Who will rule the auto world of the future?

FORTUNE -- Like few other industries, auto companies measure their output in number of vehicles built and then report that data publicly.

Besides contributing to a "mine's bigger than yours" mentality, the availability of these production numbers has created a cottage industry in massaging the figures and analyzing the results.

In addition to providing competitive data, forecasts of future production rates give suppliers guidance in planning upcoming demand and suggest where they should go to look for new customers.

But the numbers don't always tell the same story. Take a look at a pair of forecasts by two notable experts.

One is from former Merrill Lynch analyst John Casesa, who now advises governments, corporations, and investors about the direction of the auto business

Casesa extrapolated production trends from 2005 to 2009 to see what the auto world looks like in 2018 and discovered a radically changed global landscape.

Toyota, which only displaced GM as the world's number one automaker in 2008, drops from the top spot all the way down to number three.

Casesa sees Volkswagen passing Toyota (TM) to become the new number one, followed by Hyundai/Kia in second place. Both companies are expanding quickly in North America and doing well in developing markets.

Casesa's calculations contain some other big surprises: He sees General Motors slipping from second to tenth in the world, and Ford (F, Fortune 500) falling from fourth to eighth.

That may be a little harsh. GM's market share in North America with its four remaining brands seems to have stabilized, while Ford has been gaining share.

The rest of Casesa's top ten has Renault Nissan in fourth, Honda (HMC) in fifth, Fiat-Chrysler in sixth, and France's Peugeot -- a remarkable survivor given its concentration in its home country and its shunning of mergers -- in seventh.

Another surprise makes the list at number nine: China. The Chinese government is likely to force the merger of several smaller companies to create one global competitor.

The conventional wisdom

But there is more than one way to divine the future. IHS Global Insight, which produces economic and market forecasts, uses a more data-intensive method.

It makes a bottom-up forecast by looking at each company's production plans, factoring in market strength and economic growth, and making some educated guesses.

This more incremental approach produces a ranking of top global players relatively unchanged from today.

Toyota remains number one, GM is the runner-up, and VW comes in third.

Rounding out the top six: Renault/Nissan in fourth, Ford in fifth, and Hyundai/Kia in sixth.

The problem with all forecasts is that they can't anticipate discontinuous events. Mercedes-Benz, for instance, would have shot up the list after the DaimlerChrysler merger in 1999, and then dropped down again after the merger was dissolved.

Some analysts are predicting now that VW's ambitions for global supremacy may be brought low by powerful unions in its headquarters town of Wolfsburg who don't want to see production moved overseas.

The biggest question mark today, aside from the Chinese, is how far Hyundai/Kia's momentum will carry it. Twenty years ago, it was on nobody's radar screen. Today it looks unstoppable.