Matt400

[quote author=schu link=board=20;threadid=18176;start=0#msg170274 date=1060400630]
Anyone know if it is possible to make the factory computer work with the 315/70R17 tires[/quote]In the factory scan tool there is a selection for tire "revolutions per mile" the list is limited and may get you to within 3% error but is better than not changing it at all. You would need to find out the actual tire RPM form the tire manufacture because tire RPM's will vary from different manufacture molds on the same size tire.

Gumby

When the dealer recalibrated my speedo, the tool would only calibrate to 699 revolutions per mile. We did some quick math & determined the tire will turn 698 + a few tenths per mile, so he set the speedo for 699. So far, I haven't noticed much of any difference when driving in traffic.

Matt400

[quote author=Gumby link=board=20;threadid=18176;start=15#msg175290 date=1061489487]
When the dealer recalibrated my speedo, the tool would only calibrate to 699 revolutions per mile. [/quote] That is odd because my dealer plugged in 636 for my Toyo's The Goodyear P275/60R20 that many 1500's come with are also 636 RPM.

schu

699 sounds way off:

This page: http://www.bfgoodrichtires.com/asset...rain_ta_kd.pdf

Says that the 325/70R17 spins 601 times per mile.

Anyone know if the factory computer can be programed tha low?

schu

Matt400

[quote author=schu link=board=20;threadid=18176;start=15#msg175411 date=1061511941]
699 sounds way off:[/quote]I agree, in the factory shop manual the P245/70R17 LTX A/S Michelin's are 686 and are the highest RPM listed for the 03's. 699 is more like a 235/75R15

Gumby

Sorry guys, I must've had a brain fart! The calculated revolutions were 598 point something & the machine only went to 599.

doug

wow. so the factory ECM will go to 599! thats about three tenths of a percent error (not worth worrying about) if you use BFG 315s. In case anyone cares, thats about 33.7" rolling diameter for a nominal 35" tire. about right.

Lightman

is there not some tire thats just one size down from 315 that will fit without rubbing and speedo will work for?

doug

I'm told that 315s will fit on the 3rd gens without rubbing. A front lift is required for 2nd gens.

For any tire size other than stock, the speedo will be off

285s are in my opinion a sweet spot, pardon the term, between the desire for a larger tire, and the consequences thereunto apertaining ;D --

there is no danger of rubbing

The speedo is off by only 3%. My speedo is not even corrected because as the tires wear the speedo error drops.

All speedos have error, given differences between tire mfgs and of course wear (to say nothing about face calibration). With 285 tires, I guarentee that I will always have a 1-3% error on the tall side (speedo reads too slow and odo reads short). any error on the short side (speedo reads too fast) is very objectionable to me (warranty runs out too fast anyway...) so the larger tires are actually a bonus. that is, a small error on the favorable side is far more acceptable than any error on the un-favorable side. other bonuses of 285s are:

they look 100 times better than the stock 265s

my dealer supports them (represents me to DC) under warranty

being a "larger, D rated tire" they support approximately the same weight as the smaller E rated tires. thats not a bonus, just not a disadvanage ;D

schu

While I like the idea of running 285's I got my 315s for $180 each and the 285s where $250.

Also the 315s do rub if your locked and backing up but I don't notice it but twice a month so it's not a big deal.

My dealer didn't have any problems with the big tires.

I'm going to pick up a isspro speed adjuster from WTTD on these forums to fix the speedo, but I gotta wait until there is a little extra money first.

schu

Lightman

I've seen pics of ppl's stock 03's with 315's where the tire has rubbed away the paint where it rubs on one of the suspension parts. I guess it only rubs on full u-turns etc, but I bet offroad it does too, when pushed a little off camber etc. 285 sounds like a happy medium. I think we all agree the stockers look a little small.

How much of a mpg hit do you take with the 315's? I don't buy the argument that they turn less than the smaller ones causing less rpm = better mpgs. I think the added rotational mass with the heavier tires easily negates any gain in size..

doug

roger on the price difference. BFG has competition in the 315 market but none in the 285 market, and their price reflects that.

and once you start having ball joint, tie rod ends, or steering box problems, you can expect your dealer service manager to sing a little differently. But if your service manager has actually confirmed that he will represent you to DC and cover your steering and suspension components under warranty with 315s on there, then hats off to ya! ;D

If the dealer has no probs with the 315s then I'd have the factory ECM modified for 599 revs per mile and forget about an after market solution. unless of course you intend to sell the vehicle and return it to stock condition w/o involving the dealer.

schu

When talking to the dealer they said that unless they could prove that the modification was the cause of the damage they would warrenty it.

