Ideal Highway RPM's and Gearing For 37s

[boy&hisdogs]

My K1500 (5.7/4l60e) is getting lifted and geared in the near future. I plan to run 37x12.50r17 tires on NNBS stock alloys. I'm stuck trying to decide between 4.56 and 4.88.

I do probably 70/30 highway/city, only tow about once a month, and usually the towing is local. (building materials, trash to the dump, etc)

According to the Grim Jeeper calculator, and based on the claimed size of the tires I want (Toyo RT, 36.8"), my RPMs will be:
4.56 @ 70 = 2040 RPM
4.88 @ 70 = 2183 RPM

Currently I have 3.42s and 33s on steelies which sucks, since it keeps kicking in and out of 4th when the road has any incline. 4.56 would be very close to what it would have been stock with 245/75/16 and 3.73s, (2015 @ 70) which is how most of these trucks came stock.

Is ~2200 RPMs to high for sustained highway driving? Would the 4.56 get bogged down by the weight even though they "match" the stock setup numerically?

Is there a certain point in the rev range where the mileage starts to really tank?

 

[local18wheeler]

I just got my 94 k1500 suburban back from shop. They installed 4.88 bc 4.56 front diff 4.56 was out of stock nationwide and I got impatient. They also swapped the rear to a 14 bolt. I have 35" tires and at 60 she sits at 2300 and purs... but at 65 70 she gets up to 2500. Lots of power but I'm not sure what the optimal range is. I'm gonna change the tires to 37 and I think it will get to where it's all balanced out. 0-60 is perfect and I have no lack of power on inclines. I think from past experience 4.56 and 35 is a great balance on these rigs but 4.88 gives me that little extra when loaded with weight for the off highway and no hesitation on the hills.

 

[b454rat]

4.88s. Don't forget the extra rolling mass of the bigger tires. Plus these trucks are bout as aerodynamic is a brick, so the extra gearing will help. I used to drive 5 hours one way running 2500RPMs in an old malibu I had with a million mile 400. Drove from NYS to Colorado with a 97 Vortec 454 non stop doing 2300-ish RPMs. Rather have too much than not enough, then have to do it all over again.

 

[boy&hisdogs]

4.88s. Don't forget the extra rolling mass of the bigger tires. Plus these trucks are bout as aerodynamic is a brick, so the extra gearing will help. I used to drive 5 hours one way running 2500RPMs in an old malibu I had with a million mile 400. Drove from NYS to Colorado with a 97 Vortec 454 non stop doing 2300-ish RPMs. Rather have too much than not enough, then have to do it all over again.

Funny this came up, I just got my gears done a few days ago... 4.88s and no looking back! That was my reasoning too, 4.56 might be a better match for stock on paper, but factoring in the weight and wind resistance I really think 4.88 was the right choice. I turn anywhere from 2000-2300 now going down the highway, depending on the speed of traffic.

You know what's great too? The truck seems happier at the speed limit than it did before! When I had 3.42s, even with the stock tires, the truck felt like it wanted to just go faster. The engine seems really happy to sit right around 2000ish rpms. With tall gears that's a ticket waiting to happen, but now with the 4.88s thats just under the limit.

 

[boy&hisdogs]

I would like to add that before I got my gears done, (Lifted, 37s, and 3.73 rear end) I was getting 10 mpg down the interstate, and single digits in combined driving. I'm only about 1/4 of the way through my first tank with 4.88s but I'll let you guys know.

 

[b454rat]

My 2wd Tahoe has 3.42s in it, with roughly 28.5" tall tires it's great. I can cruise at 100mph without the tach bottoming out. If you have the torque, then 4.56s with the 37s might be ok..

 

[alpinecrick]

My K1500s (5.7/4l60e) is getting lifted and geared in the near future. I plan to run 37x12.50r17 tires on NNBS stock alloys. I'm stuck trying to decide between 4.56 and 4.88.

I do probably 70/30 highway/city, only tow about once a month, and usually the towing is local. (building materials, trash to the dump, etc)

According to the Grim Jeeper calculator, and based on the claimed size of the tires I want (Toyo RT, 36.8"), my RPMs will be:
4.56 @ 70 = 2040 RPM
4.88 @ 70 = 2183 RPM

Currently I have 3.42s and 33s on steelies which sucks, since it keeps kicking in and out of 4th when the road has any incline. 4.56 would be very close to what it would have been stock with 245/75/16 and 3.73s, (2015 @ 70) which is how most of these trucks came stock.

Is ~2200 RPMs to high for sustained highway driving? Would the 4.56 get bogged down by the weight even though they "match" the stock setup numerically?

Is there a certain point in the rev range where the mileage starts to really tank?

My 96 and 97 K1500s with 5.7/4L60E's and 3.73's, LT265/75 Firestone Transforce AT's (which are one of the taller 265 tires) run at ~2200/75mph as indicated on the tach. Both trucks are Z71's and came with P265 tires. If I had my druthers I think gears in the 3.90-4.10 range would be better for driving in the Rockies. I'm not sure how much that would bump up my RPMs though.

My 96 has almost 270k on the odometer with the original engine that still runs strong, so I'm assuming that 2200 RPMs hasn't prematurely wore the motor out.

I'd recommend the 4.88's........

 

[Frank Enstein]

 

[geeeee89]

I have 4.88s with 35s and an nv4500. 5th gear at 70 mph is right at 2500rpm. Won't hurt a thing. Consistently get 12-13 mpg with this setup. Plenty of power for towing, great for running around town, and not bad at all on the highway. I can actually use 5th gear going uphill or against a headwind now. I definitely wouldn't want any higher gear than 4.88s for 37s

 

[stutaeng]

I was going to suggest the 4.88s. I've got a CCLB SRW 5.7 (K3500) and a 6.0 (Suburban 2500 2wd) both 4L80e/4.10s and run about 2300-2400 @ 70 MPH. Initially I thought RPMs were high because I came from a C1500 4.3/3.42, but I really like the 4.10s now. In fact the C1500 is now a project truck, and I'm going to swap to at least 3.73s, along with an engine/tranny swap. Would of preferred 4.10s, but the AXN axle I got has 3.73s, so that's what I'll run. All my trucks run LT265/75/16 tires.

Last week I drove out of town on the suburban through a toll that goes around Austin area. Speed limit on that toll is 80MPH and a short segment up to 85 MPH! RPMs were in the 2600-2800 RPM in the 80-85 MPH, and still felt like there was power left and had acceleration. My K3500 is in rough shape, at 75 MPH it starts to feel like the dash is going to fall off! So that one stays for local duties. LOL. But both feel like they are in a sweet spot for engine power on the highway, to me at least.