That dealer is an idiot, and should not give advise if they dont know what they are talking about,
IF YOUR REAR SPROCKET IS BIGGER ([bold]its the equivelant to bigger rear tire[/bold]) So the more it would slow the spinning of front sprocket because more ground is traveled at back tire.
It's always wise to be very careful about dismissing people as ignorant, or idiots.
A bigger rear tire would actually have the opposite effect of the sprocket change considered here. A bigger rear tire means a larger tire circumference, which means more ground would be covered for a given revolution of the wheel, and hence, a given revolution of the sprocket. This would have the effect of yielding a lower displayed mph, and lower mileage, for a given sprocket combination.
To put it another way, increasing the rear tire circumference has the opposite effect of increasing the tooth count on the rear sprocket, which serves to reduce the wheel rotation for a given countershaft sprocket rpm.
This is the principle Scoot Russell was frequently exploiting at Daytona when he would purposely keep the bike leaned over coming off the banking, in order to keep the engine rpm up that extra little bit longer, because he was essentially making the effective circumference of the tire smaller, or, having the same effect as increasing the tooth count of the rear sprocket.
But yes, the dealer is an idiot.