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6.5 Ideal Cruise Rpm?

S

scottm

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the oven, Scottsdale AZ
Assuming a basic chevy 2500 4x4 truck, stock or minor upgrades, 32 or 33 inch tires, not unusually heavy, not towing. What rpm is the engine happiest at a steady 75 mph? Now assume you come to a moderate hillclimb - speed drops to 60, trans drops out of OD, what's the happy rpm now?
 
2400+ for me. That was with GM-4, now an HX35. Will be doing some highway driving tomorrow and can report back. Gotta keep the engine wound up for it to be happy.
 
1800-1900 RPM for max fuel economy, 2250 roughly for good towing power.

stock GM turbo, manual transmission. I imagine a bigger turbo you can cruise freely at a higher RPM, but in my case, not so much.

Those RPMs are why I hate 4.10 gears. go 58 mph empty to get good mpg, and go 65-68 mph loaded to stay in the powerband. makes sense...not. :)

I will be trying out 3.42 gears on one of the trucks to see how I like it. my thoughts are run 2250 in 4th gear direct (3rd on 4L80E) when towing heavy, and run 1800 RPM in 5th gear OD (4th gear on 4L80E) for good fuel economy and still decent speed.
 
I like my 3.73's in T. Stock tires, mods below including ATT. 50 is about 1600. 60 is about 1900. 70 is about 2200. Have gotten 22+ mpg with T in an even mix of 60mph, 70-75 mph, and city driving. With current driving style not as good of mileage! Previous owner stated he got around 20mpg regularly. So mods, light foot and ATT probably worth 1-2mpgs. X2 on the 4.10s, subtract about 2 mpgs. Gertie turn over 2000 at 60 with stock tires and 4.10s! Last tank it got just under 18 with a stock turbo and minimal mods.
 
For 35's the 4:10's should work out pretty good for a daily driver/light tow use vehicle. If your going to tow heavy alot probably not such a good choice, but I doubt you tow that heavy very often with a blazer.
 
Peak torque and thus economy is 2200 RPM give or take with the factory turbo. Any RPM over 2200 and you are simply wasting fuel fighting the GMx turbo. Under 65 MPH the engine doesn't work as hard as MPG goes off a cliff over 65 MPH. If your combo puts you over that RPM you have no power reserve and need a bigger turbo and that completely changes the game of where the engine is happiest at.

Speed drops and you want to shoot the transmission for downshifting and keeping RPM above 2200 as it slows you down faster than lugging it with the TCC locked below 2200 RPM. Esp on the grades out here loaded or not... Moderate hillclimb is deceiving to those not in our area seeing 7-18% grades... :D
 
It might depend on how much fuel you are burning. ~1800 rpm is economy as long as its not lugging. And 2200 feels strong. Probably where GM turbo is efficient or where both HP and torque are high.
 
I read an article once and it said find a long level road for testing. Hold your foot perfectly still in a comfortable spot about where you cruise. Try different gears and to see where you get the max speed at the lowest rpm and that is the sweet spot. I haven't done it on a real long road but can sometimes feel it when it settles on shorter stretches.
 
My 95 seems to love 2200 w/ 3.73s. All of my old mpg records were on a different app (no longer exists) and saw 20 mpg w/ 1500# in the bed at 71mph. 22 unloaded at the same speed. That was with fairly aggressive 245/75's.

My 93 got 16 at 62mph. Never did any better. Hoping for more w/ att and future IP.

Dad said my 94 got 18-20 long ago. Hasn't legally seen highway in 8yrs.

I like Roberts theory of 3.42's but I already barely get off the line w/ 4.10's pulling some of my loads. Seems an overdrive unit would work good in theory.
 
schiker said:
I read an article once and it said find a long level road for testing. Hold your foot perfectly still in a comfortable spot about where you cruise. Try different gears and to see where you get the max speed at the lowest rpm and that is the sweet spot. I haven't done it on a real long road but can sometimes feel it when it settles on shorter stretches.
Click to expand...

