Well, seriously speaking now, I doubt that the difference in mileage will compensate the amount spent on the "new" one, it could be calculated looking at the gear ratios though.

Of course, if you still want it and your current unit works well I would suggest exchanging the whole gearbox with a 6 speed one, I understant it is possible to make (but check first with specialists). Here in Europe the 6 speed ones are not so difficult to get.

If the price for the new one is really good then ... just keep both for some time and then sell the less interesting one ;-)

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BMW X5 3.0i e53 (2000)
Mazda MX5 1.8 Unplugged NB2.5 (2004)
Skoda Octavia 1.6 MPI GLP-Etanol-Gasolina 1Z5 (2009)
Audi A4 1.9TDI QUATTRO ATJ (2000)
Peugeot 306 GTI-6 2.0 (1998)
Yamaha XJR 1300 (2000)