Hi Guys,

I just got my SCT SF3 and loaded my first tune on to my truck (canned I don't have a custom yet ). Anyway, I have 275/65/20 on my truck. So found a calculator that told me 592 revs/mile. I adjusted the tuner to 590 and went for a drive. My speedometer is reading about 5 miles faster than GPS and my ECU is reading about 2.5 miles faster than GPS. I'm using Torque to see my GPS and ECU speeds. Any suggestions? Should I just play with the revs/mile to get the ECU and GPS speeds together and just know that my speedometer is off a little? Doesn't sound right but I don't know what else to do.

Thanks,
Matt

 

Hi,
My advice would be to put it back to stock and take it for a drive. You should be about 3-4miles off at 60 mph. Putting it to ~33" aka 610 revs should net GPS = Speedo. I have a 2010 5.4L XLT, OEM tire size was 275/65 18 (32") I have 275/70 18(33") on now and the GPS = Speedo when leaving the tire size alone.

It seems like these trucks are about 3-4% off at the Speedo from the factory. I like my speedo telling me how fast I am going, so I left it at the stock tire size. It also seems to shift better (the reason I got the programmer in the first place ). I know my lie-o-meter may be a little off, but I don't really care about that because i can adjust that in my head anyway.

Hope this helps, and report back once you get it where you want it.

 

So I looked and found an Axle Ratio calculator, plugged in my "stock" tire size and axle ratio then my current tire size. It said my new axle ratio was 3.98 vs the 3.73 the truck is stock. On my programmer I set the axle ratio to 4.00 as the other option was 3.90. Now my Speedo and GPS seem to be insync however, the ECU Speed is off. I just went on the interstate and set the needle at 70, GPS was right around 70 however the ECU (on Torque) was reading 67.7. I'm not sure what's better having the needle correct of the ECU Speed correct. I would imagine if the ECU is incorrect it's going to calculate my Odometer and Lie-O-Meter incorrectly.

 

I have the Superchip and had to put my tire size in 3/4" smaller to get the speed o to read as my GPS.
Tires measured 33 1/4''
Had to put 32 1/2'' in Superchip.
Speed o is right on now.

I haven't check the ECU....I do have Torque, maybe I'll check that just to see what it says.

 

I'd be interested to know if your ECU speed matches. That would let me know I've still got something wrong!

 

my speedo is dead nuts on the GPS mph. Get some custom tunes from Mike at 5-Star and you should be fine.

 

I got my Speedo and GPS synced, it's my ECU speed that is off from both of those that is causing issues now. I was hoping to get those fixed before I got around to a custom tune.

I've heard good things on here about Mike at 5star.

 

Hey mgabel24,
How are u checking yr ECU speed.
I have Tourque on my phone. Not real familiar with it.
I see speed, but not sure if it's GPS speed or ECU speed.

 

All car manufacturers make speedos optimistic (3-7%). It's done for legal reasons.

Make sure your ECU speedo reading is correct and deal with the idiot-dial on the dash.

 

When you go to add a display, scrolling to the S's you will see "Speed (GPS)" and the next one is "Speed (OBD)". The Speed (OBD) is your ECU speed. Also while scrolling down, in the G's they have a nifty "GPS vs OBD Speed difference". I have a screen set up that gives me GPS top, ECU middle and the difference at the bottom.