since time for replacement of my summer tires comes quickly, I was thinking (for some time) to go with changing classic Ti sizes of 235/45R19 to combination:
- front (leave OEM) 235/45R19
- rear (use wider) 255/40R19
according to Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit
the difference would be only +1.09%, which is value normally acceptable by TÜV/MOT
BUT, I have read too many complaints about using different tires sizes on front and rear axle of the 4 wheel drive systems, in our case Q4. same is written in the car's manual.
to my best understanding of Q4 (and other 4x4) system, it has to have 3 differentials:
- front axis (left to right wheel)
- rear axis (left to right wheel)
- front to rear axis
So, left to right wheel is standard for any car, enables rotation of the side wheels different speeds when car turning (wheels passing different circumferential paths).
front to rear axis enables different rotation speeds of front axis wheels to the rear, enabling front axis wheels to pass longer path (circumference) than rear axis wheels when car turning.
as far as I know, 159's Q4 uses Thorsen differentials, which are top notch technology for this price range. But Thorsen in this case does not have the meaning (or?).
so, if front to rear differential enables different rotation speeds, that means that it should have no problems with the 1.09% difference between such changes tires. Yes, when driving car on straight line it will be permanently working, but 1.09% is not big difference and it should not be meaning over-work or over-load of any kind, or?
can anybody with technical knowledge explain the technical reasons why companies do not recommend having different tires on front and rear axle of the 4x4 cars?
please follow the manufacturer's guidelines. any following of this thread's questions is on your own risk and I do not take any responsibility for any caused damage on your vehicle!
just found one even better tire sizes calculator
using this web page, found out that when using 255/35 instead of 255/40 I could save 0.6 kg per wheel of unsprung mass. unfortunately, it increases the change from +1% to -5%, which might raise some eyebrowes when speaking about this idea
255/35 is very low tire thickness (to be honest, 255/40 as well) for the Berlin streets. well, for Berlin streets Range Rover would be better solution
funny enough, 255/45 seams to bring only +2.6%, which gives more comfort but adds too much extra unsprung weight
for the moment, 255/40 seams as good compromise