Tom in Austin

Pondering the other day ... 3,000 rpm on my car is good for 80 mph and 4,000 (straight up on the tach) is 110.

Is this relationship linear, so that 5,000 rpm would be + 30 or 140mph? And 6,000 is 170? I think GTS tops out slightlyl above 170 mph so that seems somewhat consistent with the redline rpm.

76FJ55

You can assume it is linear. there will be a very slight decrease due to tire slip ratio as more drive force is required with increased wind drag, and if you have an auto you will also loose a little to slip throught he torque converter. if you disreguard the slip speed is proportional to engine RPM... speed = rpm X trans gear ratio / final (differential) ratio X tire circumfrence (in inches) X 60 (converting min to hr) / 63360 (converting in to miles).

3000(RPM) X 1 (trans) / 2.54 (dif ratio) X 75 (inch circumfrence) X 60 /63360 = 83.9 MPH

James Bailey

YUP !

Tom in Austin

Cool!

danglerb

Its a pretty safe bet that peak HP is where it will top out, which is a fair amount below redline at I think 5700 rpm for a GTS.

bmf944

Tom sent you a pm. 944 here in austin, gimme a shout.

Blake

Vilhuer

GTS top speed is 171mph. On 5sp that is 6270rpm and on automatic 5830rpm. Max hp is at 5700rpm.

Vilhuer

http://www.cartestsoftware.com/cartest4.5/index.html

Feed all known data in and result isn't far from reality.

James Bailey

also depends on the size of the tires these fit the rear of the old very brown 1980 and the MPH varies greatly

IcemanG17

http://www.autolounge.net/calculators/gearing.html

I think yours is an automatic so a 2.54 rear end vs 2.73 for the 5 speeds (since top gear is 1-1)...using stock sized tires 3000rpm = 87.95mph..but this is assuming no torque convertor slip...which I found to be somewhere between 0-200rpm

Ketchmi

Hey Sterling,

Mikes top speed would be the same as our 89' we are currently putting a 2.20 gearset into that has 630rwhp. It will be redline limited in the mid 220mph range unless he has raised his redline. 295/35-18" (26.1") @ 6400rpm would be approx 226mph. The loss through the torque converter (in this car) at 6400rpm would be minimal as would the loss through traction. I would guess the high side of 220mph. That's not taking into account any tire growth either...

danglerb

Stall speed on the US converter is 1950 rpm, as in engine rpm 1950, TC output zero. What your saying is the loss goes down as the RPM goes up? When does it happen?

mark kibort

yep, about 155mph in 4th gear with a 2.2 and a 26" tire. (3.2overall ratio)
The speedo is slow from the factory, so going to 25.5" tires from the stockers , will make the speedo actual. Also, the rpm vs speedo doesnt change with tire size changes.

GlenL

Righto. It's real speed that changes.

Eyeballing my car it'll redline (6500) at about 182 MPH. I've only done 147 indicated, but it's pulling hard there. 2.75 rear end, if you were wondering.

The trick, as our ORR friends have alluded to, is that the power needed goes up really strongly with speed. The power consumed to move a body through a liquid varies as the cube of the velocity. (Well, roughly.) It takes 8 times as much power to go 200 MPH as 100 MPH.

VehiGAZ

Which is why the 226 mph theoretical top speed mentioned above is never going to happen - too much air to push out of the way too fast.