Olson

I own a 2003 2500HD w/6.0 4X4 Ext. Cab SB. I have 285/75/16 Brig. Duelers, and front torsion bars turned up. It sits high and I think this is causing my gas mileage to go down. I have added an exhaust (stainless 2.5" true duals cat. back w/ no x or h pipes and 3.5" magnaflow 18" long tips w/ 2 magnaflow 8" round mufflers), and a K&N FIPK. These did not help the gas mileage much, but the Hypertech programer sure did. This programer brought everything together on my truck and I am so happy I did it. The power is an unreal difference. Those tires sucked all the power out of that 6.0, until I programed it. I will never understand why Chevy chose the trans. ratios this high with this motor. It is sad, because this thing would really hum along with the 1500 trans. gear ratios. It does 52mph in first and 87mp in second with 4.10 rear end. Those are some high numbers for a truck made for pulling. I am running it on 93 octane now and Hypertech recommends the 180 t-stat. It seems to run just fine without it here in the cold weather, so I am not changing it unless it starts pinging when the weather gets warmer. What will a different t-stat do, why is it recommended, will it hurt gas mileage? How much does the power go down if I switch to 87 octane? Is the 180 t-stat recomended for 87 octane?

--Bob

Snake Eater

Welcome to the site Olson. There are alot of knoledgeable people here. I think your current exhaust is killing alot of power and causing you alot of the millage probs. Your engine has alot more backpressure on oneside of the engine than the other. A good X-pipe would help you get back some milage. I wouldnt be to upset with your transmission. Most guys here wish we had the 4l80E. They can withstand twice the HP and abuse as the 4l60E. The whole 180 degree thermo thing is because with 93 tune you are running with the timing advanced and heat along with increased timing can cause some detination at higher rpms which will cause the computer to retard timing and you can loose power. It just helps it run a little cooler. Do you know about the BB mod? It is like running a 160 thermo.

Olson

I was told by two mechanics and the exhaust place that an x pipe won't make a difference because I am running cats. They said the x pipe should be placed on the hottest part of the exhaust and that is where the cats are. As far as the tranny goes, I am happy with how it is built, but not with the gearing. First and second should be geared lower to get the truck moving from a dead stop faster. This would help with towing also. The HD comes with 245's on them. I guess the engineers know the 245's would get burnt off if they geared first and second down, so they went with higher gears than the 4l60e. I had my dealership change my tires from 285's down to 245's just to see how much of a difference it made. From a dead stop, the tires would spin into second gear. I had to let off because the tires weren't mine. I also did a 10mph romp and broke them loose. I'd have to say the truck wasn't very drivable with those tires because of my mods. With my 285's on the stock 2500hd, it would not turn them over at all. It was a dog because of the trans. gearing. That tire swap proved to me why GM went with taller gears in first and second with the HD's tranny. My conclusion is that GM should get a 265 with the 245 load rating and change their tranny gear ratios in the 2500HD's. This would help the 6.0 out a lot. The bigger tire would help keep the tires from spinning and the gearing change would give you better performance, towing and driving. I like the 6.0 and I am convinced that the gearing is what is hurting this truck.

F8L Z71

I don't think it is hurting it much at all.

You have the benifit of a great motor/heads. Use em

BLK6.0HD on this site has similar mods to me yet he smokes me left and right. I don't see the gearing holding him back at all. If anything I think it is your mods holding you back. I'll agree that the HPP3 can help out at part throttle but WOT you wont see much of a peak gain. I've used 2 of those pieces of crap now and never got more than 1hp on the dyno.

Olson

He has a stall. This gives him higher rpm at launch which equals higher hp at launch. You would get somewhat the same effect by changing the gear ratio of a stock HD. The truck would launch at the same rpm, but rev out faster with a lower gear ratio giving you more hp sooner to get the truck launched. I just hope GM does something to help this problem. I could get the same effect by changing my rear end, but then the third and overdrive gears would be so low that my highway rpm would be too high. I guess my whole point is that I know how to get around the problem I have with this set up, but I want GM to make it better so I don't have this problem at all. I can see the potential of this truck bone stock with the gear change by GM. I feel that 265's and lower first and second gears would make this truck a whole lot better. I have heard people say this truck is under powered stock. I don't think it is underpowered, just over geared. The easiest fix for me would to be to go down to 265's, but my 285's look nice. I will wear these out and then go down to 265's. If GM geared this truck for 265's out of the box, then my 285's wouldn't be a big deal.
--Bob

BADMOFO

I think you must have been doing something wrong. I ran my truck 6 passes at the track and the average ET was a 15.73. Then, I added only the HPPIII and ran it a total of 5 more times, the following week, and the average was a 15.21! Thats over a half of second better, same weight, same amount of gas in truck, very close weather, only change was the Hypertech. I find it hard to believe that 1hp knocked a half of second off my truck, but hell, I once saw a guy pull a rabbit out of his hat! I know the HPPIII is not LS1edit, but you must be smokin' dope if you think I'm gonna believe its only worth 1hp!

F8L Z71

I said from the HP alone. Not the shift point gain which can easily be done by manually shifting

If you got it all from HP then your would have to have gained 40-50HP to get .5 in the 1/4 right?

Olson

You can't manually shift an automatic and gain a half second. I understand that you might have just put the Hypertech program in without changing the shift points or rev limiter. Did you program it for 93 octane? The HPP3 website dyno shows significant hp gains at low rpm even on 87 octane.

F8L Z71

You can gain almost half a second by manually shifting if you keep the truck in 2nd gear and not hit 3rd before the end. i've done it. All it takes is 3.73s and 265/75/16s.

BADMOFO

I'm just telling you what happened. The shift point gain cannot be done by manually shifting. When I shift my truck (at WOT) the transmission shifts when it wants to, not when I shift the lever.