I am getting a set of Coopers 285 70 17 and was wondering if anyone knows the correct number to use for the speedo? Their website doesnt say anything about it and I would like to have it changed before I take delivery of new truck.

Thanks
JIM

 

I used 616 when I put my Coopers on, seems to be right on the $$. If anybody wants it I made a S/S that all you do is punch in your tire size and it spits out your pinion factor.

Tom

 

616 sounded kind of low to me. Toyo's site shows 636 for the same size.

LT285/70R17 121S
(33x11.50R17) 301320 7.5-8.5-9.5 49.6 17 32.76 11.50 15 2755 636

 

My dealer reset mine after I added 285 BFGs......to 630 I believe it was.

Discount Tire Co. showed the tire to be 635.89 revs per mile or something like that. The mechanic that did mine made a mark on the ground and my tire and i did one complete revolution and he got out a tape measure to measure it and then divided it into a mile etc.....and came up with 630.

 

This is the formula I used.

For diameter:
(Tire size X aspect ratio / 25.4 = sidewall height)
285 X .70 / 25.4 = 7.85

(Diameter = 2 X sidewall + rim)
7.85 X 2 = 15.7 + 17 = 32.7” (Tire diameter)

(Circumference = Diameter X Pi)
32.7 X 3.14 = 102.68” (Tire circumference)

63360 (5280 feet X 12 inches) / 102.68 = 617 (New pinion factor, depending on how you round this may fluctuate up or down a very small amount)


The only reason I used this method instead of actually measuring as mentioned above is I assumed a Engineer at Dodge/Chevy/Ford more than likely would not go measure the actual tire under load and thus the stock pinion factor was figures as above. I could be 100% wrong. I did check it over a 5 mile stretch of highway using the mile markers and it looked to be just about as on as one could want.

 

BFG's AT shows 634 rpm, and Toyo AT at 636, so I'd think about changing the 616. Just can't be right. My 35" Toyo AT's are 596.

 

Mine is set at 634 for BFG 285/70/17 AT and is spot on.

 

My brother has a GPS, I'll have to get that joker and run a test or two now. Now I am curious since there is such a wide difference in what the manufacture says and what doing some math says.

 

Is this something we can set, or is it a dealership trip?

Bill

 

I am really confused now. Over on the TDR they are sayin 616/617 as well as 1stRam but their are others saying 636. I am picking up the truck Friday and would like to have the number before I take delivery.
Maybe I will call Cooper and see what they say

Thanks
Jim

 

630 with bf goodrich ta/ko"s- i am dead on

 

Toyo 285's on mine set at 636, and I have Garmin GPS. It is perfect.

 

Well I called Cooper and they said 636 was the number to put in for 285 70 17 so after taking delivery I ask the selling dealer to make adjustments and they said it wasnt possible--------their DRB was able to put tire size in only.
After getting frustrated I went to local dealer and told them I wanted 636 and after a few minutes they came and got me showed me the DRB and it only takes tire sizes and not a number. I also had him silence the seat belt chime while I was there.
I asked about the hi idle feature and he lookrd it up enabled it and it works----but not like I have read on the forum. I was stopped at a store with truck on and after a few minutes it idled up on its to 1000 rpm-----cruise was not on.

JIM

 

duece- trucks has two high idles. 1) the automatic high idle, like you experienced- when it is cold enough outside-the truck will automatically bump up to a 1000 rpm's. 2) the manual high idle - which is set through your cruise control buttons-nice feature! (dealer needs to turn it on) also- my dealer was able to enter the pinion factor on his scan tool to recalibrate my speedo- find another dealer!

 

Guys, Let's see.... the tire is 285 70 17 New. Your speedo will be correct for the first mile and will get worse as time goes on. Those tires have about a little over 1/2 inch of tread on top and 1/2 inch tread on the bottom. The tire is currently 17+ 70%x285x2/25.4 =32.7inches in diameter. When it is worn out it will be 31.7 inches in diameter. Do you want it to be accurate new, worn out, or on average. What size were your tires stock? Mine were 235 80 17 which is .1 difference from your worn out size. My outside tires are 265 70 17 and are .1 smaller than my stock tires. Your speed will only be out about 3% or 3 mph in 100mph I think. I wouldn't sweat it. I'd enjoy that my warranty is going to last 3% longer. ks