RajunKajun59

So, I measured my tires standing diameter as it sits on the truck, which is 33.3-33.5" between all 4 tires. as far as I can see there is some serious squish from the reported specs. I am running 68psi.

If you calculate the revs per mile it comes out to 602.5 or so, I used both the 600 and the 605 setting on the SCT and it is still ff by 2-3 at 70. seems good enough right.(Speed-o-meter is faster) Seems weird I would need to go lower than a 33" tire to get it to read correctly.

Sadly my OCD is kicking in, and has me wondering should I fine tune it, will it affect anything (better shifting, slightly, maybe), is it worth it?

In the end OCD is CDO and will win sometimes, LOL

Anyone else with 295/70-18s and an X4 can you chime in on your setting, thanks.

My speed-o-meter was off this much from the factory, too.

RLXXI

Ford specs on speedo are +- 7 mph so at 70 you could be at 66.5 or 73.5 and be in spec. It's also a little known fact that cops wouldn't stop someone unless 5+mph over the limit. Last couple leo's I asked said they won't normally stop someone until they get between 7-10 mph over.
.

Sirikenewtron

Mine is about the same,
- 2ish at 70 mph from what speedometer actually says, I too had to go down in size quite a bit on my X4 to get it that close. I'm also ocd, my stock tires were dead on

johnday in BFE

iTrader: (1)

Rajun, I've got 315/70/17, according to the BFG website, I'm at 34.4" diameter. Again, according to them, I'm turning 605 rpm as well. How did you figure your rpm?

Shakkrr

I put on 305 55 20 and was told by sct a number to plug in which was way off and also told a number for them from discount tire which was way off, I ended up just changing 5 revs at at time between the two and kept a log and wrote down the speed and how far off it was, I used garmin gps for true speed which I knew was right to set the revs right. I found on mine it was about a 1/2 MPH per 5 revs took a few days to do.

RajunKajun59

Measure the standing height, from ground to top of tire while loaded. That is your loaded Diameter, multiply by pi then divide by 12 to get your loaded rolling circumference in feet, then divide 5280 feet by you loaded rolling circumference to get your revs per mile.

johnday in BFE

iTrader: (1)

I'll try that, it might be interesting to see what I come up with, versus BFG's statement. I'm going downstate tomorrow, I'll see if my wife will give me a readout on her GPS as well. I know, I'm likely the only one here "without" a GPS!! LOL

I think you mean divide 5280 by the loaded rolling circumference? I estimated 34" tall tire, and came up with, 492 rpm. A long shot from what BFG says, 604rpm.

LOL, oh dopey me!!! I redid my calculation, and used the real pi, 3.14. 593rpm. I forgot that pi was 3.14. Haven't been in school for 47 years!!!

TJPlatinumEB

Have you looked at the tire manufacturers website to see what they show the revs per mile to be?

I only ask that because I recently put 305/55/20 (33.25" roughly) Toyo AT2's on my truck and Toyo's website says 650 revs per mile for that tire. I put that in the SCT and am spot on all the way up to 80+ mph on GPS.

Also with your tires being almost the same size as mine, I would think you would be closer to my number than the 600 or so that you are at now.

RajunKajun59

Yea, whoops. Thanks for catching that. - Going fix it.


Mine are actually 34.5 tall, loaded they are 33.5. Cooper doesn't report a revs/mile number interestingly enough. I am just going to playing with it until it reads even w/ my garmin.


Interesting


I wonder if the shop changed the my speedo and din't say anything, but I would think my SCT which had the 93 Perf/Tow tune loaded, would have thrown an error if they did.

woychc

For my 276/65-20s I was the same, couldn't get it quite right. I think for the 275/65-20 it should have been around 600 revs per mile, however that didn't work so I emailed 5* and they said to use 635. It's bang on using it.