RPM Question

[gmenor]

Maybe it's just me but, I don't like to push my RPMs too high. I've read a lot of other peoples replies pushing 2400+ RPM range which to me is a lot. Maybe I'm off track here but, high RPM's on a diesel engine is a bad thing. On the highway I will do 70 mph @ 2200 RPM setting the cruise control. What is considered too high for the 6.5 TD under constant load? I know mileage and modifications are the considerations and factors that needs to be taken into account. Has anyone blown a 6.5 engine because of too high of RPMs? What is the normal base line RPMs for a 6.5 with phaser gears under load? Has there been a poll to find this out? I don't pull a trailer so I am interested what all you all's RPMs are hitting at 65 - 75 mph.

[patrick m.]

i run 75 on the interstate all of the time, with bounces up to 80 according to traffic. 75 for me is 2500rpm.
i have 184k miles on the same eng.
if i have a really heavy load i will run in third gear 55-60 at the same rpm.

i bet everyone is going to be very close to this number.

[gwaidman]

I try to keep it at around 60mph. Thats a little over 90kmh here in the great white north. RPM's are 2250 or so (4:10 axle). I rarely rev it past 2800, even getting on the freeway, and this is far below the factory redline. 3000+ RPM seems to much for a diesel. Of coarse, once I get my exhaust, chip, and boost controller, maybe I will have to change my train of thought...

[Dvldog 8793]

Howdy
I have a Gearvendor 2cnd overdrive so take that into consideration. My truck is used for 2 things: Cross country family trips and hauling steel and heavy equipment for my welding business.
So I wanted/needed the best of both worlds with one truck.
Rpm towing with 4.10s in 2cnd with GV on = 2200-2300 at 55mph
Rpm highway with no load(except 3 kids, 1 wolf and 1 wife :D ) double overdrive on = 1850-1950 at 75-80mph
I never really pay attention to MPG with a load as my loads vary so much. I've gotten as bad as 7-8mpg pulling a goose neck with a Dodge RV on it :D ! On the highway unloaded my rig gets around 20mpg at the stated speeds. I've done some work to the truck but not as much as others. I do have Phasers. I have a REAL heavy foot around town and I REALLY like the way my truck sounds tongue.gif :D tongue.gif
So trips into the 3000rpm range in town are probably too frequent! "Real Mature!" as my wife would say! A 6.5 tuned up with big clean pipes sounds really mean.
I'm done rambling SORRY ;)
L8r
Conley

[JoeyD]

gwaidman, i have 4.10 gears in my truck and 60mph is 2000rpm. Yours is 2250?

[dieseldummy]

I had a friend with a 94 and he ran his pickup around in 1st gear wide open for a month. His mom said that he could get a new pickup when his old one died, so he thought he would blow it up. It never happened though and if I remember correctly it had over 200k mile, most of which were hard trailer pulling miles. My pickup has 4:10's and I never run under 65-70 on the road, and have a tendency to like to hear the turbo whistle around town. I have 130k and no problems yet.
Justin

[gwaidman]

Joey D, well,... I think its 2250, but I will check on the way to work tomorrow. Was a 4.56 axle available on these trucks?

[gmenor]

Dvldog,
You're running 1850-1950 at 75-80mph? I thought I read somewhere that the optimum highway mpg was @ 1800 rpms. Maybe I should check into the Gear vendor 2cnd overdrive. Not knowing what it is, can it be installed by a backyard mechanic? Can you send more information on it.

[Dvldog 8793]

Howdy
They call it a "Gear Vendor under/overdrive" Don't ask me how they get the name as it never really undrdrives anything. It can be used in all gears so technically my truck has a 8 speed auto. tongue.gif I think it's more realistic to call it a double overdrive as that's how it gets used most.
They supply GM and some othe makers with the unit for commercial applications and claim it can handle more torque than the transmission. I've never had any problems with mine in 100,000 miles. the transmission runs cooler when pulling if I use the gearvendor. I put mine in, took about three days. I made my own driveshaft and did all the work. Wasn't terrible but it wasn't what I'd call "fun" either. :D MANY sacrifices were made to the mechanical knuckle-blood god :D :mad:
the best place to find out info is on the web, do a search on Gearvendor. There is another company called US Overdrive, I'd check them out also. COST is something to consider. My unit was about $3200 total w/ install. The only way it pays for itself is if I figure that without it I would drive 70-75 on the highway anyways. ALL things being equal, the gearvendor probably cost MPG to run as it is one more thing that the engine has to operate. As it is I'm very happy with it, it allows me to cruise down the highway and pull whatever load I want.
It's really fun to sneak up on people at 70 and then put-it-to-the-rugs! :D she kicks out of OD and the rpms go from nice/quiet 1850 to look-out-here-I-come 2600!! If I time it just right I can do it when the 6inch exhaust tip is right by the drivers door. tongue.gif
But really now thats mean and I would never do such a thing to the lesser vehicles on the road. ;)
L8r
Conley

