Speedo gear change-issues with chart

[FrankNBrew]

I have an issue with the speedo gear charts I see published on various sites. Here's what I have:

I went from 245/75/16 MTRs to 285/75/16 MTRs. 245s run 684 revs per mile. 285s run 635 revs per mile. I had a 32 tooth speedo gear, which was accurate to within 1.5%.

Now, according to the speedo gear charts I should be running a 31 tooth speedo gear with the 285s. I figure it should be between a 29 and 30 tooth speedo gear. Here's why:

635/684=0.928
32*0.928=29.696

This being the case, I should use a 30 tooth speedo gear to be within 2%. As the tires wear, the speedo should become more accurate. With a 31 tooth gear as the charts suggest I will be 4% off from the start, and the speedo will fall further out of accuracy as the tires wear.

Am I missing something here? Am I using fuzzy math? Help me out!

[Ralph]

What gears are you running?

RR3

[FrankNBrew]

I have the stock 3.55 ratio with 33" MTRs. I had 31" MTRs.

[Gojeep]

The problem I have found is there is a lot of difference between brands and even more between imperial and metric sizes. The metric ones work out much more exact that the former.
Use this formula from my site: Old wheel diameter x Old speedo gear teeth = then divide by new wheel diameter = teeth on new gear.
Your old is 30.5" high and the new ones are 32.8"
So the it would work out like this.
30.5x32=976 Divide by 32.8 = 29.75. That is the number of teeth ( 30 ) you should run!

[FrankNBrew]

I used the revolutions per mile because I got those from a reliable program used to calculate RPM based on tire size. I also made the calculation using the tire diameter published on the Tirerack.com website. Both gave me a figure of close to 29.7 for the correct gear, so I will be going for the 30 tooth speedo gear. Thanks for the replies!!