Mechanical Speedo Correction Box (help with math)

So I have a 2.7L 5 speed (W59) had a stock axle ratio of 3.58 (BO2A) and stock 225/75/15 tires. I have since changed to 4.88 gears and 33" tires.

I am trying to calculate how far off my speedo/odometer is. I know in some cases you can change out the speed gear inside the transfer case but considering my drastic gear and tire change I don't think they make the correct sized gear to fix my problem. I have a mechanical speedo so one of those magic boxes wont work.

After living with it for about a year now I am officially sick of it and need to find a way to fix the problem. Reduction boxes seem to available at a fairly affordable price and I am surprised I haven't heard of them on this forum yet.

I have seen a few conversion charts/calculators on the web but I'm having a hard time finding out how to factor in the gear and tire size change in one calculation (sorry I am incredibly bad at math). So does anyone know how to do the math to find how far off I am in a percentage?

 

Why not just use GPS? That should put you in the 1-2 mph range.

 

http://speedometer.mobi/

 

http://www.tciauto.com/tc/speedometer-gear-calculator/