Hi everyone. Been a while since I was here. Quick question or two. What is the difference between engine rpm with a 4sp verses a 5sp.? Eg: If my 4 sp is has the engine spinning at 3100 at 60mph, what would the rpms be with a 5sp at the same speed?
Also, Would the shifter from a 4sp work with a 5sp or does that have to be changed too?
Thanks

The formula relating road speed and engine RPM is:

RPM = (MPH x Gear Ratio x Axle Ratio x 88) / Tire Circumference

where the tire circumference is measured in feet, gear and axle ratios pertain to your car and 88 combines several conversion factors.

So, for instance, lets say your car has:
215/60R/15 tires, tire circumference = 6.40 feet
5th gear in a Getrag = 0.72
Axle ratio in a Getrag = 3.61
Road speed = 60 MPH

Then

RPM = (60 x 0.72 x 3.61 x 88) / 6.40 = 2144

The same formula using the Isuzu five speed instead gives you 2045 RPM since 5th gear = 0.74 and axle ratio = 3.35

Edited to add: As for the shifter, it does have to be changed out for the five speed shifter since they're quite different. You'll also need the cables too since they're specific to each transmission.

[This message has been edited by Bloozberry (edited 11-09-2009).]

Ok, thanks. That answers one of my questions and sort of answers the other. I'll need the shifter I see. I've got a chance to buy an Isuzu 5sp with cables. I've got a 4sp in my 84 now. I was just wondering how much of an overdrive would there be with the 5sp. I guess if the 4sp is a 1:1 and the 5sp is a .72:1 then assuming the final drive is the same, then my engine rpm will be dropped from 3100 to about 2232.
Thanks

Here are some common Fiero tire cicumferences for people who have different tire sizes than the ones I used in the example above:

185/80R/13 = 6.454'
215/60R/14 = 6.283'
225/60R/14 = 6.448'

So then do I understand this correctly? There isn't really much of a difference in engine rpm at highway speeds between a 4sp or a 5sp.

[This message has been edited by the big guy (edited 11-08-2009).]

Unless you're going from the 4.10 four speed (2790 RPM at 60 MPH) to either of the five speeds or the economy four speed, then you're not going to gain much in reduced RPM in top gear at any given road speed. The other three are within 100 RPM of each other. What you DO gain in the five speeds is the ability to keep the revs closer to the engine's power band when accelerating and shifting. In other words, there isn't as deep a hole that your engine has to come out of when you up shift from one gear to the next higher gear, because each gear is a little closer ratio-wise to the next one in the five speed than it is in either four speed. This means better acceleration, but does nothing to improve fuel economy if that's what you're looking for.

quote Originally posted by the big guy: If my 4 sp is has the engine spinning at 3100 at 60mph

Are your sure your tach is accurate?

I have no idea if the tach is accurate or not. It idles at about 7-800 rpms. It revs out and I sure wouldn't want to rev it any higher than when the tach says the 5000 rpm redline. At 120km/h (75mph) the tach is sitting right on 3500rpms. The engine doesn't seem to mind this speed, actually it purrs right along. I'm getting over 30 mpg and the power is good, I just thought the fuel economy might be improved a bit by lowering the rpms. This engine speed would also be a concern if I was to install a 3.8 or v8. Although the power would be incredible, the fuel would just pour through a v8 spinning at 3000+ all the time. I'll do some calculations with the numbers provided by Bloozberry and see what I come up with.
Thanks again.

Transaxle information http://www.calgaryfieros.com/OSGdocs/tranny.html

Automatics were only a 3-speed - no OD. You'd have to swap in a 4T60 4-speed auto - THEN you'd have OD.

On my Indy, I changed from a 3-speed auto to a 4-speed auto, then changed again to a Getrag 5-speed manual.
Much better. https://www.fiero.nl/forum/F...3/HTML/000077-3.html

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World of Wheels Winners
My 3.4L S/C 87 GT
& Super Duty 4 Indy #163

Speed calculator with all the Fiero manuals and the F40. Plus you can input your own gear ratios if you prefer. Just enter the RPM and the tire size and go: http://www.cowspatoot.com/utilities.html

Yeah, the '06 F40 was a .623:1 sixth, while the '07 was .707:1 sixth, with a 3.545:1 final in both.

See Cave... Gear Ratio and AOD tranny (AOD in option... AOD because workbook Speed vs RPM)

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top of every forum page...)

Thanks for the replies. A lot of useful info. I have a donor car as well. The engine and trans are already out of the car. Are there any ID numbers on the transmission to tell which type of 4 speed it is? I never drove this power train as I bought it with a blown motor. It's an 84 as well but now I see there were two 4 speeds offered in 84. I was just wondering if there was a quick way to determine which one the spare is.
Thanks

The most reliable way is to put it in fourth gear using the levers on the trannmission and then mark any one of the splines on the input shaft, like the top one. Then, temporarily install one of your tripot joints into one of the output shaft holes on the tranny. Again, mark it so that you can reliably know that it's made one complete turn. Lastly, start rotating the input shaft by hand until the tripot has done one full revolution, all the while counting how many times the input shaft was turned.

If the input shaft had to turn:

0.73 x 3.32 = 2.4 turns, then you have the economy gearing 3.32 differential or;
0.81 x 4.10 = 3.3 turns, then you have the standard gearing 4.10 differential.

Thanks Bloozberry. It's good to know the numbers. The power train is still in the cradle complete with engine attached. I was just wondering if there was a model number tag or something that would be a quick look.

I'm pretty sure there isn't a tag on the tranny, but under the front hood on your RPO code sticker (the list of numbers and letters on the driver's side wheel arch) will have one of the two following codes somewhere on it: M19 for the 4.10 ratio, or MY8 for the 3.32 ratio. It's not 100% fool-proof since the tranny may have been replaced at sometime during the life of the car, so your best is still counting the revs.

Thanks Blooz, that's as close as I'm gonna get for the info I was looking for. The car that my current power train came out of has the M19 code which matches the rpm reading I get at 60 mph. My current driver car has the MY8 code. That's the cradle I've got out back with the blown motor in it. Although not fool proof like you said, I'm not gonna pick up a 5 speed and shifter if I've got the same gear ratio here. I'll just use this tranny if I'm gonna do a 3.8 build.
Thanks again, you guys were a big help and saved me from collecting more junk.