Fuel Economy and torque converter after lift and tires

[spalding1028]

about 2 weeks back I went ahead and installed Mark's 3" suspension lift and threw some 265/70/17 Cooper Atp's on. I was expecting a decrease in fuel economy, but not to the extent that it is. After I had the tires mounted, I ran it for a week with what seemed like little issues before taking it to a stealership to have them reprogram the pcm for the larger tires so my odometer and speedo were correct (it was showing that I would get 17-18 mpg, down from 20-21 at 70). After I got it programmed it seemed fine, shifts were slightly sluggish (it would hang at 3k for a second then shift) but I noticed a huge drop in highway performance. If I go 70 my tcc will lock up once i get up to speed, but will unlock with almost any incline. When the tcc does lock, I get 15-16mpg and when it unlocks, I'm running 12-14 mpg. It could just be that these things lifted don't like driving at 70, or the stealership reprogrammed my pcm wrong. I'm going to see if I can drive at least a good 30 miles on the highway (miles are marked fairly accurately) after resetting my trip and I will see how they compare. My normal commute is just under 5 miles on the highway, and after driving 4.7 miles, my od read 4.4, so I think it could be that they didn't program it correctly.

Any ideas other than a bad reprogram? I'm getting the pcm4less tune next month and hopefully that'll help at least with gas mileage.

[DirtyBacon04]

Well, ideal speed for best fuel economy stops at 60 (45-60). Assuming you're running 3.42s? The added size of the wheel can cause extra strain on your xmsn, hence the pause in shifting and tcc unlocking easier.

[bartonmd]

Interesting... The mileage should hav gone down some, but if you were getting 17-18 based on ODO, it should have actually been higher than that, once you add the miles back in, that he ODO didn't count because it was slow, because of the larger tires.

That kind of performance is what Kyle was getting with 35" tires, 3.73 rear end, maxed out suspension and a 3" body lift.

Is your rear end 3.42, by chance?

Mike

[fishsticks]

Even with gears, my TC will unlock if I hit a good rise on the highway. That's with a lot more tire than you though.

How much weight do you carry?

[Trail X]

That does seem like a steep dive in fuel economy.

First - I'd question what the dealer did to the PCM, not only because it's producing poorer results, but mainly because as I recall, reprogramming the PCM for larger tires is not at all a simple thing to do with pre-CAN vehicles. As I understand it, it was part of the programming and not an easy variable.

Get a scangauge, it's the best way to get an accurate speed... and you'll also be able to monitor your manifold pressures (which trigger TCC unlocks), and monitor the TCC lock signal.

[chevycrew]

My wants to run with the converter unlocked all the time now. But that's 40s and 4.56 gears...

[djthumper]

You can only adjust the tire size in the PCM on 2006 and newer... I am going to bet they made adjustments in the PCM but there was no place to change the tire size.

[spalding1028]

I'm running 3.73's Carrying maybe 30lbs in the vehicle, but removed the spare a week ago (before I really paid attention) due to it not wanting to drop. I took it to Ballas in Toledo. My buddies got a high end snap on tool, we're gonna see if there's any way to look at what size it's been programmed to if at all possible.

[ScarabEpic22]

You can only adjust the tire size in the PCM on 2006 and newer... I am going to bet they made adjustments in the PCM but there was no place to change the tire size.

The dealer is the only place that actually CAN adjust the tire size on 02-05 P10s (or so everyone says).

Im going to play with my 02 again this weekend on my trip to Idaho and try to get it corected for the 265/70R16s I have on it. Not off much, but Im pretty sure I can get it closer.