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Topping out at about 90mph

kiwigtx said:
I don't see where the OP says his Tachometer is verified for accuracy?
65mph at 4,000RPM tells me that it's a factory tach, and those things can be a wild as a bucking bronco.
Click to expand...
The tach inaccuracy possibility has already been brought up several times in the thread,
oh observant one. :lol:
 
Easy to verify the speed. Use a cell phone with the Waze app. Now will the car not go past 6500? If it keeps pulling the tach is a likely suspect. If not: A the trans is slipping. B its in 2nd gear. C the rear axle ratio has been verified incorrectly. It’s that simple.
Doug
 
moparedtn said:
The tach inaccuracy possibility has already been brought up several times in the thread,
oh observant one. :lol:
Click to expand...
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Without pitchers, this thread is difficult at best.

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Topping out at 90mph and 6,500 RPM, screams out 4:56's to me.....or 13" wheels.

My old 383 Charger auto would do 90mph in second.

My old 70 Road Runner 440 + 4-speed would do almost 90mph in second.
 
What kind of vacuum are you pulling at highway
speed?
Would be weird to think exhaust being plugged
but you never know?
Usually with exhaust problem you wouldn't have
that kind of RPM, I'd still check.
 
Have a you got a buddy or wife etc who can drive alongside you at 60 mph in a modern car to verify your speed?
Maybe jack the rear end up and turn the wheels one rotation counting the driveshaft turns to verify the rear end ratio?
If both those come up OK then it points to the transmission slipping, unless the tach is out of whack.
 
It’s pretty basic really.
The gears and tire diameter have been verified.
The math doesn’t add up....... which means either:
- the tac is off
- the speed is wrong
- something in the trans/converter is slipping a lot

Well...... or aliens are involved.
 
I have found gps to be pretty accurate. We have used the Garmin and two phone apps and got within a mile per hour. Factory speedo should not be trusted for accurate info even with a scanner and test port on new vehicles. The old truck can be off as much as 15 mph at any given time.
 
delete the app...update your phone software...download a different app...then go for a drive and see what mph youre at



watermelon
 
Orange71SE said:
I have a 1971 charger that is topping out at about 90mph (monitored with gps app that is fairly accurate +/- 5mph) mild 440/727 with a cheetah full manual valve body forward pattern. Has a hughes 2k stall converter. Rear end is a dana 44 with 3.54 gear (46 splines on ring gear and 13 on pinion). Rear tires are 275/50/15, measure about 25in when car is on the ground. I know my rear tires are too small but i cant even get on the freeway at 60mph because this thing is cruising at 4k rpms. Any ideas?
Click to expand...
Tires are too small and toss that diff. It wont last behind a 440. Find a good 8 3/4 and if you want to calculate gear and tire size I use 25 divided by 335 times 4500 divided by 3.54 equals mph. ( tire size / 335 *rpm/gear ratio= MPH) 29 inch tire would make a world of difference. 355 are a good street gear 323 good hwy and cruse gear in that car. Good luck.
 
Orange71SE said:
I have a 1971 charger that is topping out at about 90mph (monitored with gps app that is fairly accurate +/- 5mph) mild 440/727 with a cheetah full manual valve body forward pattern. Has a hughes 2k stall converter. Rear end is a dana 44 with 3.54 gear (46 splines on ring gear and 13 on pinion). Rear tires are 275/50/15, measure about 25in when car is on the ground. I know my rear tires are too small but i cant even get on the freeway at 60mph because this thing is cruising at 4k rpms. Any ideas?
Click to expand...

Well,
Here is something "I" found out about my 70 Superbee....
All "Los Angeles" approved Mopars (383s and 440s, not A-12s, 6-Packs and Hemis) sold in the Los Angeles area were actually de-tuned...per factory.
And henceforth, were down on power.
My Bee was one.
My Bud's Roadrunner was sold out of San Bernardino/Riverside County and his 383 Roadrunner was never affected.
Has to do with something from the Distributor
BUT, as Cranky said, other things are also happening with your tach etc.
 
PRHeads said:
It’s pretty basic really.
The gears and tire diameter have been verified.
The math doesn’t add up....... which means either:
- the tac is off
- the speed is wrong
- something in the trans/converter is slipping a lot

Well...... or aliens are involved.
Click to expand...
This!!! One (or more) of the three. Pick one.
 
This thread seems to be going around the mullberry bush over and over. I think it's been well established there is no way you're turning 6,500 RPM at 90MPH and the transmission is in third gear unless you have some major slippage. I believe your GPS reading. I've never had mine show more than a MPH or two off one way or another. I agree with another poster that your rear axle choice is suspect and I would swap for an 8 3/4 but that doesn't solve your problem. Either your transmission isn't going into third gear or it's slipping like crazy. You need to figure out which it is. Something that hasn't been mentioned is what kickdown linkage are you using? A buddy of mine has a Lokar kit and his transmission will stick in second. It needs to be adjusted to get it to shift properly. Bottom line, you need to look at your transmission. Something in the driveline is costing you. The math doesn't lie. With the RPM you're claiming, it sounds like you're still in second. The "glitter" in your fluid is cause for concern though.
 
Does the oil smell burnt? When you pull the Trans dipstick after driving is it smoking hot? Thermal gun reading of the trans pan? I would expect some serious heat buildup if there is some sort of issue in the transmission making it slip that bad.
 
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Man I really feel for you; a lot of the comments on this thread are... confusing...

Getting only a 500 rpm drop on the shift from 2nd to 3rd, is the only info we need, assuming that is reality

At 70mph in 2nd tach should be roughly a little below 5k; the downshift to 3rd at 70mph should get you to roughly around 3300 rpm (per the calculator, and other comments).

You should be seeing more than a 1500 rpm drop at high rpm from 2nd to 3rd in a 727, right everyone?

I can’t take mine out for a spin yet, but other folks can

In 2nd gear with the specs provided (tires size, rear end, & 727 trans) per the calculator:
60 mph ~ 4K, 70 mph ~ 4,700, 90 mph ~ 6k

If your specs are correct, looks like you are stuck in 2nd gear; the values you posted line up with that pretty well.

The tach could be giving false info or the converter is doing weird stuff or a problem with 3rd could be the issue instead

Being that you tried 2 converters with the same result, the transmission seems like the likely suspect vs the converter

If you want to confirm the tach reading & rear-end ratio (doesn’t seem needed) you just need reads at a few speeds in first & second (above the stall speed), where it sounds things are makes sense.

The calculator is linked below

I believe a ratios are: 1st is 2.54, 2nd is 1.45, 3rd gear is 1

If it roughly lines up, the tach should not be a problem in what’s happening and the rear ratio is what’s claimed.

Feel free to post values if you want help with it; especially if they aren’t confirming your readings

http://www.csgnetwork.com/multirpmcalc.html

If the trans/converter is the problem I’m not your guy, but there should be folks in here to assist.

Do you have a standard 727 or the hd version with more bands? (3 vs 5 or something, right everyone?)

Wonder is it’s just a linkage problem with the shifter; am not familiar with your set up so take that with a grain of salt

Looking forward to what it turns out to be; and hope this helps.

(Everyone: If there are issues with my info above please feel free to rip it apart, however specially site the issue and the correction)
 
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