Gear Replacement - Which Ratio

[Jim Allen]

Best not to consider the gears as "increasing towing capacity" just as making it easier for the truck to do it. I saw dramatic drops in towing and hauling trans temps on many trucks with lower gear ratios.

As to towing with medium-big tires and a 6-inch lift, I'm with Meep.... doesn't make for a great tow rig. Coupla things you can do to equalize it a bit: go with a rim increase and a shorter profile tire. If you rims are 16s or 17s now, go with a 17 or 18-20 inch rim with the metric equivalent of a 35 (305/70R-xx). The shorter profile tire will help with the towingby being a bit more stable but will still have the height. Also, investigate the LEAST amount of lift that will support that tire size on your truck. I suggest calling the lift kit mfr. and asking them about towing with the lift you are interested in. They may have some advice or options to make it work better.

 

[RichardSenn]

We can throw towing out the window, I was really just curious about it. I'll be pulling the trailer twice a year if it is really that bad I can use the Tahoe.

 

[sw99]

Originally Posted By: RichardSenn


I've been thinking of going with Standard or Yukon gears, those have been the two most recommended.



I know when I did gears and made my choice, USA standard was the same a Yukon. They just dont do the final lapping process that makes the gears quieter. Same alloy and strength. I run mudders so minor gear noise the last thing I worried about.

 

[95busa]

I had 4.56-1 gears on my 2000 F150 with a 4.2L V6 and stock 255/70-16 tires. It made towing a 5000lb trailer very easy, and every day driving was much enhanced as well. I got 13mpg towing anywhere from 55-60mph, and unladen MPG went up in every scenario up to 65mph, and stayed the same at 70-75. 4.56-1 is a straight AWESOME truck gear with modern overdrive transmissions. Don't be put off by the old musclecars with TH350s and 4.10's spinning to the moon on the highway. I loved my 4.56s, and I will run them in the future without reservation.

 

[sw99]

I know from some past experiences, if I settled on what I thought would be the best gear choice. I always wished that I would have gone a bit lower. to get you closest to factory with a tad more power, 4.56's to me are the best choice for a 35" tire. Let us know what you decide on.

 

[spasm3]

My 4wd avalanche has the 3.73 rear. I really only tow with this truck. I am considering going to a 4.56 gearset. Currently with the 3.73 i run about 1800 rpm in od(.69) and 2600 rpm in 3rd gear(1.0). Based on a 31.6 diameter tire. If i go the the 4.56 set according to my figures i will run about 2200 in overdrive and 3150 in 3rd gear. This does not seem outrageous to me so i am considering doing this gear change. Has anyone else done this with a v-8 overdrive truck?

 

[Ken2]

Another calculator:
http://www.ringpinion.com/Calculators/Calc_RPM.aspx

Randy's Ring & Pinion is a great shop to do business with. I've found their staff to be very knowledgeable. They had some OEM shims in stock where the dealer would have had to order them from a distant warehouse.

If you're already running oversized tires, your speedometer and odometer are running slow. You're going faster and making more miles than your speedometer and odometer show (and showing worse miles/gal than you're really making). Correctly regearing will correct this.

 

[supton]

Originally Posted By: spasm3
My 4wd avalanche has the 3.73 rear. I really only tow with this truck. I am considering going to a 4.56 gearset. Currently with the 3.73 i run about 1800 rpm in od(.69) and 2600 rpm in 3rd gear(1.0). Based on a 31.6 diameter tire. If i go the the 4.56 set according to my figures i will run about 2200 in overdrive and 3150 in 3rd gear. This does not seem outrageous to me so i am considering doing this gear change. Has anyone else done this with a v-8 overdrive truck?


I'm not sure what kinds of weight you are pulling here, at what speeds, etc. A while ago when I gave some thought to a V6 Silverado I ran into this issue; deep gearing was a great idea but ran into the large jumps potentially working against me--I might wind up going slow again, despite better gearing, as the transmission couldn't use the next lowest gear.

In your example, at whatever speed it is, currently 1800rpm is 4th, 2600 is 3rd and 2nd is 4300. Dropping from 3.73 to 4.56 changes to 2200rpm in 4th, 3150 in 3rd and 5100rpm in 2nd. Might you run into where 3k is not enough but it won't downshift to 2nd due to the high rpm? Your engine is likely making great torque at 3k but in the hp is what gets stuff done, and I'd think it'd run all day at 4k. You might not like how it keeps kicking down on hills but it might actually climb hills faster in the taller gear, assuming you need more umph than what is available at 3150 rpm.

