Lean limit methane-air flame propagating upward in a 24 mm diameter tube was studied experimentally and by numerical simulations. Gas velocity field was measured using Particle Image Velocimetry (PIV) method. The experimental measurements are compared with the results of numerical simulations and with previous similar measurements performed for a standard flammability tube. The experimentally determined lean flammability limit in 24mm diameter tube was 4.9 +/- 0.03% CH4 by volume, against determined earlier flammability limit 5.1-5.2% CH4 in a standard tube. Maximum stretch rate value was found to be at flame leading point and was higher in a 24mm diameter tube than in a standard flammability tube. The observed extension of the lean flammability limit is attributed to the strengthening effect of positive flame stretch on lean methane-air flames, characterized by Lewis number, Le<1. Numerical simulations of limit methane/air flame in a 24mm diameter tube demonstrated presence of negative flame speed at the flame leading edge.