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RPM to speed ver gear.

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I'm wondering what speed people are going at what RPM. today i hauled a 35' 5er weighing in around 15000lb. and i was paying real close attention to my speed vs rpm.i didn't use OD while towing.2nd gear was 40 mph at 2700 rpm.3 locked was 55 at 2000. in od i turn about 1700 ish and go 67ish. i'm starting to think i don't have the 3.55 gear the sticker on the glove box says i do.thought and imput would be great.

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A 2nd gen with 3.54, automatic and stock tires should be running about 1900 rpm's at 70 mph. Did you correct your speedometer with the smarty for bigger tires?

  • Author

My tires are stock. 265/75/16. Checking the speedo with a garmin rhino 530 it's spot on. Although the fuel mpg thing in the overhead council LIES!!! Us there a way to fix this? It says I get 25-27 mpg on a regular basis.

No, they are known to be inaccurate in the first place but when you add the smarty or any other power enhancing device they get way off.

  • Owner

Just use a ScanGauge II they can be calibrated for any fuel system with any mods and show close to perfect MPG numbers. :whistle:Math formula for instant MPG's...Forumla: Speed (MPH) / Flow (GPH) = MPG

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Yes! I want one, but money hasn't grown on trees got me in a few years, first on my list is gauges, then edge comp, then tranny, well maybe not in that order, but I will get one someday. Wife told me to look for newer truck maybe this one will be sold.

  • Owner

Problem is 3rd Gen cost more when they fail (injectors) vs. VP44's on the 24V engines. The life span of 3rd gen is about half of a 24V. So you would be better off putting money into the 24V with some gauges.

Yes! I want one, but money hasn't grown on trees got me in a few years, first on my list is gauges, then edge comp, then tranny, well maybe not in that order, but I will get one someday. Wife told me to look for newer truck maybe this one will be sold.

I would change this order to: Gauges tranny Edge Comp :) the 47RE's suck balls, stock.
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    It sounds to me like your spot on. 55 Mph locked in third at around 2000 to 2100 rpm. 4Th gear at 70 mph is 1800 to 1900rpm. My speedo is also corrected and checked with a gps. When towing heavy I generally lock out OD unless I am on the freeway. I will run third gear up to 2300 rpm steady, maybe a tad higher if needed.

    Wow. My 5speed + 3.54s on 265/75s was ~2200 @ 70mph.. I know that because the mufflerless 7"stack droned like hell. LOLSent from my SPH-L710 using Tapatalk 2

    Over drive. [ATTACH=CONFIG]5984[/ATTACH] [ATTACH=CONFIG]5985[/ATTACH]

    Haha! Ill take a pic of mine in the morning. You'll laugh! Sent from my SPH-L710 using Tapatalk 2

    And put your damned seatbelt on!Sent from my SPH-L710 using Tapatalk 2

    Yes, you have 3.55's. Mine was the same with the auto. Im about the same now with the 5 speed and since dodge calibrated my speedo for the bigger tires.

    I calculated a few times and you can basically say that any truck that does 70 at 2100RPM+ is 4.10. If your tires are huge then it is possible to be under 2100 with 4.10's but for sane tire sizes this works.

    Mainly because the highest O/D (0.75) with the smallest tires OEM (245/75) with 3.55 is 2050RPM at 70. 4.10 would obviously rev much higher. If you go the other route to see if this rule of thumb holds up wtih an auto (0.69) with 4x4 tires (265/75) and 4.10's you get 2100 so the rule of thumb works, barely.

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    I think they dropped the ball on the gear choices in the second gens. 4.10s are probably a necessity for the guys hauling at or close to max gcvw, but IMO the 3.55s being the only other option are just too high. They're ok for cruising down the highway Empty but that's not the reason I bought my truck to begin with. The 3.73 Would have been a much better option IMO.

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