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b1jetmech 01-31-2011 02:30 PM

Noob from Michigan...looking to gain MPG from Suburban
Hello all,

Currently live in Michigan and have a 99 half ton suburban 4x4.

I have a goal of 30 mpg from this beast and I know it's achievable through some serious mods(No, not selling the thing) but doing a 6.5 diesel conversion, remove the fan and replace with electric fans.

Already have the gearing of 3.42 so that will put the RPM's at 2,100 @70 mph with 245 tires.

Along with the diesel conversion the a 4L80E will be swapped in because it can handle towing much better...this vehicle is a "Draft" horse.

Would like to make a belly pan for underneath. It would mostly be a 2 dimensional pan because of the straight frame. this would be benifical because we all know a LOT of drag comes from underneath and be nice to straighten out some of that wind.

Want pics???


MetroMPG 01-31-2011 04:38 PM

Welcome to the forum - do you already have the diesel lump?

If you haven't already, you might want to look at Big Dave's modded Ford F-350 diesel (28 mpg) to get an idea of what's within the realm of possibility for a large, heavy truck.

Details: The Red Baron - 2000 Ford F-350 XLT Fuel Economy -

fuel log: 2000 Ford F-350 XLT Gas Mileage (The Red Baron) -

Yours will likely be harder to get to those numbers since its less aero than his, with its tapered "fastback" bed cover. The Suburban drags a much bigger wake behind it, which kills highway mileage.

Also, I'm pretty sure Big Dave has honed his driving techniques to squeeze out those numbers. That's half the battle right there.

Good challenge for sure.

b1jetmech 01-31-2011 06:13 PM

Hey metro,
Thanks for the welcome and info.

That is some impressive numbers from "Dave" considering his truck weighs close to 4 tons!

Guess I should raise my goal for the mileage increase.

I know it can be done because I did another Suburban diesel conversion using a Cummins 4BT.

Best MPG achieved from the 4BT was 26 highway and 20-21 city which isn't bad.

I decided to do a gm diesel because they bolt right in and their inexpensive. Just have to know how to take care of them and they will take care of you;)


I would like to do a yanmar 3 cyl. diesel conversion to a metro, I think 70 mpg would be achievable.


Frank Lee 01-31-2011 06:41 PM

Oddly, the net says it's under 5200 lbs. :confused:

rmay635703 06-11-2011 08:34 PM

If you were doing a 1989 or older 6.2 diesel conversion I could see 26-28mpg highway without special techniques. My old 6.2 with a 700r4 got 26mpg highway without special techniques (but I had 2.8 gears).

The 6.5 is NOT a 6.2, the C code 6.2 was designed for fuel economy period, the 6.5 and J codes were not. Whether you can make a 6.5 into a 6.2 I don't know but the two motors are tuned for totally different things.

For a 6.5 to behave like a 6.2 you would need smaller pre cups, higher compression, a detuned turbo (or a delete of the turbo since high compression + turbo = boom) You would also need to advance the timing.
I would also recommend moving from 16" tires back to the stock 15" size tires the old rigs had. I would also lower the front end a bit (or the whole lot).

To get higher than 26mpg with a suburban you will need to do the following
1. Full Hypermile
2. NV3500 or 4500 MT transmission.

You could try other things but the absolute max for a modded suburban is normally around 26mpg for the untrained with an AT and appropriate gearing.
Beyond that would require special techniques and a MT.

slowmover 06-11-2011 09:56 PM

The 6.2 and 6.5 were not so hot as tow vehicle engines either. I'd spec the trailer QUITE carefully.

rmay635703 06-12-2011 02:36 PM


Originally Posted by slowmover (Post 244592)
The 6.2 and 6.5 were not so hot as tow vehicle engines either. I'd spec the trailer QUITE carefully.

Nah, they were just fine, but remember they like to tow slow and steady. They will tow fast and hard just fine too, just like a 350 but then you have to be rebuilding the darn things every 100k or better.

My 120hp 82 c code diesel went the last 100,000 miles half of which was towing 10k+ of two enclosed trailers (one camper and one enclosed trailer). Much of which was right to the floorboards getting 10mpg going 55mph until we found an air deflector raised the top end to 65mph/12mpg (all with a turbo 400 3 sp). The first 100k I couldn't speak for.

It was slow & underpowered but I could make it up any hill and thing is still on the road today.

A few things to remember, the best longest lasting 6.2/6.5 rigs are the ones with the least HP and the best set of gears. (and pure luck) The weak spots are the crank and harmonic balancer with older units suffering from bad heads and bad bottom end webbing.

If I were setting up a 6.5 as a tow rig, I would up the compression (different precups/heads), turn down the IP a good amount, lower boost, advance timing and make sure to have both an oil and a xmsn cooler and play it nice on the throttle. In my case I would never own a GM diesel without an MT and you will learn why if you use one for any extended period with an auto.

There is still a guy in TN that uses old 5.7DX's to tow lots of weight because they get better FE than more capable rigs, as with anything if you put a good set of gears and a set of guages in the right places, you can do almost anything you want with any motor but you have to watch it to make sure they don't fall apart from abuse.

If your the boost it and floor it all day type of guy a 6.2/6.5 is definately not for you unless you detune the thing to the point of being incapable of grenading.

Like I said originally the only reason to have a 6.2 is for FE and occasional towing. They tow slow but will get you where you want every time so long as you use COMMON SENSE.

They were not designed to be a cummins and you shouldn't try to use one as such. They are more like a VW 1.9 diesel that gets better FE and can haul something.


slowmover 06-12-2011 11:39 PM

Nice post, Ryan, thanks!

b1jetmech 06-13-2011 12:55 AM

Hey thanks for the response!

After a few months of driving with the converted diesel I'm averaging 20 MPG. That's up from 11 which isn't bad I must say.

I think it can do better because a couple of thinks need attention. First the injectors HAVE to be replaced. Sometimes I get grey smoke which means unburnt fuel so the injectors after having almost 200k miles on them dare not providing the best spray pattern.

If that helps but the mileage doesn't improve much then the injector pump will be looked at.

Then engine runs great. There is NO blow by which is unheard of with these but being a non turbo didn't have to suffer the abuse of turbo because GM kept these engine's compression ratio very high. Just like Ryan said, Too much compression plus boost = BOOM!

I do have a turbo set up but won't be installing it anytime soon. I'm having the urge to convert back to gas. Want to try a 305 tune port from the late 80's early 90's because they got good mileage and having tune port gave it a real good low end torque but we'll see!

Another item on the to-do list is turn the fuel up. I have a hard time accelerating up to highway speeds because the motor runs out of steam about 55 mph. :turtle: The fuel curve is way below because this engine doesn't belch smoke. I have never seen black smoke under any load. so an 8th of turn on the IP should wake it up a little.

over all I do like it but have the tune port ideal in the back of my mind we will see on that one.

Ryan, If I pull the heads for anything I just might install some 6.2 pre cups. I have heard of 6.2 Burbs getting in the mid to upper 20's in MPG. The differences between the 6.5 non turbo and the 6.2 is larger bore on the 6.5 and cylinder heads. 6.5's have smaller exhaust valve to prevent the cracking between valves on 6.2s.


b1jetmech 06-13-2011 12:58 AM

BTW the I sold the 99' Suburban and bought a 94' one.

Like the 92-94 burbs the best!

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