1968 442 convertible with TH400 and stock (assumed) anti-spin rear end. About 5500rpm at 80mph. This is too high. Is this normal? Should I change ratio on the diff? The 400 can't take it sustained at this high rev.

Bill

 

What is your rear end ratio?

 

Sounds very high is it going into third gear? I mean I guess you would have something like 4.56 maybe 4.33s??? Back there ??? I dunno there is a formulae to figure it out but f iguring your close to redline and top speed is 80 , I think you need to make sure the tranny is shifting into 3rd or reduce rear end gearing if your not racing etc, IMHO or add overdrive

 

It should not be running 5500 rpm at 80 mph. You need to find out what your rear end ratio is and if you have a higher stall converter. Are you sure third gear is not gone?

 

I went over to www.Richmond gear I put the figures in at 80 MPH and 5500 it says it estimates at 5.73 rear end gear so unless you have a car geared for the 1/8 mile with a SBC in it lol there is something not right with the tranny as 70 holds said maybe third gear is wiped or it isn't shifting into 3 rd is the selector working properly? Do you feel 3 distinct shifts? Another good point brought up was the converter . There is a code on the axle tube that would tell you the ratio it is suppose to have, or you could jack up the rear and count revolutions but something don't seem right not much help I know my friend but we are trying

 

Check your tach. My motor should be at 3300 at 75 mph and my tach reads 4500.

I'm pretty sure if my tach was right I'd be blowing up my motor everytime I hit 5500 rpm's

 

If you're not getting 3rd gear, it is likely the transmission governor. However, check the kickdown switch first; that is an easier fix.

 

Definitely getting two shifts (first to second and second to third). I'll look into the tach calibration. It's an original tick-tock-tach. I noticed the rpm value jumps up about 15% when I turn the lights on. Grounding problem?

 

what does you ears tell you? does the engine sound like it is at redline ( wot ) or does it sound like your doing 80 MPH with everything normal??? the wrong tach reading was an excellent tip btw, I would think your ear should tell you pretty close if your rpms are that high at 80 mph..

 

Clearly the tach is inaccurate. There is no way you are turning that many RPM.

Temporarily wire in another aftermarket tach & you will see some more realistic rpms.

 

You've obviously got some problems, as when running a 3.73 axle, I was doing about 3500 rpm at 70 mph, in top gear of a TH 350 tranny. If to much converter stall, car will hardly move from a dead stop, until you get the rev's up.

 

My tick-tock-tach was way off. I calibrated it best I could, (adjustment screw in the back) but it was too sensitive to make a fine adjustment. A small turn caused a big jump in RPM. Check your RPM with a quality tach before you do anything.

 

If you do indeed end up needing less gear.I have a restored 3:08 posi rear from another 68.You could cruise all day long with that.

 

Alot of aftermarket tachs have a small electrical micro-switch to be switched for 4, 6 or 8 cyclinder motors......check into that..