speedometer is 8MPH off

[ashleesmach1 28]

when the speedometer in my car reads 60mph, my GPS says the car is actually going 52. I have confirmed the gps speed with 2 different gps's.

the car has an fmx. the rear is a 3.50 to 1. the tranny driven gear is a 21 tooth gear..... what  driven gear do I need? or do I have to change the internal drive gear? help please. thanks

 

[j69302 11]

52 mph is 86.7% of 60mpg, or 13.3% low. So you need to make your driven gear spin 13.3% faster.

 

13.3% of 21 teeth, is 2.8 teeth.

21 - 2.8 = 18.2, or 18 tooth gear.

 

 

Another way to look at it,

100% - 86.7% = 13.3 % of 21 teeth = 18.2 So try a 18 tooth gear and recheck.

 

Also you can check ot this calculator http://www.classic-cougar.com/tools/speedocalc/speedocalc.php

[jholmes217 65]

Are your wheels and tires stock height with what was originally on your car?  If not, then find out what the height difference is, then car CJ Pony Pony parts or some other vendor that sells Mustang speedo gears, and they should be able to fix you up with a more accurate gear, or go with a tire that is the stock height.  Had the same problem on a 67 Mustang when I changed wheels and tires.  

[skidmarky 50]

Here is good info http://429mustangcougarinfo.50megs.com/speedo_gears.htm

[ashleesmach1 28]

I shud add the tires are 26.28 inches tall 235/60/15

the stock gear was 3.00 and the driven gear was 19 tooth. I will try the 18 tooth

thanks so much

[ashleesmach1 28]

52 mph is 86.7% of 60mpg, or 13.3% low. So you need to make your driven gear spin 13.3% faster.

 

13.3% of 21 teeth, is 2.8 teeth.

21 - 2.8 = 18.2, or 18 tooth gear.

 

 

Another way to look at it,

100% - 86.7% = 13.3 % of 21 teeth = 18.2 So try a 18 tooth gear and recheck.

 

Also you can check ot this calculator http://www.classic-cougar.com/tools/speedocalc/speedocalc.php

funny, I used the calculator and it said I needed a 21 tooth gear. that's what I have.....

[ashleesmach1 28]

what is the drive gear of the fmx tranny I thought it was 8 but I don't really know

[det0326 179]

funny, I used the calculator and it said I needed a 21 tooth gear. that's what I have.....

the smaller the number of teeth driven gear (the one on the end of the speedometer cable the faster the speedometer will turn making it register faster than the car is going. 

[ashleesmach1 28]

ok My car has 26.28 inch tires, 3.50 rear end, fmx tranny with a 8 drive gear, supposedly. According to this calculator http://www.classic-cougar.com/tools/speedocalc/speedocalc.php it says the car needs a 21 tooth driven gear. well that's what I have and it's 8mph off. like I said, at 60 on the speedo the gps says the car is doing 52.........help?=(

[j69302 11]

ok My car has 26.28 inch tires, 3.50 rear end, fmx tranny with a 8 drive gear, supposedly. According to this calculator http://www.classic-cougar.com/tools/speedocalc/speedocalc.php it says the car needs a 21 tooth driven gear. well that's what I have and it's 8mph off. like I said, at 60 on the speedo the gps says the car is doing 52.........help?=(

 

The speedo calculators are not always accurate, however, you there is another way to check to see if you have the right gear.:

 

For the time being, ignore the speed and check your ODOMETER reading. Drive a known distance(a long distance, 100 or 200 miles, does not have to be at a time, just keep track of actual GPS measured distance), and compare it to your odometer, if it is close, then you DO have the right gear, and your speedometer's magnetic drag cup is out of calibration.

 

If the odometer does not read correctly. then try the 18 tooth gear, or, apply the same math to the odometer reading as I did to the MPH reading, a buy the new gear.

 

FWIW, my odometer is 4.2% lower than actual distance. However, the needle indicates eabout  10% low. If I were to fix it, I would buy a  gear that drives it 4.2% FASTER.

 

The odometer is directly geared to the speedo cable, and the transmission. The speedometer, is a magnetic drag cup, there is no direct mechanical linkage, and there can be that the magnet has weekend, causing more "slippage", friction in the needle, or the return spring stiff.

 

So first, get your ODOMETER to read correctly. then have the speedometer checked out for proper magnetism and return spring tension.

[SA69mach 39]

I had an inaccurate speedo in my 70 after installing a T5 and 3.70 rear gears, and some 275/60/15 tires. Did all the calculations and mathematics. Had the speedo tested and calibrated by tachman.

After all that, I ran it on a dyno and kept changing out the speedo cable gears until I had it as accurate as I could get it (2 mph low at 60 mph)

 

Just get one of these kits and do the same thing with your GPS as a test unit.

http://www.cjponyparts.com/speedometer-driven-gear-kit-type-3-17-18-19-20-21-tooth-aod-aode-automatic-transmission/p/SGK2/

 

All the maths may not help you. As you say, they all point you to a 21 tooth, and that is what you have in it now! What you do know is that you need to get a gear with LESS than 21 teeth, so the speedo cable spins faster at 60 mph. How much depends on a few factors. Try a 20, then a 19, then if necessary, an 18 tooth gear.

They will not be wasted money, because every time you alter the tie size, rear gears, you will need to adjust your speedo by use of different gears.

[ashleesmach1 28]

I guess I'll have to do a trial and error. I try an 18 tooth gear and work from there. thanks for all your suggestions, advice and help....

:)

[det0326 179]

If the speedometer is calibrated correctly as mentioned above and he is running actually 52 referenced from a GPS and his speedometer is registering 60 seems to me he would need to slow the speedometer down with a higher number teeth rather than speed it up with smaller number of teeth. 

Dave

[SA69mach 39]

If the speedometer is calibrated correctly as mentioned above and he is running actually 52 referenced from a GPS and his speedometer is registering 60 seems to me he would need to slow the speedometer down with a higher number teeth rather than speed it up with smaller number of teeth. 

Dave

You Sir, are correct.

23 tooth gears are available, but the wear rate is high. It will be interesting to see how long it lasts. My apologies to the OP. Kudos to det0326

[j69302 11]

Whoop you are right, I got it backwards.

 

Try a 23 tooth gear.

[ashleesmach1 28]

oh boy LOL confusion is setting in aargh!....:)    ok I'll try a 23 tooth first

[det0326 179]

You Sir, are correct.

23 tooth gears are available, but the wear rate is high. It will be interesting to see how long it lasts. My apologies to the OP. Kudos to det0326

sometimes I have a moment of enlightenment but most of the time I am as dumb as a rock.  LOL

 

Dave 

[ashleesmach1 28]

does NPD carry the type 3 23 tooth speedometer gear?

[SA69mach 39]

Don't know about NPD. Maybe

 

CJPony Parts does

http://www.cjponyparts.com/speedometer-driven-gear-type-3-23-tooth-automatic-and-manual-transmission/p/HW1290/

[RPM 1,190]

Nice catch! At least somebody's paying attention. 188 views including me every day, and one guy can finger it out.

[Pakrat 1,043]

 and one guy can finger it out.

 

Hey now, let's keep it clean! ;)

[RPM 1,190]

Hey Ashlee, did you ever get your speedo gear figured out? I too have an FMX with 3.50 rear gears.