was looking into what my 4.9 with an 86 4spd would do on rpm and top speeds and came across this calculator thats pretty niffty. i used 205/60r14 for the tire size and found that the trannies rated as follows

fastest top speed at 5500 rpm

1. 84 econo at 159.9 mph
2. isuzu 5 spd at 156.3 mph
3. getrag 5 spd at 149.1 mph
4. 85/86 4 spd at 131 mph
5. 84 perform. at 116.6 mph

best fuel economy at 55 mph

1. 84 econo at 1891 rpms
2. isuzu 5 spd at 1935 rpms
3. getrag 5 spd at 2028 rpms
4. 85/86 4 spd at 2307 rpms
5. 84 perform. at 2592 rpms

if there are any errors just point them out i tried my best to make shur it is accurate but i recently found out i was human and not a grey soi think i make mistakes from time to time
of course this will just tell you top speed not how fast youll get there

heres the website
http://ctny.audiworld.com/m...gears/gear_calc.html

[This message has been edited by Niterrorz (edited 12-06-2011).]

My only concern is the top speed estimates. I don't think the stock engines have enough hp to reach 5500 rpm in "high" gear. I know that my 87 GT with its getrag at 70 mph is turning ~2500 rpm in 5th gear, to reach 150 mph or 5000 rpm that wont happen. Wind drag, light front end and 135 hp.. Now my IMSA with its ZZ4 v8 getrag 5 speed, at 70 mph and ~2500 rpm probably has the HP (350 hp) to get to 5500 rpm and some speed too fast for me to drive.

I do know at 55 the GT does get great economy cruising at ~2000 rpm. When I bought the car in 1987, the national speed limit was 55 mph and You could cruise a long way with its 11 gal gas tank.

quote Originally posted by Old Lar: I don't think the stock engines have enough hp to reach 5500 rpm in "high" gear.

Exactly.

The top speeds Niterrorz posted are theoretical top speeds since none of the engines have enough horse power to reach red line in top gear. For example, various magazines tested the 84 Fiero with the four speed performance transmission (4.10:1) and ran out of steam at 103 MPH (Car & Driver and Road & Track Sept '83). The '84 econo trans was never tested by any of the magazines that I have, but given it's mechanical disadvantage it's top speed would likely be even lower.

In Nov '84 R&T tested a four speed V6 and found the top speed to be 125 MPH

In Oct '87 R&T tested a Formula with the Getrag and V6 and found the top speed to be 125 MPH as well.

As you can see, top speed is dependant on more than just gear ratios.

Also, 5th gear in the Isuzu will not support WOT at high speeds. It's gears are very small in diameter and thin, during one of my 135 mph runs I pushed 5th gear out the end cap. (with a warmed over 2.8).

And many Fieros had 15 inch tires, which ups the entire charts numbers.

ya my 4.9 is in an 84 for 14 inchers so i just listed that

My tach must be off.
235-45-17
70mph
3750rpm
'88 GT 5sp

quote Originally posted by n7vrz: My tach must be off. 235-45-17 70mph 3750rpm '88 GT 5sp

Take it out of 3rd.....

[This message has been edited by weaselbeak (edited 12-06-2011).]

This is kind of a cool website, just plug in your settings, check it out:

Gear Ratio Calculator

I like this one as you can input any cruising speed and/or any red line or shift RPM you want to see in the results, and it graphs it for you.

Moonwell's Gear Ratio Calculator - Muncie 3.65 Example

It also tells you where you'll be for RPMs after a shift at your desired shift RPM.

[This message has been edited by carbon (edited 01-23-2012).]