6.7 and 7.5

[keithm]

Will the flange that is bolted to the front of the pinion interchange between these two axles?
When I installed the v8 and the aod, I thought the 3.45 7.5 would be ok to use. Problem is engine is turning 3400 at 75mph. I was given a 3.08 6.7 which was a simple swap, but the flanges have a different pattern, if they interchange I'll do that, if not I'll have to find the yoke that bolts to the flange and see about a joint that will work.

Thanks

 

[Mac]

With that RPM at that speed, sure would think you're OD is not working to start with. No way would I swap down to the 6.7" especially with the V-8. People with a good V-8 can ruin the 7.5"
Dave

 

[85_Ranger4x4]

Are you correcting the speedometer as you mix and match rear gears?

Tach is set for an 8 and not a 4 or 6?

 

[Captain Ledd]

That does seem awfully high. I'm siding with the others that something else isn't right.

Granted I'm running a M5OD, but even with my 29" tires and 4.56 gears I STILL only turn 3,000 @70mph - in overdrive.

 

[keithm]

Tach is V8, 7.5 was a 3.45, 6.7 is a 3.08. This is a stock 302, only changes are a 4 barrel intake and afb carb, Idea is to keep it clean, simple, reliable. Flange yoke was standard 1310 u-joint, bolt pattern was slightly smaller, drive shaft was 1.250 short, that was fixed with a spacer from fab tech. I decide on staying stock after a friend put a 347 in his with an aod, a 9 inch posi rear end. The truck was nice but just wouldn't hook up, mine is an everyday driver that I can show. I don't drag race and I don't haul heavy loads so 6.7 works fine.
I have been checking speedo with chase car, and changing pinions accordingly.
I'll be doing tach and speedo testing tomorrow and will list my findings.

Trans is working perfectly shifting smooth and thru all 4 gears.

 

[85_Ranger4x4]

Something is fishy here...

With 28" tires my F-150 that had 3.31 gears and a 4R75W (fancy AOD) was turning nowhere near your RPM.

With 3.73's, 31's and a THREE SPEED I am turning about 3k at 60mph in my Ranger. Never had it up to 70 but I don't think it would be turning much faster than your setup.

 

[keithm]

Got the 3.08 installed and have been doing some mileage test, speedo is correct, at average of 55 mph 18 mpg seems to be the average. The tach is showing 2600 in od. At 75 3200. Still higher then what I thought it would be. I'm at a point where I'm going to put a second tach on it and see if they are anywhere close

 

[Mac]

Better get that other tach and check, something is way off. Either tach wrong, not going into OD or you are running 15" tires. I'm running 3.73 gears and at 55 mph in OD, running about1600 rpm and at 72 mph running 2100 rpm. Check the chart in the library.
Dave
http://www.therangerstation.com/tech_library/gearandtirechart.shtml

 

[Captain Ledd]

Your Tach, speedometer or transmission is not correct. I just ran some calculations on a spreadsheet that thus far have never proven themselves incorrect across a good dozen vehicles that I've checked them against.

IF it's in OD. 75mph, in OD, 3400RPM, 3.45 gears, AOD trans - you would be running a tire right around 17" in DIAMETER.

Before you do any more part swapping and waste $$$ and time, re-check some things.

Tach hooked up correctly, if trans is going into OD in particular.