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I’m new to the forum but have already spent a little bit of time reading earlier articles and posts (many of them very informative). My friend and I were having a discussion on gear ratios and bigger (35”) tires, specifically for a 2015 6.7l Cummins Ram 3500 (single rear wheel) with the Aisin transmission (stock gearing of 3.42). Over the course of the discussion we had several unanswered questions about re-gearing for that truck and for bigger tires in general: Why is a numerically larger gear ratio (like 4.10) referred to as “lower” gearing while a numerically smaller ratio (like 3.42) is referred to as “higher” gearing? It seems counter intuitive. I understand that the higher ratio has more gears on the ring gear and less on the pinion, which allows for better towing and slow-speed crawling performance. Conversely, I understand that a lower ratio has less gearing so to speak but is better suited to highway efficiency. Where do the “lower” and “higher” aspects come in? Can re-gearing to a numerically higher gear ratio help offset some of the mpg losses normally associated with bigger tire sizes? I understand that some amount of efficiency degradation is unavoidable when switching to bigger tires. But I’ve also heard that re-gearing can help put the transmission back into its optimal RPM band and also helps to reduce drivetrain strain caused by bigger tires. Is that correct? I only ask because I've seen a lot of Ram HD owners complain about significant mpg losses when making even a mild transition from stock tire sizes to 35's. What is an accurate way to determine the ideal gear ratio for a given tire size? I’ve seen an equation mentioned in several forums and youtube videos: Ideal Gearing = (New tire size * stock gearing)/(old tire size). Going with that equation (assuming stock tire size of 33”), the ideal gear ratio for 35” tires seems to be ~ 3.63. The closest conventional gear ratio is 3.73. Would that be ideal for a multiple use (offroading, towing, highway cruising) diesel-equipped Ram 3500 wearing 35” tires? What RPM band should we be aiming for with this new 6.7l Cummins? I realize that much of this topic was discussed in an earlier post by Moses Ludel back in 2013 (5.9l Gearing Article). His article was focused on a Ram with the 5.9L Cummins and the 48RE transmission. The newer Rams’ stock gearing and engine outputs have changed a bit from those earlier powertrain setup’s, so I’m wondering if his scenario is directly applicable to what my friend and I are dealing with.
