easy. just go into tyre size and bump it a bit then go for a drive and check against your navigator gps speed. Im running 35's and I think a went up to 34 in AE and that got me within 2kph against the gps.
https://www.ford-trucks.com/forums/9...-question.html
there is a PDF link in post#4

 

Brian,
Once again many thanks for the info.

 

Brian,
Just curious what RPM are you getting on the alledged 100 kmph

Cheers,
Patchy

 

I did not pay attention to RPMs, not used to having a
tacho, I'll have to check that.
Are you wondering if the diff ratio has been changed?

 

If the diff ratio has been changed it puts a different spin on things
My truck is stock standard and about 1800 rpm is 100 kmph

Cheers,
Patchy

 

Thanks.
There is no evidence that any work has been done on the diffs.
I will check the tacho next time I have the truck out for a run.

 

Why i ask i'm running simular tyre size you are, and to me, to be out by 12kmph seems to be alot to me if everthing is stock


Cheers,
Patchy

 

Oh Well, give it a run the open road
I still like to know what the rpm is

Cheers,
Patchy

 

1800 Im about 105 Kph

 

At 1750 I am bang on 100kph, but I have a manual (.72 top gear, instead of .71 in the auto) and 275/70R18's (a little bit smaller than your 315/75R16's)

IIRC, changing the diff ratio in these won't correct your speedo, as it reads off the wheel speed sensors.

 

ABS ? ya mean.
Im going to change my diff ratio's,i think brian said we have 3.73 gears,not sure what the next lower ratio is 4.11 ? but that's where im going,dont like my trucks response and fuel consumption now that I have 35's on.

 

Yes, ABS sensors.

4.10 is the next ratio (it's just the pinion teeth number that change).
I don't have first hand experience, but doubt 4.10's will improve your fuel consumption.

 

Its gotta around town,open road @ 100kmh's prob not,i reckon ive gone up 3 ltr's per 100 since I went from stock rims and 31's to these 35's..

Im going to take these 35's off and stick my stock rims and tyre's back on and do a fuel consumption test,i'll let you know.

Atm im doing 18.5 ltrs in 80E and I drive it like an ol lady

 

just checked again. 1800 rpm is 108kph (70mph). And Im the same with the fuel when switching to 35's and 5" lift. I have gone from mid to high 15's per litre to mid to high 17's. 18's when Im dragging anything that looks at me

Bruva 4.10's is the next step. But i would do some research. Alot on Powerstroke Army say and dont do squat.

 

Thats what AE does with the tyre size calibration. It adjusts the sensors.