tvette

Go to the tires web site , it will give you the revolutions per mile to set. also leave the rear end gear ratio stock do not enter anything. Worked for me gps is right on.

RajunKajun59

I thought I would give myself some 5.10s, lol. Just kidding.

Cooper doesn't post the revs though.

BC119

It might just be an SCT thing because my livewire TS does the same thing and it used to drive me insane. I have 35.1" tall Toyo Open Country AT2s and the speedo is dead accurate when entered as a 33.1" tire. My truck was 3 mph slower than the speedo when I entered the exact revs/mile given at Toyos website.

justjimmy

Mine does the same thing. My concern is the MPGs will not be accurate unless the speedo is. I'm **** about correct speed and MPGs. I also understand I have a truck...lifted and with taller, heavier tires. I just like to know what I'm really getting for MPGs and the "actual" speed I'm going.

Like you, I'll have to play with it to get the correct speed.

I use a Garmin GPS to set speeds. I assume that's the best way to verify actual speed?

dcfluid

The odometer is extremely accurate. The speedometer is intentionally running faster than actual speed.
Multiple GPS devices and trips confirm this.
Obviously a safe guard to prevent lawsuits from people getting tickets or crashing and blaming a too slow speedometer.
You can have an accurate odometer or an accurate speedometer but not both at the same time.
On the bright side Ford hasn't tried the too fast odometer trick to end your warranty or lie about fuel economy. Both more issues that bring lawsuits.

justjimmy

Not to be argumentative but why can you not have a accurate speedometer and odometer at the same time?

dcfluid

You can. Just not the way Ford has them set up. I don't know how you would get them in sync, the tuner can't do it.
Maybe a programmer that makes tunes can compensate for the difference.

justjimmy

I see.

Brokemillwright

I have 295/70/19 st maxx, I went thru the same thing. I probly reflashed the truck 8 times trying to get it perfect. My sidewall claims 34" and they measure 33.5, so I think that's what I ended up doing. Speedo is dead nuts/ +1 fast depending on how I'm sitting.

Also, I noticed the gps seemed to tale a few mins to completely catch up and track me. Most of the flashes were load, hit 70 fast and if it was off I'd relaod. I'd rather have my speedo being a hair fast for a grace period because I normally add 5mph to any speed limit

Cowpig

so the sct x4 tire sizes actually do something? reason I ask is that my truck came with 245/75-17's ... I put the 295/70-17's on recently. that should be about 33.3" I've tried that setting, and up to 1.5" smaller or larger and nothing seems to change the speedo - it's always 5-7mph off (ie; 83 mph on speedo = 75 mph on GPS). I let it get to a speed and set the cruise then give it a few miles

I was thinking the x4 wasn't actually communicating the tire size to the truck/pcm since smaller, correct, and larger than actual wasn't having any affect.