6.2 Diesel Cruising RPM's

[Eraser475]

I am trying to figure out a ballpark figure about what my RPM's are at when I'm going 60. I have tried this: http://www.dieselhub.com/tech/rpm-calculator.html , and it gives me around 1500 RPM. This seems really really low. I have a 6.2 Diesel, 700r4 transmission, 3.08 rear end, and 29" Diameter tires. Can anyone give me a figure at about what I'm currently at RPM-wise?

[Oregoon]

Diesels are low rpm motors. I'd guess that figure is pretty close to accurate.

Does it sound like it's lugging?

[Steve-W]

Yep seems perfectly normal to me.
My daily is an Audi A4 with a 2 liter diesel engine and a 6 speed manual box. At 60 mph it runs 1300RPM's in 6th gear ��

[68Timber]

This is the one I usually use: http://www.wallaceracing.com/gear-speed.php

It agrees, 1500 rpm at 60. It also lets you plug in other rear gears to compare.

[Corts60]

You want your engine to be at peak torque when cruising to achieve the best MPG.

Peak torque on a 6.2 is around 2,000 RPM.

[rs74]

Originally Posted by Corts60

You want your engine to be at peak torque when cruising to achieve the best MPG.

Peak torque on a 6.2 is around 2,000 RPM.

So with a lower gear he should see better MPG's then? You would normally think that with a higher gear it would get better mileage. Maybe more so if it was a gas engine.

[Eraser475]

Originally Posted by rs74

So with a lower gear he should see better MPG's then? You would normally think that with a higher gear it would get better mileage. Maybe more so if it was a gas engine.

Usually from what I hear, 3.08 gears weren't really beneficial in stock gas engines because you had to floor it just to get rolling, so mpg suffers badly. My dad said that because it is a 6.2 and they have the low end grunt that it can get away with being so high geared

[Corts60]

Originally Posted by Eraser475

Usually from what I hear, 3.08 gears weren't really beneficial in stock gas engines because you had to floor it just to get rolling, so mpg suffers badly. My dad said that because it is a 6.2 and they have the low end grunt that it can get away with being so high geared

Believe it or not, the first diesel trucks came out with a standard 2.76 gear ratio. Talk about gutless. They were looking for MPG but the truck couldn't move out of it's own way. An optional rear end was 3.40 and was still not a great match for the non-turbo but helped a bunch.

[Eraser475]

Originally Posted by Corts60

Believe it or not, the first diesel trucks came out with a standard 2.76 gear ratio. Talk about gutless. They were looking for MPG but the truck couldn't move out of it's own way. An optional rear end was 3.40 and was still not a great match for the non-turbo but helped a bunch.

I haven't heard of a 2.76 before, only a 2.73. Was that when the olds 350 was around?

[Corts60]

Originally Posted by Eraser475

I haven't heard of a 2.76 before, only a 2.73. Was that when the olds 350 was around?

Yep, 2.76 ratio, code HC2 was the standard diesel axle. Your only other options were a 3.07 (HE3) or 3.40 (HE4). This was for the 5.7 diesel.

[Dieselburb86]

My dad has an 86 GMC with the 6.2 and 3.42 gear ratio! I got 21 mpg cruisin down the highway. BTW it was a 2wd suburban.

[Eraser475]

Originally Posted by Corts60

Yep, 2.76 ratio, code HC2 was the standard diesel axle. Your only other options were a 3.07 (HE3) or 3.40 (HE4). This was for the 5.7 diesel.

Thanks!

Does anyone know what speed mpg really drops off at? I want to head to Rapid City, SD for college and getting there includes around 6.5 hours of straight interstate driving. At around 70 with the same setup, will. I be doing well mpq wise, or will the speed kill it?

[Edahall]

I've got the same setup as you and best fuel economy is 55 mph. 65 mph is the point when fuel economy starts to really plunge. The lowest I have ever got was 17 mph driving between 85 and 95 mph. About 10O mph is terminal velocity on one of these things.

[Edahall]

If you're on a really tight budget, keep it at 55 mph. At 70 mph vs 55 mph, you'll burn about 20% more fuel but of course get there a bit faster.

[Eraser475]

Why does it just drop so quickly? Is it because resistence increases as speed does?

[Edahall]

Originally Posted by Eraser475

Why does it just drop so quickly? Is it because resistence increases as speed does?

The answer to your question is poor aerodynamics. Remember, these vehicles were produced when the maximum speed limit was 55 mph. The worse the aerodynamics, the faster fuel economy will drop off with speed. It'll also limit top speed.

[rs74]

Let me ask this question. This diesel I just bought came out factory with 3.08's and 235/75's with is a 28.9" tire. If I put a smaller diameter tire on then wouldn't that effectively lower the effective gear ratio. Does that make since. I used a gear ratio calculator and it was saying that if I put on a 27" tire then it would change the effective ration from a 3.08 to a 3.3:1 ratio. I realize the gears are still 3.08 but the overall ratio would change with a small dia. tire. Right?

I was thinking about lowering my truck and putting a smaller tire on. So if I do that then I can get closer to the 3.42 ratio that is suggested would give me better mileage. Hope that all makes since.

[rs74]

http://www.jeep4x4center.com/calculators/?utm_source=cj

[Bigstevex4]

Some thing else people dont take into acount epesialy with high comprestion long stroke diesiels. I s the amount of Horse power it takes just turn the eng at say 3000 rpms. theres a point when your eating up power your making.

[nonstop]

Just throwing this out there - what about throwing a turbo on there? It will give an all around increase, and used units can be found for the price of a moderately priced set of tires. Never did one on a GM, but woke up my 6.9.

[slotard]

Originally Posted by Edahall

I've got the same setup as you and best fuel economy is 55 mph. 65 mph is the point when fuel economy starts to really plunge. The lowest I have ever got was 17 mph driving between 85 and 95 mph. About 10O mph is terminal velocity on one of these things.

You managed 17 at 85-95? That's pretty impressive. I don't think I could even approach that at any speed.

[slotard]

Originally Posted by Edahall

The answer to your question is poor aerodynamics. Remember, these vehicles were produced when the maximum speed limit was 55 mph. The worse the aerodynamics, the faster fuel economy will drop off with speed. It'll also limit top speed.

They were designed before the 55 national limit. That came into effect in 1973, I'm pretty sure the design work was finished before it.

[crashz]

To the OP,

One thing to keep in mind driving an older square some distance is to check/change all fluids and filters. I did the same thing you are, only 1/2 the commute (3 hrs), on a weekly basis and quickly realized why you need to check the fluid in the rear axle. Burnt up the pinion bearings.

Mine was an 83 K25 (different from my sig) with 3.73 gears (I think), 6.2L diesel, and th350 trans. At 70 she was at top speed, but ran happily for years with my foot on the floor. It was a nice old truck, but rust had taken her at a time when I was in school and could not fix her.

One thing I like about the 6.2 is that they smooth right out on the highway. Mine always seemed to run best on the highway at a constant speed. Always loud, but happy

[D13]

Remember a 3.08 with overdrive is like a 2.56. Having said that, my 87 gets best MPG at 58 (fast enough to keep the converter locked on little hills) and really doesn't care to go much over 70mph (quite frankly the truck itself is the limit not the motor).

[Edahall]

Keep the 3.08's for top cruising speeds and to pass other much newer SUV's with 95 mph limiters.