Diesel and petrol have completely different final drive ratios due to the difference in the rev range of each engine. Not a good chose to swapI think both those numbers were fitted to Pandas, 500s and Puntos (and Ford Kas). Diesels and petrols use different clutches, so the boxes might have some differences. @koalar might know?
Also, it seems the same 'box in the Punto uses a different final drive to the others, being a heavier car, so make sure it's from a Panda/500/Ka or the gearing will be wrong.
Ah but doesnt that also depend on how fast you cruise. Ive had cars in the past that irritatingly used less fuel at 80 than 70mph. I do agree with you in principle. If they spend a whole pile of time deciding what the best compromise is for each car designed I feel its not very likely that modifications will make a substantial improvement, and if you might ge tsay more speed you will pay for it somewhere else like poor economy or ride and handlingchanges that are not desirable. Trouble with gearboxesit they often look the same but are quite different internally. SImilarly I dont understand the must have a short shifter kit brigade.Many people think that a higher ration gearbox gives better fuel consumption. In reality, they make the fuel mileage worse because the engine can never get into its most efficient rev range.
Sorry, it has been a few days before I could get to ePER for a browse.Does anybody know the difference between a 55196336 gearbox fitted in my 2006 fiat Panda and a 551963361 gearbox.