The standard tyre on a 6.9 is a 215/70/14. Overall diameter is 656.6 mm.
It would be inadvisable to increase the width of the tyre, because if the suspension allows the car to lower too much, the edge of the mudguard could collapse onto the tyre, especially if the front wheels are not in the straight ahead position.
My car has the same width wheels as the 14'' Bundts for the 6.9 , with the same offset, but they are 15" wheels from a W126, with the 15 slot pattern.
For some years, the car was fitted with 215/65/15 H tyres, overall diameter 660.5mm.
Relative to the standard spec. 14" tyres/wheels above, the oveall gearing due to the larger tyres, is increased by 0.6%. This would trend towards, and eventually equal, the standard diameter, as the 15" tyres loose tread depth, due to wear.
There is less compliance in a 65% tyre than there is in a 70% tyre, and so the suspension bushes have a slightly harsher time. If ANYONE has lower profile tyres it is advisable to keep the car on smoother roads.
My car has recently been fitted with 225/60/15V tyres, so now I have to be even more careful to stick to smoother roads, and
to park the car with the front wheels in the straight ahead position.
The overall diameter of the 225/60/15 tyres is 651mm, and so the overall gearing is now reduced by 0.85%, and so the car should accelerate better, but I expect the better performance would not be noticeable. I also expect that fuel economy would be 0.85% worse. As the tyres wear down in tread depth, instead of approaching standard specification for diameter, that trend is now set to diverge further away from standard.
I have found by experience that 36 psi gives the best tread life for tyres fitted to my 6.9.
I would guess that may be a little too much for a lighter engined 2.8, 3.5 or 4.5 litre car.