Ford Power Stroke Nation banner

1 - 8 of 8 Posts

·
Registered
Joined
·
2,761 Posts
Discussion Starter #1
I re-programmed the speedometer when I went to a 285/75-16 tire. I installed a Nitto Terra Grappler. As per Nitto's specs, the diameter/height is 32.76.

PSOM Formula:
20186/tire height in inches = XXXX then XXXX / .6666 = program code

The math
20186/32.76= 616.17826 / .6666 = 924.35982 (Program code is 924)

So I entered 924 into PSOM. Went for a ride and tested the odometer against the highway mile markers. The speedometer was off by a tenth....hmmmm

So I went home and used a transmission gear, rear gears, tire height formula = final drive rpm. ( RICHMOND Street Performance Calculator )
4th gear on an E4OD is .71
Rear gear is 3.55
Tire height is 32.76
Speed is 70 mph

I entered the values and the computed rpm at 70 is 1810. The rpm displayed on the AIC was 1776 at 70 mph, hmmmmmm.......
I set the cruise control at 70 mph as per the speedometer. The GPS unit was saying I was doing 65/66 mph, hmmmmmmm........

I did not try speed up for the AIC to go to 1810rpms. I know if I did, the speedometer would have gone to around 75ish.

So I went home, jacked up the rear of the truck, put a flexible measuring tape around the rear tire, put the truck back down and came out with 102.75" circumference / 3.14 = 32.70637.

Back to the original formula for PSOM and the program code is 925 hmmmmm

I did not enter the 925 code yet.

Does anyone know if it would make any difference being off by one number? Or should I forget it and continue driving it with first code I entered. I have used up already 2 memory places. I think there is a total of 6 "lives" I can use.

Opinions???????

Billy T.
[email protected]
 

·
Registered
Joined
·
69 Posts
off by one number will make a slight difference but prolly not worth losin a "life" on the odometer.I recalibrated min and its about 3mph off but im not sure how many times its been done.Ive done it 3 times myself and the previous owner may have done it.So im going to live with it until a bigger tire change.>>>>>>brian
 

·
Registered
Joined
·
69 Posts
also i basically have the same height tires and the same gear and at 70 im running like 2100-2200 rpms.I do have a manual trans and im not sure how the ratios compare.>>>>>brian
 

·
Registered
Joined
·
69 Posts
Perhaps i should know this but what do you mean by "lives"??
you can calibrate your speedo 6 times,then you have to get a new one........so keep track of your speedo's "lives"......:D
 

·
Winners never quit!
Joined
·
502 Posts
Never heard thatbbefore.. so after the 6th time does it just quit working or??
Mine has never been re calibrated, to my knowledge so i should be good lol.
 

·
Registered
Joined
·
3,025 Posts
"Lives": You are only permitted to recalibrate the speedo so many times (6 I believe). After that it will not reprogram. As far as the tire height goes, you can put a chalk line on both the street and tire, roll the truck for 10 revolutions and measure the distance. Divide that by 10 and you should have an accurate rolling circumference. Divide that number by pi (3.141593....) and you get a rolling diameter to use in your calcs.

Just a note on the formula that everyone uses for the code calculation: The factor 20186 I think is incorrect (JMHO). The factor should come from the following: (5280 ft/mi) X (12 in/ft) / (pi). Do the math and you get 20168 (not 20186). It's easy to transpose the 8 and 6(which seems to me someone may have done). It really doesn't make all that much difference in the calculation (roughly 1 code number less for the diameters we're talking about), but as an engineer it's always bugged me.

Cheers!
 

·
Registered
Joined
·
319 Posts
how do you reprogram It?
 
1 - 8 of 8 Posts
Top