VSS speedo gears by tires sizes from the Fiero P22 parts manual. Pages 1-5 4.337 Group.
I combined all the data from all the years into this. Let me know if I made an error.

Looks like there are 2 VSS gears for manuals, and 2 for automatics. The drive gears are different, 10 tooth and 35 tooth, so I don't think the VSS gears are interchangeable between automatics and manuals. ie: using a RED gear in an automatic.

MD9 TH125C Automatics will have the large GREEN 10 tooth drive gear.
The smaller VSS speedo gear will be:
BLUE 30 tooth - tire sizes P185/80R13, P185/75R14, P195/70R14, P215/60R15
WHITE 31 tooth - tire size P215/60R14

MG2 Muncie/Getrag 5-speed transmissions will have a NATURAL (tan) 35 tooth drive gear.
The smaller VSS speedo gear will be:
RED 29 tooth - tires sizes P185/80R13, P185/75R14, P195/70R14, P215/60R15
PURPLE 30 tooth - tires sizes P215/60R14

M17 Muncie 4-speed transmissions will have a NATURAL (tan) 35 tooth drive gear.
The smaller VSS speedo gear will be:
RED 29 tooth - tires sizes P185/80R13, P185/75R14, P195/70R14, P215/60R15
PURPLE 30 tooth - tires sizes P215/60R14

MT2 Isuzu (?) 5-speed transmissions will have a 35 tooth drive gear (color unknown).
The smaller VSS speedo gear will be:
RED 29 tooth - tires sizes P185/80R13, P185/75R14, P195/70R14, P215/60R15
PURPLE 30 tooth - tires sizes P215/60R14


If you calculate that you need a 25 tooth speedo gear - you're out of luck unless some other GM vehicle using the same VSS had one for even larger tires. GM may have had other speedo gears for their transaxles in other cars (Beretta, Grand Prix, Bonneville etc) with larger tires sizes (less teeth), but I could not find any information on whether they used the same VSS or what other gears may have been available.

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3.4L S/C 87 GT www.fierosound.com
2002/2003/2004 World of Wheels Winner &
Multiple IASCA Stereo Award Winner

[This message has been edited by fierosound (edited 09-30-2008).]

So this means that if I am already running P195/70R14 stock on my auto trans I can run a P215/60R15 without needing to change anything?

[This message has been edited by Miker (edited 07-15-2008).]

quote Originally posted by Miker: So this means that if I am already running P195/70R14 stock on my auto trans I can run a P215/60R15 without needing to change anything?

Yup. The outside diameters of the tires should be the same.

quote Originally posted by fierosound: Yup. The outside diameters of the tires should be the same.

not exactly the same

http://www.1010tires.com/Ti...or.asp?action=submit
make the comp
there is 1.29 inches more for the 15"

Which gear should I run in my 1984 SE with 87 GT tires with the bigger in the back?
it is off by 2.5 mph at 60 mph I noticed, close but off, any ideas?
I might have a blue one at home? please email me directly if anyone knows? BUFFBILLSNO1@AOL.COM
Dave

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D A V E ' S
1984 FIERO SE
T - TOPS
AND
1986 1/2 FASTBACK GT

So according to that chart a 205/60R15 would be a closer match to the 195/70R14 with only 1/4 inch difference.

What size tires came stock on the 15 inch rims?

If there is such a difference in sizes does that mean the Fiero's that were equipped with different tires are running an actual different speed which is not reported correctly on the speedo?

quote Originally posted by Robert 2: not exactly the same http://www.1010tires.com/Ti...or.asp?action=submit make the comp there is 1.29 inches more for the 15"

Looks like only 0.4" difference on diameters when I input the numbers. With the 215/60R15 he'd be doing 59mph when the speedo shows 60mph. I would assume GM would have decided it's "close enough" since there would be a variance in diameters on the same tire size even between different manufacturers.

[This message has been edited by fierosound (edited 07-15-2008).]

quote Originally posted by Miker: So according to that chart a 205/60R15 would be a closer match to the 195/70R14 with only 1/4 inch difference. What size tires came stock on the 15 inch rims? If there is such a difference in sizes does that mean the Fiero's that were equipped with different tires are running an actual different speed which is not reported correctly on the speedo?