[cruzer]

I run around 70mph pulling 8500lbs at about
2500rpms w/ 4:10's. Empty run 70+. Pulling at 75+mph really puts mileage close to single digits. On the very rare occasions when it downshifts at about 60mph I'm looking close to 3000rpms.

[gmenor]

Drove down to Fayetteville yesterday @ 70 mph my RPMs was at 2150 and at 75 mph RPM's was @ 2350. After seeing you all's RPM at 2400 + I guess I not pushing my Suburban to its fullest extent.

[C.K. Piquup]

With an auto trans(the majority)you`ll probably never need to see 3,000rpm unless you manual-shift or peg the speedo."Normal driving"probably never over 2,500.Breaking my truck in(5spd/4:11)I never went past nor needed past 2,000.I think speed limit was still 55 in MD.I was probably going 60-65.Don`t hold me to these numbers,I just know I never went past 2,000rpm(it was`92).Now,when I`m between 2,500 and 3,500+ it`s going thruogh the gears.Either hauling from stop at bottom of a hill or just being"High-Spirited".Can`t get the kid outta that right foot.

[moondoggie]

Good Day!

For my experience with Gear Vendor mpg, you might want to look at my post in What's your fuel mileage with a load? (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=004439#000014) (Click in colored text.) I figure I gained about 2 mpg. The biggest gain was you can't hear the engine at all at highway speeds.

Blessings!

Brian Johnson, #5044

[rjschoolcraft]

Redline on these is 3500 rpm. The maximum power is quoted at 3400 rpm. I run mine over 3000 rpm on a regular basis when accelerating with a load. With the Kennedy chip, my shift points in first and second are actually a little above 3500 rpm. When engine braking on a grade, I let it rev to redline without fear. The lower speeds you're used to are on big bore inline sixes that are throwing a lot more mass up and down in the cylinders. My dad's 4020 has a 404 ci inline six. The 6.5 is a 395 ci v8. That's about the same displacement spread over two more cylinders. That's why the 6.5 can rev higher than the John Deere... plus the Deere uses heavier components throughout the reciprocating assembly. The Deere max is about 2500 rpm for sustained road travel.

[KallyI]

At 70 mph, my truck is running at 2150 rpm, with the 3:73 diff.

[C.K. Piquup]

I agree with RonnieJoe,running to 3,500rpm doesn`t hurt.GM would have set redline lower if that weren`t the case.Redline is 4,000rpm(check tach),Penninsular rates theirs @3,600/GM @3,400.Of course,as with ALL engines,running at lowest possible RPMs is certainly the best for long service(unless you lug it).

[gwaidman]

JOEY D....Okay, checked RPM and it was 2150. I have a 5spd manual trans as well. FYI.

Cheers!

[gmenor]

Thanks for the information everyone. I will probably wait on the transmission kit after the exhaust upgrade. Wonder if that will affect the RPMs and gas mileage. Anyhow I am out in LA (not Lower Alabama) for the week and will be back in the beast Friday.

[charliepeterson]

There is nothing like the power, speed or sound running flat out off the line @4000 rpm's thanks to JK's chip! I've got no carbon build up on my valves.

[Marty Lau]

65 mph= 1800rpm
75 mph= 2050-2075rpm

3.42 ratio

[gmenor]

16ga, why is your RPM's so low and what type of MPG are you getting? I going to have to check out what type of rear end I have.

