight or even overnight in Rome. You can trying sleeping at the airport. It has been done before. Assuming you have a fairly long lay over in LHR. A better option might be to see if you can get an earlier connecting flight. Even if you have to pay a few extra dollars, look at it as an insurance premium. Just to make matters worse -- late day flights have a much lower on-time percentage than earlier in the day. The second leg is risky for being on time.

"will we need to exit immigration and customs and then come back through security to make our next flight to Paris?" - yes.

What is the minimum check-in time for your Rome - Paris flight?
Plus 1 hour AVERAGE to disembark and go through immigration (customs takes no time).
Plus risk incoming flight may be late.
Plus how far from arrivals to check-in? Is it even the same terminal?
I don't think this is possible.

Is it not possible to change the LHR-FCO leg on your current trip to LHR-CDG? You may want to at least ask.

Had you looked at SFO-LHR and then eurostar to Paris? Looks like 'walk up' fare is under $200 for eurostar

Are you able to change/cancel your FCO-CDG without a huge penalty?

Does your your FF plan allow changes without punitive penalties?

I will not say you cannot make your transfer, but you will need the heavy-duty extra thick prayer mat

If your flights are unchangeable, you may want to have a "plan B" at the ready.

Good luck!

These flight schedule things are always a puzzle to me. If you would tell us whose FF miles you are using, what day you leave SFO, where your stop along the way is and from where and when you plan to return, we be able to make more suggestions.

Using Google Flights, I see some pretty cheap one-stop SFO-CDG options in July. However, RT non-stops SFO-CDG are sky high.

It's a pickle for sure.

Yeah joe, that was my other thought. Simply get off after landing in London, and Eurostar to Paris. But airlines typically don't like you not completing your booked itinerary, so be careful with that approach, as it might get you flagged and potentially impact your return flight, assuming it's on the same airline as the outbound ticket.

I was NOT suggesting just walking away from the LHR-FCO leg, rather could OP change his trip to end in London and then take eurostar from there rather than the risk of the tight connection in Rome.

If OP booked his trip as open jaw , his return booking is toast if he misses an intermediate flight.

We're trying to save money not staying in a hotel over night at the airport hotel and paying more for a Saturday ticket out of FCO which are more expensive than Friday flight out to CDG.

Ah yes, but it's going to be more expensive by far to buy the Friday night ticket now, miss the flight, then have to add a hotel and buy a last-minute ticket THEN for Saturday morning departure to CDG. You think the Saturday ticket is expensive now -- wait until you have to buy it the night before you travel!

Our last time landing at FCO, our United Flight was 2+ hours late, immigration took about 1 hour due to all of the other transatlantic flights that had landed at the same time. If you are going to catch a flight within Schengen Area you need to have two hours to clear security etc after you exit passport control. I would look at making changes if you can.