II's an inch and almost 5/16 on circomference then for ten turn of a wheel you make more than a foot of travel imagine thousand and thousond of turns

[This message has been edited by Robert 2 (edited 07-15-2008).]

quote Originally posted by Robert 2: II's an inch and almost 5/16 on circumference then for ten turn of a wheel you make more than a foot of travel imagine thousand and thousond of turns

The speedo gear information is from the P22 book. That's what they installed based on tire size. I say again, GM likely didn't worry about the speedo being off by 1 or 2%. I think you get that much variance between the time you mount new tires and wear them out.

Just looking the tire information on the link you posted. You got circumference and diameter figures mixed up.

195/70R14 O.D. is 24.74
215/60R15 O.D. is 25.15

The 215 is 1.64% larger, so the speedo will be showing slower than actual speed. In this case, it will show 59mph what you are actually travelling at 60mph (or 100mph at 101.64mph) Are you saying the figures should be greater than that?

In any event all you have to work with anyway is the speedo gears that are available. If you calculate that you need a 25 tooth speedo gear - you're out of luck unless some other GM vehicle using the same VSS had one for even larger tires.

[This message has been edited by fierosound (edited 07-24-2008).]

Charts like the speedo gear and the tire size come in very handy, but like Fierosound said, they only put you in the ballpark and aren't precise. I made several changes on my 85GT and used charts to get it to an acceptable representation of speed. I changed from the original 14 inch tires to 15 inch lace wheels, then installed 245/60/15's on the rear. Then I installed an 86 Isuzu 5-speed into the car. With the blue 30 tooth VSS gear, at 70MPH I was blowing all the Atlanta traffic away. I was about 15% off, or for each mile traveled, I was showing only .85 miles. Great for keeping the mileage off the old Fiero, but it could cost you in court.

I changed to a 29 tooth and dropped the difference down to probably less than 1/10th MPH error. At least that's what I get by checking the odometer against the mile markers on the highway. Sometimes it falls exactly on the mile, and others are off by 1/10th. I mark that up to errors on the part of the DOT. So, just to be safe, I estimate I'm doing 66 when my speedo says 60 and I'm doing 77 when it reads 70.

A source of much of the confusion here appears to be that fierosound is focusing on a tire diameter measurement wherea Robert 2 is focusing on a tire circumference measurement (which is simply diameter times the constant, pi, or 3.1416.

It turns out that the car's new speed (whether in miles per hour, kilometers per hour, or anything else) can be expressed as being a function of tire diameter, or it can expressed as being a function of tire circumference. It doesn't matter which, as long as one is using the correct formula. For example, all other things being equal...

If focusing on tire diameter, use the formula...

Speed with New Tires = [New Tires' Diameter/Old Tires' Diameter] X Speed with Old Tires
If focusing on tire circumference, use the formula...

Speed with New Tires = [New Tires' Circumference/Old Tires' Circumference] X Speed with Old Tires
Both formulas will yield identical results because tire circumference is simply the tire diameter multiplied by the constant, pi.

quote Originally posted by project34: A source of much of the confusion here appears to be that fierosound is focusing on a tire diameter measurement wherea Robert 2 is focusing on a tire circumference measurement (which is simply diameter times the constant, pi, or 3.1416. It turns out that the car's new speed (whether in miles per hour, kilometers per hour, or anything else) can be expressed as being a function of tire diameter, or it can expressed as being a function of tire circumference. It doesn't matter which, as long as one is using the correct formula. For example, all other things being equal... If focusing on tire diameter, use the formula... Speed with New Tires = [New Tires' Diameter/Old Tires' Diameter] X Speed with Old Tires If focusing on tire circumference, use the formula... Speed with New Tires = [New Tires' Circumference/Old Tires' Circumference] X Speed with Old Tires Both formulas will yield identical results because tire circumference is simply the tire diameter multiplied by the constant, pi.

Thank you ,

Thanks for the info. for those of us not wanting to get an aftermarket programmable electronic speedometer.

quote Originally posted by project34: A source of much of the confusion here appears to be that fierosound is focusing on a tire diameter measurement wherea Robert 2 is focusing on a tire circumference measurement (which is simply diameter times the constant, pi, or 3.1416. Both formulas will yield identical results because tire circumference is simply the tire diameter multiplied by the constant, pi.

Thanks. It still shows the difference between a P195/70R14 and P215/60R15 is negligible and why GM used the same VSS speedo gear.

[This message has been edited by fierosound (edited 07-24-2008).]

If you really need a 25 tooth gear I can probably make one for you. (My post on making a speedo gear)

I have made several for automatic trans but not a 25 tooth or any for the manual transmission. Shouldn't be a problem.

Ray