What are your RPMs @ 60mph w/ OD on ?

 

Using Bill's calculator with a 3% squish, you get 649 revs per mile. If you increase the squish to 5%, you get 655 revs/mile. No squish is 640. This agrees with Discount Tire's calculator and the formulas in the PHP manual.

The squish will be dependent on tire pressure and vehicle weight. More weight and less pressure increases squish. It's also the rear wheels that count, because the speed sensor is back there somewhere. There was a rather heated discussion quite a while back regarding exactly where it is.

I suppose tire construction could have a slight effect on the squish too.

- Jack

 

Hey guys,

I just got my new wheels and tires (pics to come soon) Anyway, I am trying to adjust my speedometer with my X Cal 3. I went from a 275/55/20 tire to a 275/70/18 with a 18x9 wheel. I am trying to figure my revs per mile so that I can input it. Trouble is, when I've gone to a few sites with tire calculators, I am not getting the same results. What's up??? How do I figure this out? I tried it on 610 and it is showing about a 3+ mph difference. Should I try a 605????

Also, to those of you with X Cal 3's Can you only adjust the revs by increments of 5 only? I did not see anywhere where I could change the digits by any less than that. I hope somebody can chime in and help

 

Oh yea, btw...

I am running 3.73 gears if that matters?

 

610 is about what you have with perfectly round tires (no squish due to weight). Try 620, which is just a bit over 3% squish. Don't use 605, that will make your error worse (fewer revs per mile - so the computer thinks your truck is traveling farther/faster with each revolution).

- Jack

 

You think 620? What would 615 give me? I am just wondering if I should try 615 first or go straight to 620? Thanks for the help!

 

it's all relative ain't it?
I mean come on "squish" ya maybe it does but not a sunstantial amount

 

Ok, so I found out that I should be using 624 as my number for my revs/mile. Problem is, the X Cal 3 can do either 620 or 625. What would you all recommend? Thanks again!

 

Actually, I calculated 617 with 3% squish (which is closer to 615). The 620 figure is a 4% squish. I told you to try 620 because you said the speedometer read 3mph fast, and I know you have to make a fairly substantial change in tire revs to move the speedo much. But, think of what I'm suggesting here - 615 is only 5 more revolutions in a whole mile! 610 is 10 additional revs. Neither is a lot in the great grand scheme of things. Try either one - you'll probably be pretty close to accurate. Choose the one that's best. You can't hurt anything at either setting, so experiment.

You might also keep in mind that Bill Cohron says (I've confirmed this with a GPS) that the truck's speedo reads 2mph higher than true in the 70mph range. I have a Gryphon, and when it and the GPS say 75, the speedo says 77, and, the truck's odometer is dead on, so it's sensing distance traveled correctly. Maybe Ford doesn't want us to be caught speeding.

- Jack

 

Thanks Jack! I will try 615 and see how that goes. If I need to, I'll go to 620. Again, I appreciate the help!

 

Well? with 615, I seem to be off about 3 or 4 mph. I guess it's time to try 620 then. Is there any chance of fixing this to be dead on or do I have to live with the fact that I will always be a couple of mph's off with new tires??

 

mine still not right. i talked to guy who owned the truck and he says its all stock. now in the vin number there is a h9- and i read it means it has 3.73 gears but the sticker on the inside door says 3.55. so not sure. i give up, at least i know i am showing 5 mph slower than what i am running!!

 

I just found Bluejay's post where he lists the axle codes:https://www.f150online.com/forums/tr...tial-pics.html and H9 says you DO have a 3.55 differential. You should also have a tag on the differential and the first number on the bottom row should confirm the 3.55 gear ratio (the "3" may be punched out when the tag was mounted, but the "55" should be visible).

So, if you set that gear ratio to 3.55 and a tire rev setting of about 650 makes you 5mph slower than actual, you have to reduce the number of revolutions in a mile so that the PCM will think the truck has moved farther per revolution. I'd try 640 and see what happens.

- Jack

 

i have it set on 3.55 right now. With it at stock value (whatever that is). If i set it on anything other than that its way off and says im running 70 when i know it cant be any faster than 55 or 60. I drive the same roads every day to work and the speed limit is 50 so ive got a pretty good idea at waht 50 is and it shows 70. I know im not going 70 mph!

 

Please don't mind me, I was poking trouble in the wrong direction. and please don't feed into my idiocity!!!
Squish is a real factor. Calculate for it and you will see!

BUT.... BUuuuuutt..
Do not.. factor squish if you fail to check your tire pressure regularly, or have less than 30% tread life. C'mon that would be like having virus protection and no internet!!