285/75 = Disappointed in street performance

vhato · March 11, 2006

Sure we all know you dont buy a 2500HD looking for mileage and you don't buy the Vortec 6000 for power, but I was very satisfied.

The 245/75's were pizza cutters, but I could burn them down easily and passing was a breeze. My average economy was an EASY 14.1 MPG on my work commute.

I upgraded to 285/75r16 BFG AT/KO. The tires are absolutely gorgeous and the truck looks brawny. A little crank on the T-bars and some wax on the wheels really make this truck look awesome.

Anyway, with my same driving patterns I can't break 12.6 MPG on my work commute. And not only that, but passing is no longer an easy task for the Vortec 6000. The truck is actually slow. 6.slow as someone calls it.

While in cruise at 65 MPH, if I press the button to accelerate, the truck downshifts into 2nd gear just to get to 70 MPH. A mere 5 MPH increase. I can't possibly see how anyone could/would tow anything of significant weight at Highway speeds.

I can honestly say, going from 245/75 (29.79") to 285/75 (31.91") would EASILY justify a gear change from 4.10's to 4.56's in an HD w/Vortec 6000. I would probably get my mileage back as well as power.

I am planning a Dyno Trip soon to see what has happened. I recalibrated the PCM for the increase in tire size. I am SERIOUSLY considering creating new High/Low Octane Tables as well as MAF Frequency timing increase for peak performance running 93 Octane Fuel. Currently my truck is tuned for peak power running Chevron 87 Octane Fuel from a few select stations I can trust.

Anyone else experince this?

mmmikkke · March 11, 2006

I did the same change from pizza cutters to 285's on my '05 2500hd. But I was already running about 12-13 mpg at 70mph and I didn't see any change when I changed tire size.

I had 4.10's in my '85 K20, and then I had 4.56 gears in my '95 K3500. In my opinion, the lower ratio (4.56) made a noticeable difference in deceleration behavior. With 4.56 gears, the truck did not coast as well, it tended to need light throttle in order to behave. Coasting is noticably different (better) with a 4.10.

Dropping my cruising speed from 70 to 65 did enough to reduce wind resistance that I got back to reasonable mileage this way. I got about 14 mpg during a recent trip from PA to MI by cruising at 65, and because most of the other traffic runs about 70 in that area, it means that I could do most of the trip in the slow lane, and I had no competitive drivers doing mini-races to the next clog on the road (all 3 lanes clogged by sub-70 cruisers driving abreast).

Part of the "benefit" of having a big truck like this is that you can go a little slower than the rest of the traffic (65 mph) and nobody messes with you.

PS: I did the tire swap soon after buying my truck, and never even towed my toy hauler gooseneck trailer with the smaller tires. So I don't know if the bigger tires made my towing mpg less. Towing with the 285's I usually get around 9-10 mpg at 60-65 mph.

SilveradoWV · March 11, 2006

Bought my truck used and it had some cheap 285's on it. It is an 8.1 so I thought it had loads of power when I got it. Never was satisfied with those tires so I went to 265's not long after having the truck. I guess tire quality has alot to do with my results but overall I like 265s. On other trucks I've had, tire size has ranged from 245s to 285s, I never really liked either one. For some reason a 265 is the perfect fit for me, doesnt look as good as a 285 but better than a 245...

mmmikkke · March 11, 2006

...While in cruise at 65 MPH, if I press the button to accelerate, the truck downshifts into 2nd gear just to get to 70 MPH. A mere 5 MPH increase. I can't possibly see how anyone could/would tow anything of significant weight at Highway speeds...
<{POST_SNAPBACK}>

Instead of pressing and holding, try 5 blips of the button, spread out over maybe 30 seconds. That will raise your speed by 5 mph (1 mph per blip) without forcing a downshift to 2nd.

JRKRACE · March 11, 2006

I went this route also. Had 285's on 16 x 8 wheels and a PCM tune. Yes, it was OK and performed somewhat well, but I switched over to the stock 245's and I could not believe the difference in smoothness and acceleration. Got rid of the 285's and looking for 265's. I figure it's the best of both worlds.

michigan2500hd · March 12, 2006

Dont forget to factor in the fact that even though your truck says you are getting 12.6 mpg your getting more than that because your 285's are about 7 % larger than the 245's. Still any time you go to a larger tire you increase rolling restance and unsprung weight resulting in mpg loss.

