what is top speed with 391 gears?

scottish440 · Post by **scottish440** » Sun Mar 25, 2007 11:07 am

I am putting a car together and will be using a 391 posi rear
and a 727 with a built 400 BB

My question is: What will be my top speed in 3rd gear?..

..or my feel like top speed...you know with these big block it
seems like you get to your limit sooner than a small block!!

72GY9Runner · Post by **72GY9Runner** » Mon Mar 26, 2007 10:54 am

Depends on what your rear tire size will be, and what you want to have for maximum RPM.

My wife's Cuda with 205/70/14's, with a 3.91 diff has been to about 100 mph, but it was screaming pretty hard by then. There's no tach in her car so I'm guessing it was between 5500 and 6000 at that point.

I also know that my Valiant revs about 3300 @ 60-65 mph with the same ratio and tire size as above.

I plan on running the same gears with my Runner, but with a 275/60/15 which should get the top end revs down quite a bit. Not sure what the top end will be yet though.

JosephGiannini · Post by **JosephGiannini** » Wed Mar 28, 2007 11:21 am

http://www.juric.org/mopar/tools/topspeed.php

72GY9Runner · Post by **72GY9Runner** » Thu Mar 29, 2007 11:23 am

That's a cool link.

Any idea what ratio the transmission would be? I assumed 1:1 since they aren't overdrive. and got a top speed of 127 mph with my chosen tire size and 3.91's. If I keep my 3.23's, I end up with a top speed of 149mph at 5800 rpm.

And my Cuda on 14's will apparently top out at 119 @ 6000rpm.

JosephGiannini · Post by **JosephGiannini** » Thu Mar 29, 2007 2:42 pm

Actually they make a over/under drive gear vendors or cope racing transmission
and the gear ratios in a 727 are the following

1st 2.45
2nd1.45
3rd 1.00

if memory serves me right

however if you look for acceleration you want a larger gear
3.55,4.10,.4.55,4.75,5.25,5.75

But it wont give you high end top speed it will reduce
The capability to reach top end. However acceleration is vastly improved
With acceleration fuel mileage decrease.
So I like a 2.71 gearing myself
If I could i would get a custom 1.7 or 1.6 myself

http://www.gearvendors.com/
http://www.coperacingtrans.com/odunits.htm

pigman · Post by **pigman** » Fri Mar 30, 2007 11:20 am

My 472 CID with 4:10 / 4 speed / tops out at about 104 MPH, but it does indeed get there fast.

landon1 · Post by **landon1** » Sat Mar 31, 2007 1:39 am

question...i put in the info and used the 1:1 for transmission final ratio and that isn't right at all....i've had my 440 satellite pushin 170 with my buddy in a hopped-up newer trans am behind me...so why does it say 150 something??? does this not consider engine power or does that matter? i said 3.23 rear gears, cuz i think it is still just stock satellite under there
thanks

Post by **Eric** » Sat Mar 31, 2007 5:47 pm

I'd guess you were no where near 170 mph. The speedo doesn't even go that high, and I wouldn't want to think about the aerodynamics and limitations of a stock suspension....

landon1 · Post by **landon1** » Sun Apr 01, 2007 2:19 am

idk...that's what his speedo said...not necessarily stock either...at that time, the torsion bars were turned down enough to where it sat lower than my other friends iroc z28...and the speedo doesn't go that high...that's why it's now broke...it snapped and went down to zero - hasn't worked since

72GY9Runner · Post by **72GY9Runner** » Mon Apr 02, 2007 11:57 am

If the car has had a differential gearing change, and the transmission speedometer gear was not changed to match, then the speedometer will read incorrectly. For example, if you changed from a 3.23 ratio to a 3.91 ratio, then your speedometer will show a faster speed than you are actually travelling. In addition to that changes in tire size from factory spec will also affect the reading. If the car is slammed on the ground like you stated, I suspect he is also running a low profile tire. This would drive the speedometer reading even higher.

Just my 2 cents.

JosephGiannini · Post by **JosephGiannini** » Mon Apr 02, 2007 3:22 pm

to get top speed accurately you can get a radar/laser reading.
or you can do stopwatch mathematics.

http://www.hypertextbook.com/physics/me ... /velocity/

landon1 · Post by **landon1** » Mon Apr 02, 2007 4:52 pm

i think his was lowered, but i was talking about my car being slammed to the ground...it's a cool look, but i got rid of it when i switched to sidepipes

JimF · Post by **JimF** » Thu Apr 12, 2007 9:29 pm

Keep in mind that unless you have a lock up torque converter, you're going to lose about 10% of your rpm's to slippage. And at speeds above 110 mph you're going to lose a bunch of power just to fight wind resistance.

I have a 71 Satellite running 370 hp, 410 torque, 3.23's, 245/60-15 tires, and a gear vendors overdrive that gives me a top gear of 0.78. And I think my top speed would be about 150.

- Jim

Road Runner Nest

what is top speed with 391 gears?

what is top speed with 391 gears?

Top Speed