I know this issue has prob. been covered many times, but I would like some assurance. I ride a 09 dyna street bob, and without a tach, 5th gear at 60-65 mph seems like the motor is screaming. My brother shifts his fatbob into 6th at around 70-75. He claims any less and his motor lugs. Our trans. gear ratios are exact same. Diff. tire size though. Both cruising down interstate in 6th gear at 75-80mph. his purrs along, while mine screams as if its turning 5k. Would tire size make that big of difference? Or should I not even worry about it and just ride it how I want to? If it was an issue you would think tachometers would be standard equip. on every Harley sold.