Or slight variant: hold your foot to the floor and try and break over 62 MPH towing on a steady slight climb with a headwind. This is where you have 3rd and OD available with 4.10's and TCC lockup override if you dare. This is where I found 3rd slowed me down at higher RPM and 4th peaked at 2200 RPM and that was all she had with the TCC locked up. This mapping the peak torque of the 6.5 with a GM3 turbo, fuel turned up and boost at turbo redline 14PSI. That trip is why I scrapped the GM3 and went with the ATT...

Lower RPM is of course better fuel economy to a point if you can hold the gear without excessive throttle. Your auto will downshift at excessive throttle so that's easy to know. Except with a bigger turbo you need to downshift sooner, but, MPG's are better.

With a larger turbo My 1993 is a completely different animal liking higher RPM and pulling hard to redline, but, giving up starting from a stoplight. Thr trade off for MPG over 75 hides the better economy at higher RPM, but, I note the boost gauge for economy/load reference.
 
My van loves 75-80 mph, 60-65 is ok and after looking at an rpm calculator, makes perfect sense. 65 is 1624 rpm and 80 mph is 2000 rpm. Maybe time to swap in some 3.42's.
 
My 1999 K2500 burb is 2.1k rpm at 65 mph w/4.10 gear sets, the tires are 295/75/16e TOYO's (33+" unloaded) however @ 55 psi tire pressure the loaded diameter of the tire is only 31.5" this loaded diameter is the correct diameter for calibration of vssb speedo and ABS controller.

Best mpg for me is @ 55 mph.
 
Tanner,

One thing I have felt is that the DS4s seem to have a little more grunt off the line compared to a DB2, but I might be imagining things.

I wonder if a 94-95 truck with a chip and a 1992-1994 spec NV4500 with the deep 1st gear might be the ticket for a hard launching truck.
 
The 94 pulled pretty good back in the day, my 93 takes a 2-2.5k clutch dump.

For now I'm blaming the worn IP on the 93 and poor off the line boost.
 
My 97 with 3.73 on 265's likes 2200 revs and 72 mph. It gets 20.5mpg around there. More revs than that and mileage goes away. That is empty. I haven't had a load in it lately.
 
greenmeanh1 said:
I got way different gearing i guess. I run steady 80-85mph at 3000rpm average. No issues yet at 130k miles. 26900 gets me 70-75ish.
Click to expand...

Holy Moley! That is some high steady state rpm's I hope you are running a Fluid Damper balancer if not it's perhaps the best upgrade you can do on that at risk 6.5.

On another note geared hubs do come in taller gearing or gear-sets can be changed out to taller ratio to lower those rpm's. Best upgrade is a Gear Vendor UD/OD unit can be installed for a 20% rpm reduction and turned off when not needed.
 
No gear vendor for hummer/hmmwv without changing the transfer case. It is full time 4wd. The transfer case only selects hi/ lo range and goes from applying 60/40 torque variance to locking front and rear axle allowing no slippage.

I'm not sure on the ring/pinion change out details, but the differential gearing is a torque bias assembly. Last I checked only one option.

At 2700 rpm I'm at 65mph; 3000 rpm is 80, maybe 81 mph. Idk if my tc is going into lock up...keep in mind 37"tires are stock.
Only geared hub ratio I've seen is 1.92:1
 
Will L. said:
No gear vendor for hummer/hmmwv without changing the transfer case. It is full time 4wd. The transfer case only selects hi/ lo range and goes from applying 60/40 torque variance to locking front and rear axle allowing no slippage.

I'm not sure on the ring/pinion change out details, but the differential gearing is a torque bias assembly. Last I checked only one option.

At 2700 rpm I'm at 65mph; 3000 rpm is 80, maybe 81 mph. Idk if my tc is going into lock up...keep in mind 37"tires are stock.
Only geared hub ratio I've seen is 1.92:1
Click to expand...

Humm, what t-case sounds something like an autotrac but with less tq variance. I suspect there is a cooler in the t-case so a changeout to say a NP 241/243 would require drilling, tapping for oil cooler lines in/out a hi temp oil pump and cooler, or maybe the standalone GV behind what you have.

The carriers are lockers, or?

How about an aux OD only after the t-case?
 
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