My dads friend has owned a tire shop for 40 years and he told him that if I put 285's on back and 275's on the front that it will mess up wth abs on the car and that I have to get 275's all around. Is this true do u have to disengage ur abs or is it all BS and I'll be fine??
What many overlook is the fact that ABS is affected with different overall tire diameters (non-matching sets). It may still operate BUT may not perform correctly.
It assumes that when it sees a difference in pulse widths between front/rears or left/right etc etc...that something is occuring (tires slipping etc). Thefore it may activate more often then it is actually needed which affects the function of your car hampering braking distances, not improving it.
The difference that you're considering between these two sizes (275/40 or 285/40) is VERY small (~5/16") and would be well within tolorances which you probably won't notice a thing. You could probably overinflate a rear and underinflate the same size tire and have that amount of a difference.
Another comment regarding disabling ABS is while just removing the fuse does shut it off...you now have no control or biasing adjustment for your car. We manufacture ABS delete kits...this is why we always include a proportioning valve with our kits. The valve gives the user the ability to actually tune and re-bias their brakes giving the user more control over their braking system when they stray from stock setups.