145/70 R12 doesn't belong to the most popular dimensions for summer tyres - there are tyre models available, which can be compared to 325/30 R21, 155/60 R15, 155/65 R13. Let's break down the sizing - 70 is height of the tyre, which is calculated as 70% of the tyre width (145mm), which means the sidewall height has 101.5 mm. The wheel diameter is R12, which means it fits to a 12 inch/304.8mm diameter wheel. The total circumference of the tyre is mm, which is very similar to 180/65 R16(2011.88 mm), 255/40 R21(2316.61 mm), 185/65 R14(1872.7 mm), 255/55 R19(2397.35 mm), 195/40 R17(1846.63 mm), 255/70 R18(2557.88 mm), 195/60 R14(1852.28 mm). There are multiple speed indexes available for summer tyres in 145/70 R12: S(180km/h), T(190km/h). The same goes for weight indexes: 69.