I have read about how the speedo is off. On my CRD i have to do 75mph on the dash to be really doing 70 on gps. Im not the smartest guy, but I know that's 7% off. Whats the effect on mpg?

Short version absolutely none. Longer version the error in the indicated speed on the speedometer is not related to any indicated error in the odometer and neither, apparently, have any correlation to any error in the indicated MPG on the EVIC. Hence each reading may be off in a different direction even with stock tire dimensions. Of course anything but stock tire dimensions adds another source of error.

Solutions 1) use you GPS to figure speed and odometer reading correction factors to avoid tickets and to accurately calculate mpg or 2) I've "heard" the stealership can, within limits, electronically adjust your indicated (not EVIC) speed and odometer readings to be less inaccurate.

FYI the current GDE tunes do not "fix" the EVIC error but do make it somewhat less dramatic.

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Sold to LOST member my 05 Ltd, GDE Stg II turbo + TCM tune, SunCoast TC w. Transgo kit, Steiger window regulators, Samcos, Fumoto valve, 2nd gen filter head with Lub. Spec. bleeder, Hayden clutch & 11 blade fan, inverted spare, P-1 battery, BF Goodrich Long Trail TAs, Etecno1 glow plugs, timing belt at 50K miles/8 yrs

Thank You. I'm just really trying to figure out the mpg. I am the second owner, and the the original owner said the best he ever got was 20mpg. So far, Iv done every single thing in the newbie post, and the best iv got on a tank is 400 miles, or 20mpg. I'm considering the GDE tune but not sure it will really help, as much as i drive. I mean 25 would kick booty, I just don't see it happening.

Actual mpg varies based on a bunch of factors:
1. how you calculate it. Ideally use a GPS unit on at least a 50mile drive, flat is better as is straight, to get an accurate correction factor for your odometer (e.g. X odometer miles * factor = real miles with your current tires).
2. how accurate the fuel pump gauge is and how you fill your tank. You cannot control the former although pumps are inspected and required to meet a certain standard. As to the latter you need to fill your tank to a consistent level but the only way to be sure of that is to fill until you have clear unfoamed fuel at the top of the filler neck.
3. winter vs. summer fuel and bio diesel vs. dead dinos. Winter fuel and bio over about B-10 cut into mpg.
4. engine temp. Is your temp gauge needle, when warmed up (ca. 5-10 miles depending on speed and ambient), dead vertical or no more than 1/2 of 1 tick left of vertical. If lower your tstat is failed or failing open and your engine is not operating at peak efficiency. Get new OEM tsat or do the in-line version.
5. type of driving and speed - in town and over 62 mph (ca. 1800 rpm) cut mpg - it's reported that 70mph costs 10% mpg and we are driving an aerodynamic brick
6. engine rpm as distinct from speed - if you have had the F-37 flash done you need to run speed up to ca. 64mph to get into top gear/overdrive before you can drop back and hold 62 mph at 1800 rpm.
7. altitude/terrain
8. tire pressure and rolling resistance
9. are your front brakes dragging due to "fat" rubber slider sleeves.
10. payload weight of stuff in vehicle and towing.
11. clean MAP sensor?

I'd go on but... In the real world a post F-37 CRD driven moderately should get ca. 18-19 mpg in town (10 mile commute, top speed 45mph w. average like 15mph) and 27-28 highway (62mph as above). Post GDE EcoTune all other things being equal I saw about a 20% improvement in those mgps

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Sold to LOST member my 05 Ltd, GDE Stg II turbo + TCM tune, SunCoast TC w. Transgo kit, Steiger window regulators, Samcos, Fumoto valve, 2nd gen filter head with Lub. Spec. bleeder, Hayden clutch & 11 blade fan, inverted spare, P-1 battery, BF Goodrich Long Trail TAs, Etecno1 glow plugs, timing belt at 50K miles/8 yrs

You need to establish the calibration factor for your odometer against a known source such as a GPS or highway markers. For mine it's 5.26% so I simply correct all odometer readings by this amount to get the true mileage before calculating mpg.

Can we assume that you're new to the CRD? If so, and depending on where you live, i.e. colder climate, an uncorrected 20mpg may be absolutely normal for this time of year.

Here is my mpg since new which clearly shows the seasonal trend. BTW, the ~1mpg drop between Jan-Sep 2011 was because I was running with a bad thermostat:

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