GatorSkins and tire liners

The liners will slow any tire down, but my "guess" is that they'll slow a faster tire down more than they will a tire like a gatorskin.

I would agree with monkeyclaw in that you don't really need to worry about it unless it's impacting you from keeping up with the group.  I know when I ride with faster roadies, I have to swap tires.  If I was able to sit in the pack the whole way I'd be fine, but there's no way I can take even pulls with them while I'm putting out 10-20 more watts to keep the same pace.  For an hour maybe...but not for 3-5 hours.

My guess is that it can turn something like a GP4000S and make it ride similar to a gatorskin. 

SO that's my problem... I just thought I was Sloooow...

What kind of difference are we talking about?

I have not swapped wheels for a race/ speed run so nothing to compare to. (to swap I need to change brake pads and adjust positions as the tracks do not line up carbon to alloy)

Wind resistance will definitely have a bigger impact than rolling resistance.  A strong wind can slow you down about 5+ mph, but don't underestimate the difference between a very poor rolling tire and a very good one (especially if you pair it with a latex tube).  In actual field testing, the difference can be very close to 1 mph. 

You can take this chart done by bike tech review...

http://www.biketechreview.com/tires_old/images/AFM_tire_testing_rev9.pdf

Then play around with this calculator...

http://www.analyticcycling.com/ForcesSpeed_Page.html

The way to use the calculator is to first set the slope to zero.  Then play around with one of two things.

1.  Adjust the default crr by the amount shown in the BTR chart.  So if you're comparing two tires that have a difference in crr of .001, then see what speed the default crr results in, then see what happens to speed when you add .001 to the default.  Note that the BTR chart shows crr per tire.  Theoretically, you would need to double the difference in the crr to get the effect of two wheels.

2.  The BTR chart also shows a difference in wattage per wheel.  This is essentially how many watts you lose to rolling resistance.  So again...the lower number the better.  Play around with the calculator and see what happens to speed when you compare the difference in wattages lost for 2 wheels.

A gatorskin is such a poor a rolling tire, it wasn't even tested on this review...so you can probably assume it's at best comparable to the worst tire on the list.  But just for the sake of an example lets just compare the worst tire on the list (Conti GP3000)with a GP4000S and latex tube (GP4000S is not even the fastest rolling tire).  You get a difference of .00181 in crr per tire.  So .00362 total.  Plug that into analytic cycling and the result is (using default frontal area, cda, 75kg bike and rider, and 0 slope)....

Default crr of .004 at 250 watts = 25.12 mph
Crr of .00762 (.004 + .00362) at 250 watts = 24.02 mph

If you want to get really technical, the calculator I linked can probably point you to some answers as well.  Plug in the slope of the decent (as a negative value) and adjust the air density/barometric pressure.  You may not get the exact numbers since your frontal area and cda will have to be spot on for the numbers to match, but you can at least play around with the numbers and see how the air density can vary your speed all else being equal.

Thanks for that link.  I always wondered what the difference between the GP4000S and the Gatorskin was.  But the result is still clear...that rolling resistance between tires (whether it's a Gatorskin or GP3000) can be very significant. 

If you're currently using a poor tire, I would go with the tire upgrade first (don't forget the latex tubes as well).  You may be able to get 0.5-1 mph in speed gain where an aero helmet is likely closer to 0.3-0.5 mph.  If you're already using a good tire...then it's more of a toss up.

I've had good success with both the GP4000S as well as the Vittoria Open Corsa Evo.  The Vittoria is known be be less durable, but knock on wood I haven't had any issues yet.  I only use them for racing though.  I've used the GP4000S for training and racing at times.

Recent aero testing has shown though that the Vittoria is pretty bad in the wind tunnel where the GP4000S tests much more favorable.  So I've now switched to a GP4000S up front and a Vittoria in the rear.  When both of my Vittorias wear out, I'm likely to simply go GP4000S front/back.  At this point your splitting hairs because how a tire tests in the wind tunnel is largely affected by the type of wheel it is mounted on...but since I already had both tires...I figured I'd be either gaining a little, or a wash at worst.

The only experience I've had with a Michelin PR3 is when I had a sidewall blowout during the first race I used it.  It's not to say that other tires can't have the same problem, but it left a bitter taste in my mouth and never went back.  Others have also reported durability issues, but then you also have people who use them for thousands of miles without a problem.  In any case, they are designed to be quite good and there is the new PR4 model that has been out for a couple years now.  I haven't seen them as cheap as the GP4000S though.