Yet another GEAR swap info request thread

So I'm seriously looking at swapping gears this winter. I have a '15 CC short box 4x4 with the 6.4 and 3.73 gears. I'm rolling 295/65r20 Nitto Trail Grapplers. I've seen a few posts here & there about swapping to 4.56s, but wondering if anyone that has moved up to 4.56s, runs a similar set up to me.

I'm essentially running 35s and looking to possibly move up to 37s next summer. I have about 50/50 daily commute of about 40 miles. I only tow occasionally and not heavy.



I hate how sluggish the truck feels. If I'm driving normally in stop & go traffic, it's fine. But, when I'm getting on the hwy or putting my foot in it, it just lags soooo bad. It's funny, but the truck seems to take just as long to get up to speed when empty as it does when I pull my TT fully loaded (only about 4400#) and about 500# of gear in the bed of the truck. I know the power is there, but it just bogs down. I know a tuner will help some with throttle response, but with big tires and possibly going bigger, I'm thinking my truck might benefit the most from gears right now. I also have trouble holding speed on the hwy. My truck likes to settle at 60 mph at 1400 rpm. But on slight hills (like overpasses every other mile here) it drops to 55 and holds before downshifting. If I run at the speed limit (65-70) it runs about 1500-1600 rpm and downshifts a lot. It seems to hold 75 pretty good, but I only hit that on long trips out of town at about 1800 rpm.



So, is there anyone running 35s, the 6.4, and 4.56 gears?

1 - What are your rpms at 65, 70, 75 mph?
2 - Did the gears make a big difference on daily drivability with the trans shift points?
3 - How is it at holding speed on the hwy (65 - 75 mph)?
4 - Any increase / decrease in mpg city / hwy?
5 - Any other observations you've made good or bad about moving to 4.56s? Any regrets about it?

Engine RPM / Engine Speed Calculator

this will help with #1

according to allpar the 66/68 ratios are:

5th gear is .816
6th gear is .625

6th gear, 4.56, 34.5 tire

65 = 1804
70 = 1943
75 = 2082

I've been playing this for a few days, 37s and 5.13s seem pretty good. I would think 4.88s and 35s might be nicer based on what I see. Which would be:

65 = 1931
70 = 2079
75 = 2228

FWIW, a 4.10 truck with a 32.5 tire...

65 = 1722
70 = 1854
75 = 1987

1500-1700rpm seems too low to me for a giant brick that weights 7k lbs, with heavy mud tires added to it. Plus, thers no way the MDS can be working at all with that type of constant load. I would think something around ~2000 at 70 would be ideal on flat ground, I haven't tested this, but if you leave the MDS on you should be able to get away with more RPM. It should reduce load and allow the truck to try and conserve fuel instead of being lugged down at ~1700 and feeling like its going uphill all the time. All these modern trucks with 0.6 overdrives... that's extremely aggressive for a heavy brick. It might work with a diesel, but its not going to work with a gas motor.

SilverSurfer15 said:

Engine RPM / Engine Speed Calculator

this will help with #1

according to allpar the 66/68 ratios are:

5th gear is .816
6th gear is .625

6th gear, 4.56, 34.5 tire

65 = 1804
70 = 1943
75 = 2082

I've been playing this for a few days, 37s and 5.13s seem pretty good. I would think 4.88s and 35s might be nicer based on what I see. Which would be:

65 = 1931
70 = 2079
75 = 2228

FWIW, a 4.10 truck with a 32.5 tire...

65 = 1722
70 = 1854
75 = 1987

1500-1700rpm seems too low to me for a giant brick that weights 7k lbs, with heavy mud tires added to it. Plus, thers no way the MDS can be working at all with that type of constant load. I would think something around ~2000 at 70 would be ideal on flat ground, I haven't tested this, but if you leave the MDS on you should be able to get away with more RPM. It should reduce load and allow the truck to try and conserve fuel instead of being lugged down at ~1700 and feeling like its going uphill all the time. All these modern trucks with 0.6 overdrives... that's extremely aggressive for a heavy brick. It might work with a diesel, but its not going to work with a gas motor.

Click to expand...

Has anyone seen 4.88s?
I have factory 4.10s and would like more gear, but 4.56 isn't worth the cost


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GPRAM2500-SELECT

They go from stock ratios all the way up to 5.13. So this is what has me thinking about 4.56. Coming from 3.73s, that should make a pretty big improvement.

When I had my '08 Wrangler unlimited, I was running 35x12.50s and 4.88 gears. The 4.88s were a perfect match for that anemic 3.8 V6 and a 5 spd trans. So I think 4.56s should be great for 35s and pretty decent for 37s with a big 6.4L V8 and 6 spd trans.

But, that's why I'm looking for feedback. I want to see if anyone with a 6.4L is running 4.56s and 35s or 37s.

TWilliams.... 4.56 or 4.88s cant remember. have you seen that one, the big white 6.4? He has 37s, I think it might be 4.88s though.

FWIW, I used to DD a 6.0 Silverado HD with 4.88s and 37s. I liked it, it still got around 13mpg if you stayed in the 60-65 range. Driving over 65 though killed it. RPM was around 1800 at 62, which was the sweeet spot for that setup. The useable power is great, it towed a 17ft bt with no problems at 55mph. I wouldn't try to go much faster, but that truck was 12" on 22/37 mud grapplers. Huge.

SilverSurfer15 said:

TWilliams.... 4.56 or 4.88s cant remember. have you seen that one, the big white 6.4? He has 37s, I think it might be 4.88s though.

FWIW, I used to DD a 6.0 Silverado HD with 4.88s and 37s. I liked it, it still got around 13mpg if you stayed in the 60-65 range. Driving over 65 though killed it. RPM was around 1800 at 62, which was the sweeet spot for that setup. The useable power is great, it towed a 17ft bt with no problems at 55mph. I wouldn't try to go much faster, but that truck was 12" on 22/37 mud grapplers. Huge.

Click to expand...

He went with 4.56