Tire Swap

Growler · September 28, 2005

At 46K miles, I finally got some (weak) justification to change out the original Firestone 245/75R16's AT's. I'd always wanted to replace them (I'd picked the rear end ratio on my new truck based upon eventually getting bigger tires), and from earlier feedback on the boards I expected them to wear out at about 25K miles. No such luck; in fact, at 45K miles they still had almost a half inch of tread left.

But when I got home from work last week I found that the driver's side rear tire was almost flat. I went to the gas station to air it up and found that the valve stem had failed. It would hold 19 PSI but would open up and relieve any pressure beyond that. Back home, I attempted to replace this tire with the spare, but it was froze onto the wheel and I had to use the tire jack against the frame (with a piece of wood against the wheel) to break it loose. This was a huge pain in the behind, and was only possible because it was a rear tire. Finally got the spare on, though.

OK, a failed valve stem is really not a good reason to replace still serviceable tires, but I didn't need a good reason, just a reason. So I went to Sam's and got a set of 285/75R16 BFG AT/KO's. Had them mounted on my OEM 6.5" aluminum wheels. It took them over 4 hours to do it - which is ridiculous - but when they were done I took one look at the truck and said "this is what it should have come with." Nothing required to get them on, but some Air Dam trimming required to make them turn without rubbing. Haven't cranked on the T-bars yet, but I'm probably going to try and raise the front a little, as it still has a bit of a "rake" visually.

I bought 5 tires - I have locking hubs and didn't want a mismatch spare - and I just ordered (via Ebay) a spare aluminum wheel to replace the steel spare wheel so I can do a 5-tire rotation. Not sure where I'm mounting the spare, though, because I don't think it'll fit in the OEM location on my truck without being too close to the stock exhaust. The 30" factory spare was only about 2-1/2 inches from the exhaust pipe, and even if I could relocated the factory carrier, the 285 spare would stick down further, and the road might not be too kind to an aluminum wheel under the truck. For now, it's chained in the bed standing up in front of the drivers side wheel well. Tried the same location on the passenger side first but it obstructed the "lane change" view too much.

Anyway, the BFG's ride great! Next time I can finagle a day off during the week, I was planning to stop by the dealer and ask them to reflash the PCM for a 3.73 rear end ratio, even though I have a 4.10. What are they gonna do, check to see if I really had the gears changed? It's out of warranty now anyway, so I can't see why they would care. The reflash should fix the speedo (10% taller tires = 10% change in final drive ratio = 10% low on speedo/odo/DIC gas mileage/DIC range, corrected to almost nothing with reflash for phantom 10% gear change). I don't know if they can change the Allison programming for 3.73 gears to correct shift points, or if Allison really even has different programs for different rear ends, but I haven't really noticed any issues with shift points since making the change anyway. Before I do this reflash, I'll try to figure out if the Diablo Predator would make me happier for the price. I'm guessing that it doesn't do anything for the Allison either, but it may let me adjust torque management in addition to correcting for the tire size change and getting rid of the 98 mph speed limiter (still gotta check speed rating of the BFG's, the Firestones weren't rated, which didn't keep me from finding the speed limiter).

Thus far I haven't noticed any substantial gas mileage change, either. Long term average with the stock tires was 10.9 mpg with my combination of city and highway driving (8.1L, 4.10 rear end, A/C usually on, lead foot). So far DIC is reporting 10.1 mpg, which translates into 11.1 mpg, but that’s more highway than the long term average. Theoretically, if the rotational inertia where the same the taller tires would improve the gas mileage (equivalent to a gear change to more efficient gearing). Obviously the taller, wider, heavier tires have more mass further away from the hub, i.e a substantially higher rotational inertia, as well as more mass to be moved forward - which is gonna hurt - but thus far it looks like the final drive ratio efficiency increase is going to make up for a lot of that, and the average mpg hit won't be too bad. I'm currently guessing I'll average about 10.2 mpg, but I could be off a few tenths either way.

Too make a long story short (yes, I realize it's too late), so far I love these tires. Still waiting for some rain to see if the hydroplaning is significantly worse, and for some snow to see how they work out there, but for now my opinion, as I said above, is that "this is what it should have come with." If I ever get around to washing the beast, I'll try to post a pic.

