tire revs per mile for uneven tires..help!!!

[johnofarch]

2k mustang auto trans
U7146
set to 93 oct

i am just wondering which tires should i use to set my tire rev per mile.
i dont have the same tire size, front tires are 265/35/18 and the rears are 285/35/18....
following the tire calculators online,,,,
i tried setting it to 780 followed my rears but im still off like 2-4 mph,
tried 804 now speedometer is accurate but the shifting is not,
i tried 822 following the front tires now speedometer is a lil bit accurate and shifts good but my engine really vibrates and i can really feel it when idle at drive... i just have this bbk CAI, airaid TB spacer and true dual exhaust...
do i need to adjust any other parameters?? i also dont care about the gas, i just want maximum performance.

thank you so much!!! any help will be deeply appreciated.

[Mike_Levy]

Your front number is wrong. 265/35-18 is 797 revs per miles.

[mikel]

vibrration is not acaused by the revs per mile change, sounds like you need a balance job.

Use the rear tire revs per mile for calculation the speedo.

[johnofarch]

@mike_levy, thank you so much, but i think i already tried that before and the result was way too far from the numbers i gave...
i followed the calculators that 06MonteSS(lew) sent me before and it gave me 2 different results, like the fronts are 797 and 822, and rears are 780 and 804... there's a lot of calculators online that give u different results and i even tried manual computing it. now i dont know which 1 to follow coz all results doesnt give me the right numbers, its not accurate..lol..

@mikel, the numbers i gave is the most closest to accuracy, hehehe.... yes, maybe il stick with my rear tire size... rear tire size numbers are smooth and does not cause my engine to vibrate it breaths smoothly, the only problem is my shifting, its really hard for her to shift like 1-2 at full throttle, do you know what i mean?? ... i dont know whats wrong, its like she wants to explode before shifting to second gear..do you have any ideas whats wrong or do i need to change any other parameters?? help me mike... this is one of the reasons why i tried looking for a predator tuner, to change my tire size and change my gears soon...thank you so much!!!!!

i dont think i have a balance problem, coz i just made an alignment and balanced job a month ago after i changed my tires and lowered my car.

[johnofarch]

? ? ?

[trickpro]

instead of using a calculator because like you said they all vary. try going to tirerack.com and finding your exact tire and look at the specs. they will have a spec for revs per mile.