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Speedo off by 20%,any ideas?

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WaltCasten

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This might be an easy one, but, the recently rebuilt speedo in my BN2 is off by ~20%. Apparently is has always been that way but I didn't notice until it was rebuilt. It has the correct speedo in it verified by the "cable revolutions per mile number" printed on the face of the gauge by the trip odo. I ran it against mileage markers on the highway and the odo only reads ~.8 for every 1 mile traveled. The tires are 165/15s which are only supposed to differ from original by less than 5%. I've heard there are different transmission speedo drives, but I have NO IDEA if the original trans/speedo drive is in this car.
Any ideas? Thanks.
 
Drive faster..
 
Have you checked if the speed is off by the same amount? If if isn't then the problem could be something slipping inside the speedo, if not start eliminating the stuff outside the thing.

MattP
 
Yes, the speed is also reading 20% low. When it was rebuilt, it was strobed and calibrated at 60mph. My next plan is to find out if the axle ratio compensation is done in the pinion gear in the trans or in the angle drive at the end of the speedo cable.

If worse comes to worse, the guys who rebuilt the speedo (for $85) will split the cable, splice in a gearbox and put a set of little gears in it to get it right. As an FYI for other members, this would also be a good way to compensate for rear end gear/tire size changes.
 
Same thing happend to my Spridget when I put on the low-profile Sumis. I took the car onto Rt. 80 (with mile markers and a wristwatch) and "calibrated" the speedo, by putting little tape stripes at important speeds (55 MPH, etc.). I wasn't worried about the Odo, but that could be figured out using a multiplication "correction factor" if needed. Not the "perfect" solution, but cost=free!

Note: I checked the marks later on with a hand-held GPS and they were right on....for speed and distance info, a basic Magellen GPS ($100 in K-Mart) is a handy back-up device (unless you drive in tunnels all the time!).
 
Walt,

There are at least two different drive pinions for the speedometer drive in the trans or overdrive. I have the opposite problem, mine reads at least 20% too high. It was somewhat high and then I had my overdrive rebuilt (it may have been replaced, but that's a long and different story) and now it's really obvious. Anyway, two speedometer pinions are listed in Moss' catalog.

cheers.gif

John
 
Um, wanna trade drive gears??
grin.gif
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by WaltCasten:
Um, wanna trade drive gears??
grin.gif
<hr></blockquote>

That's a possibility. Where are you located?

cheers.gif

John
 
In the Chicago area.
 
Took the weekend off.

We'll have to check the number of teeth on the speedometer drive pinion. I have a 4.11 rear end and overdrive. I measured the number of cable turns/mile at the speedometer end of the cable at 1,280. Have you checked to see what rate your cable actually turns?

cheers.gif

John
 
No, I have not measured the actual cable revolutions per mile. For that matter, I have also never checked to see what rear end gear is in my car, I just assumed it was the standard 4.11.

I think the best thing is to actually count teeth on that drive gear, hopefully it's not too big a pain to get it out.

Just out of curiosity, what number is printed on the face of your speedometer? I believe it's right above or below the trip odometer.
 
Walt,

The ratio is usually stamped on the differential housing (the pumpkin). 11/43 = 3.909 which was standard for the OD equipped cars and 11/39 = 3.545, standard for the non OD cars.

If the stampings are not there you can jack up the rear end, mark the drive shaft and one of the rear tires. With the gearbox in neutral you spin the drive shaft and count the revolutions required to spin the tire one time. 3 1/2 revs of the drive shaft or 4 revs will give you your answer.

Cheers,
John
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by WaltCasten:
I think the best thing is to actually count teeth on that drive gear, hopefully it's not too big a pain to get it out.<hr></blockquote>

Actually, it's a lot easier to check the cable revs/mile. One only has to disconnect it from the speedometer (may have to remove the speedometer to see the cable end while driving). Measure out 52.8 feet (1/100 mile) and count the turns while travelling this distance. Always go forward and do it three times. That's easier than removing the seats, carpet, and transmission tunnel to get to the drive gear. And you don't need to know the rear axle ratio, which is stamped on top of the differential. If you have the 4.1, it will look like 41/10 or 10/41.

I will look at my speedometer tonight.

cheers.gif

John
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by WaltCasten:
Just out of curiosity, what number is printed on the face of your speedometer? I believe it's right above or below the trip odometer.<hr></blockquote>

The number on mine is 980.

cheers.gif

John
 
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