Fastest connection from FCO to Termini

I cannot see how a car, negotiating Roman traffic, can beat a nonstop train with exclusive rights on the track. The train will deposit you INSIDE the station in close proximity to the train you want to change to, not outside the station with a walk to get to the inside train platforms.
People use car services to get to hotels that are at various locales other than a train station.

The Leonardo Express train takes 32 minutes and is faster than a car in most circumstances except for maybe in the total absence of traffic (like in the middle of the night) when travel times are similar. The issue might be how long you have to wait for the train, since they depart every half hour and therefore you might potentially have to sit at the FCO station for 30 minutes if you happen to get there just when the train is pulling out of the station and you have to wait for the next one.

I hope you are not planning to book a high speed train too close to your airplane arrival time. Flight arrival times, passport control times, and baggage claim are less predictable and more subject to delays than trains.

The advantage of a car service is if you are delayed, they will be waiting for immediate departure. The express train takes about a half hour, (same as a car in good traffic) but only leaves every half hour...so your journey by train can be an hour, not counting time through immigration, just wandering from gate to the train, then time to get to your train once at Termini. On a good day, it can be as short as 2 hours from landing, more like 3 hours, if your flight is late or other delays...who knows. Taking a car service or taxi may at best save you a half hour, or basically what you might have to wait for a train.

If buying a ticket, I would leave plenty of time. Best to plan on sitting and waiting for a train at Termini, having a cafe or lunch, than buy a ticket and lose by missing a train. Many even just plan on buying a ticket on arrival, sure, a bit more than buying ahead, but less risky.

I'm trying to make a high-speed train connection and I expect the
connection to be tight.

This is just a bad idea for so many reasons. Flight delays are all too common, and long lines at immigration (if not coming through another EU airport) and delays at the luggage carousels are even more common. Do yourself a favor and buy your tickets on onward from Termini on the day or arrival, or buy train tickets in advance that will allow for a free/low-cost change should you miss the train you're trying to take.

A car service is not faster than the express train if they have to deal with heavy traffic.

If your fast train is to Firenze SMN or Venezia SL, there are direct trains leaving from FCO, I think twice a day. But if your plane is late, you could miss the train and you would have to buy a new ticket. Still, a direct train will save you one transfer.

Kathy gave you good advice.

pbr,

To reiterate (and emphasize) what the others have said, it is NOT a good idea to book tickets on a high speed train to connect with an inbound flight especially as the connection is "tight". If you miss the train you've booked, your tickets are worthless and you'll have to buy more. There could be many reasons why you're delayed - the flight is late, it takes longer to de-plane than normal, queues are long at Passport control, there's a strike in progress or whatever. Especially at FCO, one can't always count on things moving along as expected.

To answer your original question, the Leonardo Express is the fastest way to get to Termini. As mentioned previously, it takes 32 minutes and is not affected by traffic. One important point to note is that there are two trains serving the airport, the L.E. and the FM/FR1. If you board the wrong train, your trip will be longer than expected! DON'T forget to validate your ticket prior to boarding the train or you'll risk hefty fines, which will be collected on the spot!

Where are you going after you land at FCO?

The thread David spoke of is at

https://community.ricksteves.com/travel-forum/italy/length-of-time-to-transfer-from-fco-to-termini

9:05 (if exactly on time) and departs Termini at 11:57.

So, let's see.

Leonardo Express trains leave the airport - mid week - at
9:08
9:23
9:38
9:53
10:23
10:53
11:23.

The 11:23 arrives Termini at 11:55. Too late. So the 10:53 is the last possible Leonardo Express, arriving at 11:25 with plenty of time.

There is also the direct train from the airport to Venice (via Florence) which leaves the airport at 11:08, arriving 11:40, also possible but tight. It is a Freccia so requires mandatory reservations and may be more expensive, but still takes 32 minutes.

I would have thought that the 10:53, which is 1:48 from touchdown might work - but I wouldn't do it. Even the Freccia is too much of a risk for me.

I don't think that a car service would save your bacon.

I'd rethink the plan.

We took the Leanardo Express last month. I was a little worried it might be pick-pocketers' heaven, but it wasn't. It was safe and efficient. And easy to find.