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Holt R

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  1. Moses, thank you for the detailed answer. I used the RPM calculator in the link you provided. I first did the baseline calculation using stock (~33" tires) with 3.42 gears, and the Aisin's final drive of .63. The resulting RPM for 65 mph (probably the ideal cruising speed for reasons you already noted) was 1426 RPM, which seems quite low for the 6.7l Cummins. For curiosity's sake, I also tried 75 mph and got a RPM of 1645, which seems closer to the engine's peak torque sweet spot (1700 RPM). I'm guessing the FCA engineers set up the gearing to allow for 75 mph at low RPM's? Even if so, I do agree that with a big truck, aerodynamics will certainly degrade fuel efficiency at that speed, even if the engine RPM is at an efficient value. Using that as the baseline, I did two more calculations: 35" tires with 3.73 gears. At 65 mph, the RPM was 1466; again low, but it seems close to OEM specs. At 75 mph, the RPM was 1692, which again puts it very close to OEM specs and the peak torque RPM. 35" tires with 4.10 gears. At 65 mph, the RPM was 1612 RPM. At 75 mph, the RPM was 1860; this seems a bit higher than what I would want for long highway cruises, no? The stock gearing RPM calculations kind of surprised me. With stock tires and gearing, it almost seems as if you'd be lugging the engine at 65 mph. But if my only concern is to put the engine back into OEM RPM ranges with 35" tires, the 3.73 gearing does seem to do that. I will also note that on this hypothetical truck we're discussing, there is a strong possibility of putting on a lot of aftermarket gear (bumpers, winches, auxiliary fuel, ect.) to the tune of 1k lbs. If that was the case, I'd imagine 4.10's would make more sense for moving all that extra weight, in addition to any extra payload and trailers, correct? If kept at a mostly stock base weight, would the 3.73's be a better choice?
  2. 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.
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