Bicycle Mechanics - New Mavic Ksyrium rear wheel issue

Hi, in need of an assist here. I bought a new set of Mavic Ksyrium SL wheels. Installed a new Ultegra 6700 12/25 cassette. Put new Vittoria Rubino Pro tires on. On 3 test rides totalling about 90 miles, it is very obvious that it is taking much more effort for significantly reduced performace than usual. Took wheel to LBS. Bearings and internal components OK. Only thing noted was that cassette does not spin as freely on new Mvic hub as my cassette spins on my prior set of wheels. Switched wheel sets to old set. Avg speed about 20 mph, max speed 29-32 range. Switched back to Mavics, average speed over same course 17-18.5 , max speeds around 25 ish. Checked brakes, not an issue. Questions: Could new tires be adding this much rolling restistance?? Could new cassette be this stiff to cause this? When I ride the Mavics I feel like someone is sitting on the back of my bike. Had heard nothing but great things about them and am very perplexed right now. WTH? Any thoughts??

Mucho thanks

Does the cassette need a spacer on the Mavic hubs? A 10 speed Shimano cassette needs a spacer behind it on most freehubs. Otherwise it will rub on the hub.

Does the cassette need a spacer on the Mavic hubs? A 10 speed Shimano cassette needs a spacer behind it on most freehubs. Otherwise it will rub on the hub.

Checked with LBS and confirmed there was a spacer (is Shimano)... BUT, that was a great thought that I had not considered so thanks much for response!

Very strange- my ksyriums (elites) seem to have the opposite effect- they're lighter and faster than my other conventional open-pro wheels. But not much- barely noticeable, but definitely better. Not 3 MPH better, though.

The cassette spin shouldn't make any difference unless you're coasting, and at ~20 MPH average speed, you're not doing alot of coasting!

I'm assuming that you checked for brake drag and that the wheels spin easily when you lift them off the ground. Maybe something is rubbing on the inside of your dropouts/chainstays? Different tires will have different rolling resistance, but that sounds pretty bad- like the tires are only half-full of air or something.

They really aren't much lighter than a set of open pro 32 hole wheels, maybe 400 grams or so for a pair depending on how the open pros are built. Also heavier wheels, in certain instances for me, are easier to keep rolling vs lighter wheels. Tires do make a big difference but I don't think the Rubinos would. Are you sure the bearings aren't binding when the QR is clamped?

Most MAVIC hubs require 2 spacers behind a 10speed Shimano cassette, a 2 mm MAVIC spacer and a 1 mm Shimano spacer.
Be sure you have both, but I doubt that is your problem. Sometimes new hubs have a bit more resistance than well maintained used hubs. But this should not make the kind of difference you seem to be having. Heavier tires will make a noticeable difference, but still not that much.
Be sure to fully inflate your tires before each ride.

+1 on the Shimano spacer AND the Mavic spacer.

Double check the rear dropout alignment - while loose ball bearing hubs can handle a little misalignment - cartridge bearing hubs cannot. Just a tad off will cause rotational binding upon closing the quick release and riding.

Rear dropouts need to be perfectly parallel for cartridge bearing hubs.

=8-)