Either tomorrow ( sept 9th ), or Thursday the 10th should be the day my XDs and Toyo M/Ts will be available for pick up. I did the math to re-calculate the tire circumference to program into my gryphon and I just want to make sure i'm as accurate I can get. Generally i'm asking for some verification.

On Toyo's site, they list the part # 360330 tire ( 33x12.5/20 M/T ) to be an exact 33" tall and 12.6" wide, To anyone else with these tires in this size or having experience with them, is that correct? Or am I better off measuring on the ground and with a level & tape measure?

Now according to redrock4x4.com's metric tire calculation chart that ends up being 320/52/20 in metric. Doing the math method listed in the Gryphon book I ended up doing the following,

320 x 52 x 0.02 = 332.8. rim size 20" & 25.4 multiplied equals 508.

both added equals out to 840.8, which multiplied to the 3.1415 squish factor equals out to 2641mm.

Does this sound correct? My 255/70/17 circumference is 2478 so its definitely a boost up in size. Just trying to prepare my speedo and mileage count to be correct for my lucky day. Thanks for any advice. I cant sleep right because of this wait

 

Pi times diameter times 24.5 will give you the circumference in mm. 3.14x33x24.5=2538.69 mm. 24.5 mm per inch.

 

hmm, thanks man. The books telling me 25.4 not 24.5, wonder whats with that.

 

Opps, typing error. Dyslexic fingers...

 

it happens. So it is indeed 25.4.

 

My guess would be that figure of 2641 would be a little high. But I have racked my brain over this several times. I played around with mine quite a bit to figure out what is correct. Mine are 305 x 50 x .02 = 305 and 20" x 25.4 = 508, so 813 x3.1415 = 2554. I also layed a board on top of my tire and measure from the ground to bottom of the board and got 31.5 inches x 25.4 = 800, 800 x 3.1415 = 2513. I split the diference and plugged in 2525 in my edge and felt it was pretty good.

Well I sent my edge off a few weeks ago to be upgraded to a gryphon so I programmed all back to stock. The other day I'm driving along and the local PD had one of those speedo machines sitting on the side of the road that displays your speed as you pass. No other cars around so I knew it was my speed flashing on it. Anyway it showed I was doing 51 and I looked at my speedo and it showed 51. So I turned around went back and did it again at 45. Same thing dead on. Then once more I went back and did it at 65 (good thing it was slow time of day and no one around it was a 45 zone) and dead on again? I don't remember wha the stock number was showing in the edge but I do think it was lower than the 2525 I was using? But obviously Ford programmed it a little high to be on the safe side.

My stock tires were the same size as yours 255/70/17. If your new tires are a true 33" that is only 1.5 inches more than my 305/50/20 or about a 38mm difference. I am supposed to get my edge back in the next day or two so maybe I can check the stock setting and let you know for comparison? I am not saying your math is off this is just something that has had me scratching my head as well? I'm not 100% sure that formula works correctly?

 

I've put on two new sets of tires in last few months. Neither of them were Toyos, so I can specifically answer your question. However, I found on both sets that figuring the exact circumference didn't really make the speedometer dead-on. You probably know that Ford programmed the speedometers to read a few MPH faster than the true speed, but I wanted to program that out with my Edge when I put new tires on. On both sets, I started with the number in the Edge owner's manual (http://www.edgeproducts.com/product_...manual-277.pdf - table on page 21). I then drove at a few different speeds and compared the speedometer to my Garmin. I then adjusted the Edge up or down a few millimeters until both the speedometer and the Garmin were exactly the same.

For what it's worth, the Edge owner's manual says a 33 inch tire is 2632mm in circumference. Again, that's not specific to the Toyo, and I know your original question is whether or not the Toyo is exactly 33 inches, but I would start there and adjust if/when needed. You could also use the chalk method to determine circumference.

By the way, I think your new tires are one of the most anticipated new mods of the summer around here. I can't wait to see them.

 

If it helps, I am running 325/60/20 Terra Grapplers which Nitto says measure a hair more than 35in. I currently have my EVO set at 2802 because they recommend 2792 for 35's in the manual. So I guess your headed in the right direction. You can see the recommendations on the Edge site if you need to.

 

I sure wish I had a GPS to verify my speed. I'm hoping I am correct. I put it at 2636mm, and according to the gryphon calculator with a 5% squish thats what I came up with, instead of the previous 2641mm. Seems to be alright so far, but on the highly my 70mph used to be dead on 1940 - 1960rpm by the gryphon, its 100rpms lower now so that leads me to believe either the millimeter size is wrong, or the extra weight of the tires helps keep it moving and at a lower rpm ( which I pretty much figured is false.. )

I'll get photos up here soon. I love them so far, 79 miles on the so far.