Haulin' 8.1 · March 12, 2006

My 8.1/Allison with 3.73's and 265's will downshift with the cruise control button anytime I try to accelerate over 2 MPH with it. It's annoying for the 5 MPH bumps.

My truck had 285's on it when I bought it and they were a very aggressive off road tread that drove me nuts. I quickly installed the REVO's in 265 and it was a definate improvement in acceleration and braking. I'm going to install some 4.56 gears soon to make the truck accelerate faster and make my sub 60 MPH stop and go commute more overdrive freindly. The only time I'll pay a mileage penalty is on the trips I occasionally make. But half the time I'm towing so who knows.

My truck got a significant mileage boost with the front end lower than the back.

Vernon

sonjaab · March 12, 2006

VHATO.......Don't forget your odo/speedo will be off because

of the larger tires. Thus showing a loss of mpgs..............

How about a tuner/PCM burn or a trip to a GM Tech 2 machine

to correct for non stock tire size?.................geo

Mark_in_Tulsa · March 12, 2006

VHATO.......Don't forget your odo/speedo will be off because
of the larger tires. Thus showing a loss of mpgs..............

How about a tuner/PCM burn or a trip to a GM Tech 2 machine

to correct for non stock tire size?.................geo

<{POST_SNAPBACK}>

Vhato is one of the foremost authorities in custom tuning on this board. (if I remember correctly) He does his own custom tuning, but I'm not sure which software he uses. So I'm pretty sure he adjusted his own PCM for tire size.

vhato · March 12, 2006

I dunno about an authority on PCM tuning, but I did recalibrate it for the new 31.91" tire.

Thanks so much for all the replies. I was hoping to get answers like these. However I was kinda suprised to see to see the replies from the Vortec 8100 owners that are the same as ours. Maybe I was wrong to think Calculated Engine was a major factor in determining downshift. Maybe Throttle position and vehicle speed play a greater role. I would have expected an 8100 to allow the tranny to stay in OD for a quick blip to 70 MPH. I can still adjust the downshift threshhold based on Engine Load and fix this problem (The multiple quick taps on the control mmmmike mentioned is easier though)...Hmm but I am now intrigued. I need to find someone locally who owns a Vortec 8100 and 285's so I can pull some AutoTAP logs. I am interested to see at what points the PCM allows a downshift. Hmmm, maybe it doesn't use calculatd engine load at all, it use MAP readings or a huge combination of multiple readings.

Whats weird about PCM tuning, is I can solve this problem easily by changing 1 or 2 settings and be happy. Then, later realize that change caused then engine to bog at other speeds. Its fun to play around and see how things work.

Thanks for the replies,

V

billcar9999 · March 12, 2006

Where is local?

vhato · March 13, 2006

Where is local?

<{POST_SNAPBACK}>

Around Baton Rouge or New Orleans, Louisiana.

billcar9999 · March 13, 2006

Around Baton Rouge or New Orleans, Louisiana.

<{POST_SNAPBACK}>

That's a little outside my neck of the woods. I don't think I have this problem. My truck has 4.10 gears. I'll pay attention to the downshifts the next time I drive it.

Are you saying this only happens with cruise control on or any time you accelerate from 65 to 70?

Bill

vhato · March 13, 2006

Around Baton Rouge or New Orleans, Louisiana.

<{POST_SNAPBACK}>

That's a little outside my neck of the woods. I don't think I have this problem. My truck has 4.10 gears. I'll pay attention to the downshifts the next time I drive it.

Are you saying this only happens with cruise control on or any time you accelerate from 65 to 70?

Bill

<{POST_SNAPBACK}>

65-70 only and I dunno what axle ratio the others had.

Haulin' 8.1 · March 14, 2006

I'm planning on taking another trip to Slidell on I-10/Hwy 12 this spring to catch up with my buddy and his family who are living in our travel trailer parked next to thier house being rebuilt. Possibly we could get together then for a few hours. My 3.73/265 combo is similar to 4.10/285. That's if I don't install the 4.56's by then.

Vernon