New Tires and ABS

Greetings,
I recently purchased new tires for my 2004 Chevy 2500HD 2wd and got a slightly larger size - specifically LT265/75 R16 versus the old tires at
LT245/75 R16.
By using my GPS as a control, with my old tires the speedometer would indicate 70 mph and the GPS would indicate 68.5 mph. With the new tires when the speedometer indicates 70 mph the GPS indicates 71.5 mph, so there is approximately a 3mph difference at 70 mph indicated from my old tires to my new ones.
My question is, will this difference cause any major issues with the ABS and/or the engine management systems. Will the ABS be more or less prone to activate under the same conditions as before, and will it noticeably effect my mileage or cause any problems with the engine and transmission (Duramax diesel and Allison 5-speed auto)?
Thanks for any insight.
Cheers - Jonathan
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I don't think it'll have much effect, but you can get a power programmer and program the tire size in to correct the problem! You can usually do this for Chevy's. Fords for example never seem to give you this option. It's not a cheap fix, around $400, but since you have a Diesel, a Power Programmer will make a pretty big Improvement in HP and Torque also. Quite a bit more then a Gas Engine.

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Greetings,
Thanks for the suggestion, but $400 isn't in my discretionary spending budget just yet... Even so, at the HP and torque this motor is putting out stock while still getting 20mpg in mixed driving I don't need any more power and don't want to jeopardize my mileage.
I could take it to the dealer and have him reset the ECM for the new tire size, but that is a matter of having the time to give them the truck for half a day - which currently I don't have available in my schedule.
I guess what I should have clarified is will the difference in the new tires' size cause any problems with my ABS and drivetrain until I can get the computer reset (maybe 3-4 weeks)?
Cheers - Jonathan

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Increasing the performance will not screw up your Mileage, will unless you start getting on it afterwards because of it. As long as all the tires are the same size, it's not going to effect your ABS at all. Getting larger tires is done all the time. I've done quite a few lift jobs on brand NEW dealer trucks which end up which much larger tires then stock with no ABS problem. The Speedo will be out a wack a bit, but nothing major. Go here and check out some Info. You'll be surprised. Increased Mileage! http://www.superchips.com/home.phtml

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