This doesn't suprise me as it is also the law:

Magnuson-Moss Warranty - Federal Trade Commission Improvement Act of 1975. Under the Magnuson-Moss Act, any aftermarket equipment which improves safety or performance, does not void a vehicle manufacturer’s original warranty, unless the warranty clearly states, in the owner’s manual, that these additional items void the warranty (I read my warranty and did not see this). The dealer must also prove that an added, aftermarket or safety item, caused the need for repairs before they can deny warranty coverage on that basis.

As far as gas milage goes it was not really affected. I have taller gearing now, but I'm also moving around more weight so I think it's a wash.

I plan on keeping the truck for 10 years (if it lasts) so I would rather pay $105 for the little box to correct the speedo than pay $65 for the dealer to fix the speedo in the computer. The truck will likely see a couple of differnent tire sizes in it's lifetime. Besides I talked to the dealer about this little box and he was cool with it. I figure he he happy I'm not putting a 100HP chip in it.

As far as parts wearing out I'm not to worried about that because the tires are not so big and heavy that they will tear up the front end of that truck. A chevy maybe, but we are talking about a very strong straight axle. If anything goes wrong in the first 70k miles it will almost certianly be a defect or recall anyway. Even if something did break and they won't warrenty it, it would still need to break $280 in parts before I have made up the cost difference to use the smaller tires. (I do my own wrenching).

schu

doug

[quote author=Lightman link=board=20;threadid=18176;start=15#msg177429 date=1062020740]
I don't buy the argument that they turn less than the smaller ones causing less rpm = better mpgs. I think the added rotational mass with the heavier tires easily negates any gain in size..
[/quote]
bingo.

basically the more weight you rotate at a larger radius, things get really bad really fast. wider, taller tires are a distinct disadvantage because they combine both more weight and larger radius, which is double jeopordy. sparing all the happy math, here are some loose approximations:

compared to stock tires, It takes about 50% more HP to accelerate 315 tires up to the same speed. It takes about 35% more HP to accelerate 285 tires. Every time you speed up, the engine has to deliver extra power to accomodate the larger tires. This is just the contribution of the tires themselves, which is small compared to the whole truck of course but for the 315, I'd estimate that this component is worth 1-3 mpg, depending on driving conditions. the faster you go, the worse it gets for the big tires because the "loss" due to the larger size depends on (1) the square of the radius (2) the square of vehicle speed and (3) weight of the tire.

Due to increased contact surface area (friction) It takes about 10% more HP to keep a 315 rolling on the highway under the truck at highway speeds, and about 7% more HP to keep a 285 rolling. again, thats just the contribution of the tires contacting the road, but probably (my opinion) this could be worth perhaps a fraction of a mpg, maybe 1/2.

A small, "lower rpms" advantage may be realizable when you run steady at, say, 2200-2300 rpm, where the engine seems to suffer from high internal inirtial losses. A 315 tire will drop rpms on the order of 10% over a 265 tire, enough to see some realization of low-rpm efficiency perhaps. Its probably measureable, and in my opinion (I haven't gone through the calculations) the benefit would be perhaps 1 mpg at the most.

So considering all the above, the 315s will probably net something like 0-2 mpg loss over stock tires. Depending on driving conditions you might not notice it and in some cases you'll notice as loss. Again, the faster you go, the worse this gets, so if you drive 95 mph with 315s you'll be in really bad shape, even if you have the gears to keep rpms reasonable!

doug

schu -- yea the M-M verbiage yada yada yada. I guess my comments are

(1) you are fortunate to have a dealer that seems to understand this and doesn't "believe" the larger tires are a substantial contribution to front end wear.

(2) all that supportive talk can suddenly come to a screaching halt if DC asks about the tires and you are complaining about tie rod ends or the something else wearing. the picture changes if you really have a warrenty dispute becasue DC can certainly argue that larger tires stress the front end and cause premature wear. (I know becasue my 02 front end is weak and had to deal with that...). the additional tires make a big contribution to front end stresses, especially steering in offroad situtions.

but it sounds like you and the dealer have a good grasp of all that, and since you're prepared to do some wrenching yourself, you're really not at risk. Indeed in your case the benefit of the 315s (you like them) far out weighs the risk. so be happy and enjoy them!