If 60' times improved, that's why the ET improved, as others mentioned, because of a wider contact patch (better traction). Sometimes a change that is not considered a performance improver actually helps. Another example would be adding weight to the trunk. Normally if weight is added, it slows the car down (roughly 100 lbs = .10 ET) But if it helps traction enough, it can improve the ET, AND consistency.
60' times are an indicator of starting line efficiency (tires/slicks, gearing, converter, accelerator pump set-up in a carb). MPH is an indicator of engine efficiency/horsepower.
Shorter tires/more gear usually helps, to a point. But the rest of the car has to be able to work with the additional overall gearing to apply it to the track and the engine needs to handle the extra rpms.