I'm just making a stab in the dark here, as I don't know the weight nor hills you face. But I'm not sure you will gain too much.

 

[Clevy]

If you are building a 4x4 that can do everything from pull a trailer to pulling a trailer down a muddy trail why even go halfway.
Get the 4.56 gears. At this point with those tires and that kind of lift are seriously saying fuel economy matters.
Gimme a break.

 

[Rhymingmechanic]

To the OP: I would look closely at Jim Allen's responses, and play around with the calculators posted just above here. My hunch is that the truck will feel doggy with 3.42s and 35" tires. I say that as somebody who has never driven a truck with the 6-speed automatic, so that variable could change things. Whatever you do, make sure the speedometer is corrected for the tire/gear combination. I found out that even correcting for an inch difference in tire size changes shift points.

Originally Posted By: supton
Originally Posted By: spasm3
My 4wd avalanche has the 3.73 rear. I really only tow with this truck. I am considering going to a 4.56 gearset. Currently with the 3.73 i run about 1800 rpm in od(.69) and 2600 rpm in 3rd gear(1.0). Based on a 31.6 diameter tire. If i go the the 4.56 set according to my figures i will run about 2200 in overdrive and 3150 in 3rd gear. This does not seem outrageous to me so i am considering doing this gear change. Has anyone else done this with a v-8 overdrive truck?


I'm not sure what kinds of weight you are pulling here, at what speeds, etc. A while ago when I gave some thought to a V6 Silverado I ran into this issue; deep gearing was a great idea but ran into the large jumps potentially working against me--I might wind up going slow again, despite better gearing, as the transmission couldn't use the next lowest gear.

In your example, at whatever speed it is, currently 1800rpm is 4th, 2600 is 3rd and 2nd is 4300. Dropping from 3.73 to 4.56 changes to 2200rpm in 4th, 3150 in 3rd and 5100rpm in 2nd. Might you run into where 3k is not enough but it won't downshift to 2nd due to the high rpm? Your engine is likely making great torque at 3k but in the hp is what gets stuff done, and I'd think it'd run all day at 4k. You might not like how it keeps kicking down on hills but it might actually climb hills faster in the taller gear, assuming you need more umph than what is available at 3150 rpm.

I'm just making a stab in the dark here, as I don't know the weight nor hills you face. But I'm not sure you will gain too much.


spasm3: Good points about considering the RPM in lower gears. Where and how much you tow is important--especially if you tow over mountains, you're probably using lower gears.

You didn't mention speeds in your post above, but your RPMs match mine exactly at 65. I have a 2001 Silverado with 3.42 gears, and they seem a little too tall for regular towing with 29-30" tires. The standard gears were 3.73, but I ordered it with the others hoping for a little better mileage.

Going down to 4.10s might be a good compromise. That would put your RPMs where my truck would be with the 3.73s

 

[SteveSRT8]

Many a lifted truck through this driveway. With any modern OD trans go 'short' (numerically high).

You need 'em, and the bigger the tire the more you need 'em...

 

[supton]

Looks like 1800rpm is 65mph with 235/75R16, which are 31" tall and 638rev/mile, and 3.73's.

3.42 at 65mph
4th->1700rpm
3rd->2400rpm
2nd->3900rpm

3.73 at 65mph:
4th->1800rpm
3rd->2600rpm
2nd->4200rpm

4.10 at 65mph:
4th->2000rpm
3rd->2800rpm
2nd->4600rpm

4.56 at 65mph:
4th->2200rpm
3rd->3200rpm
2nd->5100rpm

Wild guess tells me that most aren't going to like 3.42's, because cruising has rpm's too low in 4th yet seemingly too high in 3rd at 65 on flat ground. If one were to be conservative and pull at say 55mph instead, 3rd would result in a 2000rpm engine speed, which seems to be a nice compromise. However, hit a hill and 2nd is 3,300rpm (at 55mph); and 1st is out. With 3.42's it sounds like one wants to get a running start at the hill, and keep speed up, unless if they have a nice torquey motor.

