Rear Gear Advice
Postmaster
Joined: Apr 2008
Posts: 4,724
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From: Rogersville, TN
there is a difference in rpms from standard to auto. autos are always a little less rpms for the same speed in od about 300 at 70. which is where you all are probly disagreeing about. reguardless of tire size, the speedos are all calibrated the same and correspond to the tachometer according to rear ratio. unless you compare auto to standard.
and it seems to me that a motor that is turning 2700 rpms over 200,000 miles would have far less life left than a motor that has been running 2200 for the same number of miles.
and it seems to me that a motor that is turning 2700 rpms over 200,000 miles would have far less life left than a motor that has been running 2200 for the same number of miles.
and no, that extra couple hundred rpm wont affect the durability. diesels are made to be able to run wide open without a loss of durability, kind of like two cycle engines. you do more harm to a two cycle engine by running it half throttle than you do full throttle. youre not going to hurt your engine by cruising around at 1500 rpm all day, but youre not going to hurt it by running it hard either.
Postmaster
Joined: Apr 2008
Posts: 4,724
Likes: 4
From: Rogersville, TN
OK, looked at the truck and it is 235/85's. Also shows 45 (4.10) for the axle. Also, the MFR sticker shows the recommended tires are 215/85 so perhaps my speedo is a tad slow (I think I calculated it as being a little slow on my trip home from Florida), so that could make sense as to why I am turning a little over 2k at 60mph
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Elder User
Joined: Apr 2008
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I'll recheck it on Sunday. I am turning over 2k at 60 but probably more like 2150 or so. But if my tires are bigger than what was on the truck when it was built, then my speedo would be turning slower....so 60 on my speedo could be more like 63 actually, hince the bump over 2k.....
Posting Guru
Joined: Nov 2001
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From: Ft Scott
With the 235's instead of the 215's your gearing will be about 3.90 and the speedometer/odometer will be low about 4% or like you said 60 when you're actually doing 63
Be carefull with the 235's on a dually. I had a 97 DRW and the biggest I could run was 225 without the sidewalls rubbing. The ones you have now may be fine but if you replace them with a different brand you may run into a problem with them rubbing together and creating to much heat.
Be carefull with the 235's on a dually. I had a 97 DRW and the biggest I could run was 225 without the sidewalls rubbing. The ones you have now may be fine but if you replace them with a different brand you may run into a problem with them rubbing together and creating to much heat.
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Elder User
Joined: Apr 2008
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OK, took the truck out this weekend and it seems to be turning about 2150 - 2200 rpm's at 60mph. Or 2k is getting me about 57-58 mph. So given the fact that my truck was built with 215's (its a dually) and now rides on 235's slowing down the speedo by about 3-4%, I'm thinking it still has the stock 4.10's Sound about right?
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