Constant RPMs?

[VA_GMC]

I was out west this summer (Texas, Arizona, Colorado, Kansas) in a different truck and noticed that a lot of the speed limits are 70-75 mph (and most people run 80-85). Anyone live out that way and run these speeds all the time w/ their 6.5 TD. I would be taching between 2.5 and 3 to run these speeds...is it hard on it to run these RMPs continuously?... or do you all buy trucks with a different diff. ratio?

Just curious. Thanks.

[JCM5]

VA-GMC

I run my dually with a 4.56 rear end @ 65 to 70 mph around 2600 rpm all day long, meaning for 12 hour straight. I picked this ratio for pulling not speed. In my opinion and have done it for two - three hour periods, you could run up to 2800 rpm for extended periods of time with no issues other then mileage and wear and tear. I would suggest if you not pulling to lower you rearend ratio to a 3.73 from a typical stock of 4.11
If you pull more then 5000 lbs then leave the 4.11 or go to a 4.56 with a gear vendor unit if you like to run fast. This engine should run 3000 rpms all day long as long as you watch the temp gauges for overheating.

[BuffaloGuy]

Ditto. I ran my first motor at mostly 75-80 MPH, 2800 rpm, with a 4:10 rear end for 185,000 miles. It didn't seem to bother it any.

However, it would be better if it ran at a lower rpm. Wear and tear would be less and I suspect the motor would last longer but 185K says that it must not be that bad for the motor.

These trucks are thoroughbreds; not quarter horses! They love to run down the long roads.
Ken

[VA_GMC]

Thanks for the replies/info.

William

[john8662]

Recently upon traveling to Dallas TX I ran 75 for a lil while, My sub is geared 4.10 and it runs approx 2400 @ 70. It ran just under 2600 at 75. The fuel mileage wasn't that great either, 15 on the highway, thats not too good but consistent and average running A/C. I am in the process of getting a new set of gears to replace the 4.10, probably going with 3.42.

[Marty Lau]

Back in the early day of the DP MorePower used to talk about gearing your truck for the speed you want to travel. I swapped from a 3.73 to a 3.42 I run like 1900 rpm at 68mph and about 2050rpm at 75 and the computer cuts off fuel at 97mph about 2700 rpm before I get anywhere near redline. MorePower also use to say by going to a lower ratio you would gain about 50% on engine life. I tow 5,000lbs most everyday from may to september and couple days a week the rest of the year. What I tow doesn't have very much drag so it tows easier than my 3,500lb camper. There is used to be suggested tow wieghts for the gearing. if I remember right I think it was:

3.42 4,000lbs
3.73 8,000lbs
4.10 12,000lbs

You get better mieage and decent pull at 1800-1900 rpm. So if I had a low geared truck and needed it to pull heavier weights I would look into a Gear Vendors Unit (OUCH$$$) Or if you tow more modest weights changing gears but with you 4x4 it will cost double a 4x2 and might not be to far from a Gear Vendor unit. I figured it took about 55,000 miles to pay me back for gear change on my 4x2 so unless you plan on keeping your truck for another 100,000+ miles you maybe better to either slow down or pay the higher fuel bill. One advantage of a Gear Vendor is you can have it put on your next Rig if it fits, a friend did that with his on his Suburbans.
:cool:

[john8662]

Thanks for the info 16ga SxS,

Being a 4x4 its gonna be a bummer. I bought my burb because I wanted a 6.5. I didn't like the gearing and the 4x4. I occasionally put a small kubota (29 hp) tractor on a trailor and pull it maybee 50 miles or a truck or another suburban on a trailor and head down the highway. At 3.42 according to your numbers 4,000lbs is about what I pull, so I figure if I just do some modifications to the engine (for more power) that I should offset the pulling issue and when empty (as long I don't go crazy on power mods) get decent mileage on the highway (maybee 20 on a good day with a 4x4).

[Marty Lau]

John;
I went to your truck link and and looks like a nice Suburban. A couple thoughs and take it or leave it. According to MorePower and JK a 31/2" exhaust is all that is needed with even major mods for a 6.5TD any bigger is for looks and ego. Big deal is geat rid of that blam nabed down pipe! Also if you change gearing your Speedometer will not be right. You can get a new VSSB (which I did) costs about $150 or you can change the jumpers to make it accurate, ask MorePower the year to get that info. I don't think towing what you talking about you will have a problem with 3:42's if your pulling real heavy load you might want to do it early morning when it's cooler than late on Texas summer afternoon.

[moondoggie]

Good Day!

I used to pull a US Cargo SM625TA2 trailer (http://www.uscargo.com/uscargo/products/enclosed/snowmate.asp) with my 95 pickup. Except the Gear Vendors & 4% taller tires, it's bone stock. It pulls the trailer just fine with the Gear Vendors engaged. But remember, it's awful flat here.

Blessings!

Brian Johnson, #5044

'82 6.2 1500 4X4 1/2T pickup, 4spd man w/ OD, bone stock, 335K+, "In Rust We Trust"
'89 6.2 4X4 1/2T pickup, bone stock, 145K+
'95 6.5TD 2500 4X4 heavy 3/4T pickup, Gear Vendors Aux. OD, 183K +
'95 6.5TD 1500 4X4 3/4T Suburban, Kennedy exhaust, 203K +

[ 09-03-2003, 06:10 AM: Message edited by: moondoggie ]

[jeffreydmet]

I have a K3500 4x4 that was stock with 4.1 gearing. I changed the ring and pinions to 3.73 and changed to 285/65 from 235/75 which gives me the equivalent of 3.45 with stock wheels. I changed them myself with Randy's Ring Gears for about $500. The Gear Vendors runs close to $3000. I would have really liked to purchase the GV but just to expensive.

I haven't been impressed with my fuel mileage gains. Perhaps 1 mpg. Now 16-17 at best However it is much nicer to drive. At 70mph the engine runs at 2050rpm in OD. It is much quieter. Also you can actually pull in 3rd now and don't have to worry about breaking the minimum speed limit.