I have 285/40/18's on the back of my car with 4.10's and my rpms at 80mph are about 2800 in 5th.

i don't know what size tires your car came with but mine were 235/55/17's = 27.177"
the 285/40's = 26.976"
the 295/45 = 28.452"

the 285/40 is less than a quarter inch difference in height. Very close so your rpms won't change much.
the 295/45 is over 1 and a quarter inch taller than stock(if you had the size i stated). Your rpm's would be lower but it will effect your final gear ratio.

 

egh...not feeling the whole 2800 rpm thing...lol so 3.73s might be the way...

 

At 80 mph, 2800ish...not certain. I haven't driven the car in 5 months. i'm overseas.

 

o...ok...anybody else with that ratio (4.10) and tire size?

 

According to my calculations, using a compressed tire height of 28", you will be running 2,660 rpm at 80 mph in 5th gear with 4.10 gears.

I used the same kind of calculations before I got 4.10's, and it was right on the money. You can believe the 2,660 number!

 

what about 3.73s?

 

3.73 gives 2,425 rpm under same conditions.

 

hmm ok...thanks for the help! How do you do your calculations?

 

It's just simple math using the 5th gear rate of .675, the differential rate of 4.10 or 3.73, and the compressed tire height (usually about .5 in less than non-compressed height).

My fiancee's father put it all on a spreadsheet with several car/truck models to choose from.

 

I appreciate it...I'd hate to have gotten the wrong gear with that size of tire...if I go with them lol