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IHCsawyer 02-02-2018 04:22 PM

stock truck: how low can you go? (gear ratio)
Driving my pickup in 80 mph speed limits with the 4.11s is hardly enjoyable. Either my truck is screaming at 2500 rpm and Im the slowest vehicle on the road, or im going 65 and feel like every big rig is about to smash me and my family from behind. Yeah the gears are great paired with the G56 when towing 15k lbs, but the 5th wheel is for sale and I do not see myself getting another one. If anything Id have a light weight overhead/slide in/ truck camper.

Ive been reading mixed results on running 315s to lower rpms. Seems that most people do claim lower rpm but not an increase in mpg. And 100 rpm isnt nearly what Im going for. Maybe the 315s with 3.73s would get me enough top end to not be blown off the road by big rigs.....? Anybody with a 6 speed and 3.73s want to chime in?

6 speed RPM table

there is the only chart I could find but its a NV5600. Not sure the difference between that and the G56.

Obviously 3.73s are possible as that was the more common option. What about 3.54 with a stock pickup?
Actually using 1st isnt a problem but would I have no acceleration?
Would I be neutering my truck and crushing its ability to tow when I want to?
Im not looking to fly around everywhere, and the increase in mpg at 60/65 would be absolutely welcome

I dont want to sacrifice ALL of my ability to tow, but I want to be able to drive my $30,000 truck faster than I can my 76 CJ5

Jeff in TD 02-02-2018 09:22 PM

I think later g56 transmissions came with more overdrive on 6th.

Anyway my truck is also an 06 with G56, but with 3.73’s and I tow a 5th wheel that is probably in the same 15-16k range, and it does fine. I start out in 1st when towing, and probably am in 5th where a 4:10 truck might be in 6th, but I think overall it is a decent all round gearing setup. I generally don’t drive the thing 80, though.

If I were in your shoes I’d go ahead and do 3:73. New loaded diesel could set you back $$$, so if your truck is otherwise in good shape, and the gearing change would
Make you happy- probably worth the investment.

IHCsawyer 02-03-2018 12:10 PM

great thanks for the reply. No doubt that 3.73s will improve the situation and still keep my pickup capable as a tow rig. curiosity makes me wonder about even lower though

yep, i plan on sitting on this girl for a very long time. I havnt even broke 50k yet!

IHCsawyer 02-03-2018 12:12 PM

thought I was creative with that title, but now i think I wrote it backwards. shows what I know

Jeff in TD 02-03-2018 01:26 PM

3.42 or whatever the newer lower ratio is might be nice if you bomb around at 80 not towing or hauling, but I have no experience about how they would do otherwise.

From what I’ve seen fuel economy seems to have more to do with rod speed than engine rpm.

StealthDiesel 02-03-2018 05:03 PM

IHCSawyer, are you sure your 2006 Dodge Cummins with a G56 has 4:11 gears?

Have you jacked up the rear and confirmed by spinning a rear tire and counting driveshaft revolutions?

I'm sure only 3.73 gears were available original in a 2006 G56 truck.

As was mentioned, the early G56 has 0.79 OD ratio vs. a late G56 (2007+) which has 0.74 OD ratio.

Jeff in TD 02-03-2018 05:05 PM

Thanks for clearing that up. I know there was a change in OD ratio with the G at some point...

Mexstan 02-04-2018 05:41 PM

I have 3.73 gears and 315 tires. Been using the 315 tires for many years, even when towing my 12,000 lb fiver. Not a problem towing, even over mountain passes. At 1,900 rpm I see 70 mph. Don't like going over 1,900 rpm because then I see a rapid rise in fuel consumption. Never use first gear except when towing or if I am in stop and go traffic and need to creep along. No idea if the taller tires affect my fuel consumption very much, but they do not appear to have changed much because I still go about the same distance on a tank of fuel with the OE tires compared to the much taller 315's. The only real difference I find is that below about 1200 rpm I do not have a much giddyupgo as before, but that really is not a problem - just use a lower gear. Just returned from a 400 mile trip and the unreliable overhead shows an average of 17.2 mpg at mostly 70 mph and occasionally up to 80 mph. Went from sea level up to 9,800' and then changing altitudes after that. Started with a full tank and now the fuel gauge shows about 1/3 of a tank left.

I know that my speedo is accurate because when I changed to the 315's I had it recalibrated. My GPS confirms the speed.

IHCsawyer 02-05-2018 10:03 AM

I have not counted drive shaft revolutions. It makes sense how that works but I didnt know it was possible. And thanks for that stealth diesel! i was certain my window sticker said that it had 4.11s. It does not say that. My notes from talking to the PO say "4.10 gears". In other words the 77 year old that had shoulder surgery and couldnt drive the pickup anymore told me that. not much proof! Ill look up some more information on that and see what I cant figure out.

The pickup does have 235/80/r17s. its marginal, but it is taller than stock

I do know that my overhead is completely out of whack. It says I 31 mpg. 27 when I was towing 15k. I wonder if my speedometer is off too. People in Utah do drive notoriously fast. The speed limits are 80 and everyone is speeding. With the Tacoma I owned before this pickup I usually felt like people where flying past me. Maybe Im not driving 60-65......

StealthDiesel 02-05-2018 02:05 PM

Without knowing what your current differential gears are, this discussion will not progress.

Jack up the rear of the truck, trans in neutral.
Put one tire with the valve stem at the bottom.
Mark the driveshaft. Spin the tire one revolution using the valve stem as a reference while counting the number of times the driveshaft turns.

For even more accuracy, spin the tire 10 times while someone else counts the number of driveshaft revolutions.

Use your cell phone GPS to confirm speedometer accuracy. Or any number of free "Speedometer" apps.

Put black electrical tape over the overhead MPG readout!

LT235/80R17's are the exact same diameter as the stock LT265/70R17's tires that originally came with that truck.

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