This just doesn't seem right

[Stretch]

Jass and I have been talking about this for some time now. We are thinking my tach is inaccurate. My combo...

stock 340 with headers
stock 727 tranny with home grown shift kit
3.55 sure grip
P295-50-15 rear tires.

My speedo is with in 2 MPH @ 60MPH according to my hand held GPS.

The next three pics tell the rest of the story. Don't the revs seem high?

 

[Dr.Jass]

I never said I thought the tach was off. Those later B-body tachs are usually deadly accurate. I still think your converter's got an issue. According to the calculations I've used for years that have been correct, you should be in the mid 2,700s with a 295/50R-15, which is what you have (and nearly 3½" shorter than a 60-series 295 would be)... almost exactly where I've been saying they should be for several years now.

 

[Stretch]

Yep. 50's I fixed it. Stupid fat fingers.

 

[thrashingcows]

When I had my 73 RR, 340/727 with 3.23's and 225/70/15's, I used to run 3000RPM at 70 mph. So I think those number look fine.

 

[Dr.Jass]

My Challenger was in the low 3000s at 60MPH with L60-15s, 4.10 gears and a 4-speed. I have a dash pic of it somewhere with me going down the road around 60 or 70 per, but I can't find it. Yes, the 4-speed is more efficient as far as slippage, but with a factory converter it should only be a percent or two higher. The Black Bitch was 2,600-2,700 with 3.23s, 255/60R-15s, and an awfulmatic... when third gear showed up to the party. That car also had a 2,600RPM-stall converter in it, so I was just over stall speed. The Trans Am was around the same as The Bitch rolling on 245/60R-15s, with a 3.42:1 screw and a Super T-10. A 245/60 is almost the exact diameter as Stretch's 295/50. RPM calculations aren't perfect because they don't account for the compression of the tire with vehicle weight on it, but the difference it makes is minimal unless your tires are really low on pressure.

You could wire in an aftermarket tach temporarily easily enough to check the one in your dash. Yes, it's possible that your tach is off but the B-body tachs were manufactured by Faria starting in '71 and are some of the most-accurate factory tachometers of the era. Generally, if it's bad the RPM reading is wildly off, not just a few hundred RPM. 99% of the time, when they're messed up, it's the xener diode.

Have you ever checked the stall speed on your converter, Stretch? That might be an indication of a converter issue. The highest-stall production-line converters were around 2,200RPM, yours shouldn't be that high--more like 1,800. Another thing you might want to try is running your truck at 60, knowing you've got 3.55s in that as well, with the OD turned off. Of course, the truck's going to run at a lower RPM due to the taller rubber, but that along with your tire size would provide more information as far as the slippage you might be experiencing in the converter of the Charger. As I recall, the truck's converter won't lock with the OD off, but even if it does it's only a mild difference in RPM. I think it's worth checking.

 

[Stretch]

If I can remember who I last lent my timming light to I'll check the R.P.M.s with that. It's a Snap-on digital advance light with a tach feature.
It's been a while but I think the converter stalls at arouns 2200. I might be way off on that though. I cant remember the last time I loaded it.:shifty: I just cruse for the most part.

As for the truck, Yes it has 3.55's aswell and the converter doesn't lock when the OD is locked out. I cant remember what the R.P.M. is with it locked out. My truck has LT235-85-16's on it. They are the same hight as the LT265-75-16's that I had on it. I went with the 235's because I needed tires and these were free. A guy came into the shop and wanted new tires because he watched Dateline and his tires were "out dated" because they were 6 years old. The damn things were like new tread wise. Although they are really showing some side wall dry rot now. They are about 9 yeras old now.

 

[Not A Duster]

Not that this is any kinda scientific measurement, but that's higher RPMs than the Demon with the 3.55:1 and 275 60 R 15. I'm usually around 3000 rpm at 70 mph. (Measured on the GPS, not my wildly innacurate speedometer...)

 

[Diplomat_Wagon]

Even my Dakota with 3.55's and 235/75/15 28" tires is barely 3000 RPM in third gear with OD off and converter unlocked.