Can someone tell me the average speed of CAT5, 4, 3, 2 and 1 on 20 miles flat course.

Racing doesn't work that way. There will never be an average that means anything. You could have 2 Cat5 groups do the same course, same conditions on different days and the numbers could be anywhere depending on a bunch of things.

I've been in races that the "average" speed was down around 20-21 that were brutally hard.I've also done races where the "average" was around 26-27 and I spent the day in zone 2 just spinning along.

Bingo!

Here is another example...

Crit series, same course.

Nice dry day, smooth course, agressive riders in front, mid season, 26 MPH avg

Wet day, water on ground, slick course, people taking it easy, 23 MPH avg

First race of the season, many riders are not quite in the zone, 24 MPH avg

Averages mean nothing, staying with the pack means everything!

ok, then let me make it more clear.

Ride solo, on 20 miles flat course (no uphill, downhill) and no wind, no rain. so, just ride flat out.

So you are asking what a good time for a TimeTrial is? Where everyone is going all out for themselves, no other racers are affecting their pace?

Does that place exist? I've been riding for years and still haven't found it.

It still depends on too many other variables to give a general estimate of "this is what most pro racers are capable of". Your best bet is to look at time trial events of the pros and see what their times and average speeds are--then you will get a better idea of the speeds they are capable of holding.

The biggest difference between "us and them" is that they have the capability to produce massive amounts of power over far longer periods of time--hence the 6-8 hour classics races and tour stages. When you look at US Pro racing, you get slightly shorter events for the most part, but still longer than most of us can handle if we were even capable of the same power outputs in the first place (which many riders are).

Here is an illustration: watching the 2004 Tour de France DVD, there is a stage where a rider is away for almost the whole stage, and ends up winning by a matter of seconds over the chasing field. His average speed over the last several kilometers was well above the mid-20's, if not above 30mph. Now, this is a speed that some people on this forum are capable of holding on their own for a short period of time--but not after 4-5 hours of riding flat out by yourself.

I have a feeling you are trying to see where you would fit in to the heirarchy...is this the case? If it is...go do some group rides to see where you fall...or just sign up for a race.

Categories are not based on speed. You could have 5s that could cover your ideal flat course faster than 3s. Racing is all about maneuvering yourself to the finish line ahead of everyone else through strategy, pack riding techniques, sprinting, etc.

Seconded.

got a friend who does 28mph on that litle course and he claims he rides as strong as CAT2 racers. So just wondering

for me, I just ride road bike to commute to work, and train for my mtb

If you want to know how you 'stack-up' find a local TT and get yourself entered in it. Another way would be to ride the local TT course and compare your time against other riders in the stock class assuming you're not running aero bars.

That's pretty fast, but riding is riding and racing is racing. Can he go 34 for a lap, then settle back to 28? Can he do the above with me leaning on his shoulder through every turn? Can he jump up to 36-38 at any given time (when not he but someone else feels like it)? If so then "he rides as strong as CAT2 racers".

Then pass the advice along (join a group ride, or a race).

Forget sitting in a 28 MPH... try 31+ MPH the whole time... at least that is what the declared pace in some crits I have seen are. I would say if you can jump to 35 MPH from a 31 MPH pace then you are a cat 1/2 rider...

For me to sprint at 35 MPH is great... but then I am not a cat 1/2 yet!

He is either a strong rider or a funny character this friend of yours (heck maybe both).

This may be close to what you mean. All the results from the time trial series that I ride each summer are posted at https://www.carolinatt.org. The TTs are 10 miles in length and are run on the 1.5 mile Lowe's Motor Speedway, so it's flat. Results are posted for Cat 1 through Cat 5 riders plus "civilian" age groups.

Indoor velodrome?

Here are some times and speeds from a weekly ten-mile time trial held by a local racing and triathlon team. These races are open to the public. It was very windy this day. I wasn't there. I've done the course in just under 30 mins, though.
Time Avg. Spd.
1 21:03:00 28.50
2 22:23:00 26.81
3 22:51:00 26.26
4 22:58:00 26.12 (Tandem!)
5 23:00:00 26.09
6 23:29:00 25.55
7 23:32:00 25.50
8 23:33:00 25.48
9 23:39:00 25.37 (Fixed Gear)
10 23:45:00 25.26
11 24:07:00 24.88
12 24:33:00 24.62
13 24:39:00 24.44
14 24:42:00 24.34
15 24:43:00 24.28
16 25:14:00 23.78
17 25:25:00 23.61
18 25:46:00 23.29
19 26:05:00 23.00
20 26:07:00 22.97
21 26:08:00 22.96
22 26:11:00 22.92
23 26:11:00 22.92
24 26:14:00 22.87
24 26:24:00 22.73
25 26:29:00 22.66
26 26:33:00 22.60
26 27:01:00 22.19
27 27:03:00 22.18
28 27:16:00 22.00
29 27:21:00 21.94
30 27:58:00 21.45
31 28:04:00 21.38
32 28:20:00 21.18
33 28:51:00 20.80
34 29:28:00 20.36
35 29:33:00 20.30
36 29:49:00 20.12
37 30:08:00 19.91
38 30:22:00 19.76
39 30:52:00 19.44
40 31:03:00 19.32
41 31:21:00 19.14
42 31:35:00 19.00
43 31:43:00 18.92
44 32:04:00 18.71
45 33:17:00 18.03
46 54:12:00 11.07 (7.2 mi on Racing Trike)

A bike path