[CaseyR]

I'm a little baffled here. When I had the mechanic check the (well used) truck before purchase he said the option code showed a 3.73 rear end. I never got a chance to talk to him, but the previous owner was reportedly a distributor who logged lots of highway miles with a trailer full of canoes/kayaks so wasn't that much of a load so the 3.73 ratio makes sense.

My speedo seems to be reasonably accurate (when it settles down and isn't wildly swinging all over) as the police "check your speed" radars show it to be only about 1mph off at 70. However, at 70mph, my tach shows about 2700 rpm (with only occasional flickers) and at 50 shows 2000.

I am running 245 75 R16 which the gear ratio calculator estimates to be 30.5in. Plugging the 70mph at 2700 and the tire size into a couple of on-line gear ratio calculators shows that the calculated gear ratio would be somewhere around 4.7 to one! As I have gotten 17 to 18 mpg at 70mph cruise on my two trips from Portland to Berkeley, CA, I would guess the problem might be an optimistic tach rather than a dump truck type rear end ratio.

Can anyone tell me an easy way to check the accuracy of the tach?

Thanks

[gmctd]

Seems as though the alternator may not have the Diesel pulley installed.
Happens when alternator is replaced.
Check your dealer for the correct diameter pulley - or do a search here. It may still be posted.

[moondoggie]

Good Day!

Speed vs. rpm? You might want to look at my 9 Mar 04 post in Speed vs RPM with 4.10 rearend (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=005936)(Click on the colored text.). This is simple math.

Blessings!

Brian Johnson, #5044

[cruzer]

I run 70mph and the tach reads approx 2500rpm
w/4:11's w/245r16.5's. IIRC the alt pulley should be 62mm where the belt rides. should be 2.44".

[moondoggie]

Good Day!

The link in my previous post calculates an rpm of 2035 @ 60 mph w/ 245/75R16 tires & 4.10 gears. It assumes a 4L80-E AT in overdrive with the torque converter locked. This correlates pretty close to what I see on my tach. I did those calculations using tire diameter info from a spreadsheet someone made available on the Page a couple years ago. The numbers have worked for the calculations I've used them in so far.

Expanding on the above (4.10 gears):

60 mph - 2035 rpm
65 mph - 2205 rpm
70 mph - 2375 rpm

For 3.73 gears:

60 mph - 1850 rpm
65 mph - 2005 rpm
70 mph - 2160 rpm

(all rounded to the nearest 5 rpm)

If these aren't pretty close to what you're getting, you should fix what's wrong: tach signal wrong, odometer/speedometer inaccurate, etc.

Blessings!

Brian Johnson, #5044

[RLAM]

Your gdear vendor must be alot different than mine.I dont know what "double overdrive" is.Mine works in the first three gears and will give me first and over that is the same as second and under, second over,third over and thats it. It does not work in fourth lockup (overdrive). I think if you go back and count the gears, you will have six.Mabey you know something I dont amnd if you do,I would like to know. I would like to get a little of that 75 MPH@1800

[moondoggie]

Good Day!

RLAM: I've used the term "double overdrive" occasionally. I'll guess he's (Dvldog) calling it double overdrive (OD), as the transmission OD ration is 0.75 to one, & the Gear Vendors (GV) is 0.78 to 1 - both are overdriving their output shafts.

Apparently you have a different unit than we do. Mine can be made to engage at any speed, in any gear, over 18 mph. I generally leave it in automatic mode, but use the foot switch to switch it in or out. In fact, the only reason I use the factory electronics is 1) to be sure I don't inadvertently try to engage the GV in 4WD; & 2) in case I forget to disengage it when I slow down (it disengages itself at ≈ 25 mph). I almost never use my GV in any transmission gear EXCEPT overdrive - I'm not using it to split gears, but for increased fuel economy & reduced cabin noise.

At 70 mph in "double overdrive," I'm turning ≈ 1750 rpm.

Dvldog: I think when the GV is used as an underdrive, they literally put it in backwards of how ours is installed.

You said,

[gmenor]

Ben,
I don't do any heavy hauling with my Suburban so I don't see this as being a viable application for me. I do haul lots of tools and equipment in the back and I do commute about 600 miles every weekend. I was worried that maybe with all the driving that I maybe pushing the RPMs. Yesterday, I had a black diesel Suburban pass me at about 80 mph, kind of makes my concerns a mute point.