Please calculate diff ratio **update**
 

I have a Toyota Hilux, with a diff ratio is 4.10:1.
185R14 tyres are fitted, 25.65" diameter, 80.58" Circumference, 810.9 Revolutions per mile.

I am unable to change the diff / diff ratio, (just because) so I want to artificially change it by changing the tyre size. I have got a set of 15" rims, and can put new tyres on them. I considered 215R14, but I wanna go the 15's. I know about the speedo error, etc, so thats all cool.

I have two tyres in mind..
A) 215/75R15: 27.69" tall, 89.99" circumference, 751.1 revs per mile.

B) 225/75R15: 28.28" tall, 88.84" Circ, 735.5 rev per mile...

The tyres will fit, I am merely asking if anyone can please tell me what the effective "diff ratio" would be?

And/or the calculations for it.

I used this site to calculate the sizes, BTW..:
http://www.1010tires.com/TireSizeCalculator.asp

.

I think this should be correct:
since tire circumference is directly proportional to outside tire diameter, the ratio of "new" diameter to "current" diameter will be the same as the speedometer change, and the same exact ratio change is what you get for final differential change.

E.g., if 25.65" dia gives accurate speedo reading, the 27.69" dia tire will be 25.65/27.69 = 0.9263, so 7.37% lower speed reading. If you have 4.10 now, you will have 4.10 X 0.9262 = 3.80 diff ratio

Update

Well I did it, and thanks BadNick, for that.

I went the 215's on a set of Jeep Wrangler wheels.
Borrowed a GPS and checked the speedo.. Was more acurate with the new tyres which are 2 inches taller! 3% out, at 100 kilometers an hour, actual ground speed is 103.

A very slight problem where the wheel weight was touching the top ball joint, so a lathe was used to trim back the rim edge, by half. (The Jeep wheels have very long lips) The tyres touched the frame slightly on full lock, so I added a thicker piece of flat bar to the turn stops, and all is good. Have now done about 1500 Kms and all is good.