With the 4spd auto I'm not sure how I'd optimize the rear end. I think I'd want to keep it tall enough that I could use 2nd on tall hills, yet short enough that I could push 4th on the flats. Failing that then at least to keep 3rd from rattling my teeth (or wallet) to pieces. I think I'd only do 4.56's if I wasn't planning on going past 60mph and I was towing near max; that would be a nice 2000/2900/4700rpm going 4th/3rd/2nd. 4.10's would let you pull similar rpm's at 65mph.

If it was me I'd probably leave the 3.73's alone. You have two diff's to open up and change, yet one generally only tows in RWD, meaning, you're spending money on a diff that isn't used (by your admission this is mostly a tow vehicle). How does it feel now? Do you feel that you are sometimes stuck between gears, and changing the rpm a couple hundred rpm would fix it?

Edit: aimed at spasm3 not RichardSenn, the OP.

 

[spasm3]

Originally Posted By: supton

If it was me I'd probably leave the 3.73's alone. You have two diff's to open up and change, yet one generally only tows in RWD, meaning, you're spending money on a diff that isn't used (by your admission this is mostly a tow vehicle). How does it feel now? Do you feel that you are sometimes stuck between gears, and changing the rpm a couple hundred rpm would fix it?

Edit: aimed at spasm3 not RichardSenn, the OP.


It tows well on flat ground, but hills kill it. I'm towing a 5000lb double axle travel trailer. The truck weights about 5900lbs alone when fueled up including me. I pulled my tt from greensboro nc to hillsville virginia on nc 52 and it was a workout for the truck. Yes i probably would be happier with a diesel. But as much as the truck sits, i'm not sure a diesel would be a good idea. I only have about 65k on it and its a 2003. 4:10 rear gears would be ok but i hate to spend $1800 to change the gearing just a little bit. I actually think the 4.56 rear in this heavy of a truck would improve the in town fuel mileage.

To the op , if you are running a 35 tall tire, i would go with the 4.56.

 

[Clevy]

Originally Posted By: supton
Looks like 1800rpm is 65mph with 235/75R16, which are 31" tall and 638rev/mile, and 3.73's.

3.42 at 65mph
4th->1700rpm
3rd->2400rpm
2nd->3900rpm

3.73 at 65mph:
4th->1800rpm
3rd->2600rpm
2nd->4200rpm

4.10 at 65mph:
4th->2000rpm
3rd->2800rpm
2nd->4600rpm

4.56 at 65mph:
4th->2200rpm
3rd->3200rpm
2nd->5100rpm

Wild guess tells me that most aren't going to like 3.42's, because cruising has rpm's too low in 4th yet seemingly too high in 3rd at 65 on flat ground. If one were to be conservative and pull at say 55mph instead, 3rd would result in a 2000rpm engine speed, which seems to be a nice compromise. However, hit a hill and 2nd is 3,300rpm (at 55mph); and 1st is out. With 3.42's it sounds like one wants to get a running start at the hill, and keep speed up, unless if they have a nice torquey motor.

With the 4spd auto I'm not sure how I'd optimize the rear end. I think I'd want to keep it tall enough that I could use 2nd on tall hills, yet short enough that I could push 4th on the flats. Failing that then at least to keep 3rd from rattling my teeth (or wallet) to pieces. I think I'd only do 4.56's if I wasn't planning on going past 60mph and I was towing near max; that would be a nice 2000/2900/4700rpm going 4th/3rd/2nd. 4.10's would let you pull similar rpm's at 65mph.

If it was me I'd probably leave the 3.73's alone. You have two diff's to open up and change, yet one generally only tows in RWD, meaning, you're spending money on a diff that isn't used (by your admission this is mostly a tow vehicle). How does it feel now? Do you feel that you are sometimes stuck between gears, and changing the rpm a couple hundred rpm would fix it?

Edit: aimed at spasm3 not RichardSenn, the OP.



2200rpm at highway speed is perfect. Ideal for towing because its not revving too low and can very quickly accelerate if needed and never ever worry about having enough power or torque.

I've got a 99 with a 5.3. We put a cam in it at 150k and dyno tuned it with 370 pounds torque and 320hp. It's lifted 4" but I'm running the stock sized tires and rims(265s).
With 6000 pounds on the trailer I'd appreciate the few extra rpm on the highway. More torque which would help because there was a bit of shifting from 3rd to 4rt once there was a bit of a grade.
The way I see it is why go halfway. Might as well go all in considering the rest of the truck.