If it was the motor mounts, it would be likely to vibrate while idling. And the most common transmission problem would be slippage - my guess would be the wheel bearings - also, if you use brake pads faster than manufacturer specs, there may be a problem with the anti-lock brakes engaging improperly; either worn, damaged, or imbalanced. Volvo was/is state-of-the-art in center-of-gravity research and design - 45Mph may be a threshold for the activation of improved balance mechanism. Inspecting the Wheel Bearings is a good first step.

You should first try to ascertain where the vibration is coming from, which may be difficult at 45 MPH. Still, a good sense of where the vibration seems to be coming from can give you a place to start. The things I would consider, in order, would be: If the vibration is coming from the front of the car, it could be wheel bearings, tie rods or tie rod ends, ball joints, or front suspension bushings. It could also be that your wheels need balancing. Many tire shops will provide this service for free if you originally bought the tires there. Jack up the car and grasp one front wheel at the top and bottom. try to move it in at the top and out at the bottom and vice-versa. If there's any play, suspect your ball joints. Grasp at the front and back of a front wheel, and do the same. If there seems to be free play before you feel the steering rack start to move, suspect your tierods. The classic symptom of bad wheel bearings is noise when the wheels are rotating. It's also possible that it could be motor mounts or the gearbox mount. The motor mounts, especially the one on the passenger side of the car are classic failure points on a Volvo B-230 engine. The passenger side motor mount lives directly under the oil filter, and often gets covered with oil during oil changes, causing to to fail. When this one goes, the others do, too. The mounts can be visually inspected, and can also be checked by observing the shifter during startup to see if it moves an inordinate amount (you'd have to check other cars of the same model for comparison's sake). You might also consider the universal joints on the driveshaft, and also the mounting on the driveshaft with bushings to isolate it from the body of the car. These can wear and cause vibration, but the common symptom of this is a clunking noise from the the driveshaft tunnel on sudden accelleration or decelleration. If you note this symptoms, check these parts immediately. If they fail, the driveshaft can disconnect form the rest of the drivetrain. Depending on how this happens, and how fast you are travelling at the time, this can result in the shaft literally bursting up through the floor of the car, or digging into the road and causing the car to flip over. Not fun. -EdM. '90 Volvo 240DL Wagon "Lola" '72 Volvo 1800ES "Galadriel"