Tire having a load index LI, as defined in the ETRTO Standards Manual 2010, lying in the range between 65 and 105. The tire comprises a tread and an under layer (48) extending over the whole width of the tread and having an average radial height greater than 20% of the average radial height of the tread and a maximum value of tan δ at 23° C. and 10 Hz less than the corresponding value of the tread. Radial carcass reinforcement (60) extends from the beads across the sidewalls as far as the crown. The tire has an outside diameter OD and a nominal section width SW, as defined in the ETRTO Standards Manual 2010, that satisfy the following inequality: OD/SW>−0.00082641 LI2+0.11266 LI−0.185.