trans rear end help

[JMULTREK]

i have a 67 f100 that has been modified with a 351 c engine and a c6 trans. the top speed on it is around 45 mph with the engine at high rpm. it feels to me that it needs another gear the trans seems to chg all 3 times it should and is not slipping. the kickdown rod has been removed so it wont go into passing gear so it shouldn't be stuck there. im not sure where to look next but it needs to go on the highway i have a 15 mile commute to work and all of it is interstate. i drove it once and i was so slow i thought i would get run over by traffic. can anyone help me with this and give me in idea of where to start looking.

[sideoilerfe]

put it in first, drive it, then shift into second and note RPM. Then go to drive and note RPM again. It sounds like you're not going into 3rd. if you are then it has to be slipping unless some put 4.56 gears in your truck.

[MadMaxetc]

Ya it sounds like something is stuck in the valve body and is keeping you from shifting into 3rd.

Make sure you are full on fluid (check it when it is warmed up)

If it still does it then I would take the pan off and clean the Valve body and make sure all the springs and check balls are moving like they should.

Don't forget a filter change whlie you are there.

[JMULTREK]

Well i finaly got around to checking on my truck, i filled trans with fluid and took it on the road i shifted the trans by hand it shifted into each gear but still went only 40-45 mph. so i guess my next thing is to pull hogs head from rear end and check gearing i was told to count the teeth on large gear and small one and divide the larger one into the smaller and it will give me the gear ratio for that rear end so heres hoping thanks for all the advice if you have any more let me know. jim

[binder56jd]

i think there is a formula using the tire size and driveshaft rotations that would keep u from having to pull it to find the ratio

just won't come to mind-crs strikes again

if i can find it --i'll post it here

[binder56jd]

http://www.getahelmet.com/jeeps/tech/findratio/

[JMULTREK]

Ok i finally pulled rear end it looks to be a 3.25 gear ratio from what i can tell the large gear has 39 teeth and small gear has 12. anyone with an idea as to what i should do now? thought of changing whole hogs head but not sure what to look for in the gearing, any help would be appreciated thanks jim

[spartman]

with a 3.25 you should be able to top out at about 100 mph easily.

Sounds like your tranny has issues.