99-2door · September 28, 2005

Since you have 4.10s and the 285s have them flash for 245s and 3.73s.

It is the same effective ratio.

*somehow I missed the fact you already said that*

Bart2500HD · September 28, 2005

I recently installed some 265's on my truck. They look way better than the 245's it came with. I haven't bothered with reflaching the PCM for the different size and I doubt I will. Every time I go near the dealer it costs me too much money!

silverado3500 · September 28, 2005

I only paid $40 for them to change the tire size on my last silverado (went fom 235/75 R16 to a 265/75R16

silverado3500 · September 28, 2005

I was just thinking about what you said about just changing the gear ratio in the computer, and I am not sure that will work. . .While the effective ratio would be right, your vss/abs sensors would not be affected by the change in the gear ratio setting in the computer. At the very least you would run the risk of messing up the ABS system. There is a sensor on both front wheel hubs.

That could work in somehing like my F-350 where the only speed sensor is attacted to the pinion gear(I have no abs, and I like it that way:D ), or in the transmission.

IMO I would either have the dealer program for the cloeset size possible (265/75R16) or get an aftermarket programmer and program for the 285/75R16 with that.

Hope that helps

Jester4736w · September 28, 2005

Which # on the tire sizes equals the width? I like the hiegth of the stock tires on my 03 2500hd, but I would like to see them a touch wider? Any suggestions on size?

gmcjoe · September 28, 2005

Which # on the tire sizes equals the width? I like the hiegth of the stock tires on my 03 2500hd, but I would like to see them a touch wider? Any suggestions on size?

<{POST_SNAPBACK}>

285 = 285 MM wide

75 = the sidewall, 75% of the width

silverado3500 · September 29, 2005

a 265/70R16 wil be about an inch wider, and the same height as a 245/75R16

Jester4736w · September 29, 2005

Thx guys, im gonna check on a set of 265's...

renolaw · October 1, 2005

Soon tired of my OEM 245's, i took a deep breath and went w/ the 315-70-17 BFG A/Ts on factory H2 alloy wheels. Sure, i looked all over at different combinations and big prices and found the ticket with H2 take offs. Let me say it was worth every penny! Only lift is 2.5" T-bar crank. Fits the 35s like the factory intended, especially with the awesome backspacing. Minimal trimming and the street/trail manners are superbe.

I usually use the dash mounted garmin gps for a speedo (when i really care), but i'd like to get the computer recalibrated. School me on this please.

Here is a side comparison of the OEM crap tires and the H2 take off. HUGE difference! I found the tire/wheel combo, brand new still w/ the rubber nubs for $200 each locally. All mounted, balanced and ready to just bolt on. A screaming deal, no?

mb

ntercept · October 12, 2005

I have been looking at putting these on my 1500hd. The only thing you had to do is crank the t-bars? Any other clearance issues? I think they look great. Thanks for the info.

renolaw · October 12, 2005

u'll need an 8 lug spindle...

mb

ntercept · October 13, 2005

The 1500hd has the same spindles, 8 lug.

renolaw · October 13, 2005

ok, me doofus...

u can pick up H2 wheel combos pretty cheap. I paid locally $200 for each tire/wheel, and the BFG tires were BRAND NEW, still w/ the rubber nubs! Some on Ebay, wheels only, are going for $45 each.

good luck-- the backspacoing of the H2 wheel, made BTW by GM, is perfect-- no tire sticking out. Best part? the OEM center caps screw right back onto the OEM lug nuts-- so the finished product looks like the factory should have intended.

mb

mityike · October 19, 2005

I have a 04 2500 hd with the 3.73 rear and am looking to upgrade. At first, ithought about the 285's, but after seeing your 315's on the H2's, I think I changed my mind. What rear are you running, and will this big of a jump trash the allison? I dont need the 315's but like you said, you can get a brand new set on H2's for about $800, which is less than what the tires sell for new.

How has your truck handled, powerwise, since the swap? Any other mods done to compensate for the larger tires?