Some 4.56 gear questions!!

Ok everyone, back when I bought my duster...just before I got it the kid had the 489 8 3/4 rear rebuilt and SAID it had 4.10 gears in it.

well....

I need a speedo gear because i don't have one so I figured for the heck of it I would check to make sure it was 4.10 gears.

NOW...I think I calculated this right...so i will run it passed everyone for there input.

I put a peice of tape on the drive shaft and a peice of tape on the tires.....the drive shaft turned roughly 4.5 times to 1 tire revolution......x that by 10 and I get 45 drive shaft revolutions to 10 tire revolutions.

now I am sure since I was not using my (precision masking tape) that I could be off by .6 on my revolution .....which would give me 45.6 ....devide that by 10 would give me 4.56.

so am I right in how I came up with this? and would you all agree that 4.56's is the gear I have with the number of rotations I came up with?

and also ....who sells speedo gears in the 4.56 range for my 904 in my duster?

year one stops at 4.10's and summit does not seem to have them either.

Thanks in advance everyone!