Speedometer Gears

[JDAB]

I have an '86 SVO. When I bought the car, it had 15 inch rims with the wrong offset. Timing the speedometer with a stop watch on the interstate indicated that when the speedometer read 60 mph the car was actualy doing 65-66 mph.

I found a set of SVO wheels at Fords at Carlisle and had them straightened and refinished at Wheel Worx near Baltimore, Md (Great job). I ordered a set of correct size radials from the Tire Rack. I thought that the speedometer would be correct; but it appears to be off by about the same amount.

I figure that either the speedometer is out of calibration or that the transmision or speedometer gear was changed.

The Transmission has been opened as I can see reddish brown RTV sealing the tailhouse to the main body. I'm not sure if a tailhouse from another transmision was installed with a tailshaft speedometer drive gear for a different car. The nylon gear on the end of the speedometer cable has 19 teeth.

Is 19 teeth the proper number of teeth on the nylon gear at the end of the speedometer cable? If not, what number is correct?

If 19 is correct, how can I tell if the proper drive gear is mounted on the tailshaft?

Or, Does anyone have another idea as to the speedometer error?

Thanks,
John

[svono50]

Since nobody replied, I will throw in my $0.02.

Your stock drive/driven speedo gear combo should be 6/19 tooth units. If your drive gear was replaced it could have easily been replaced by a 7 tooth unit, which is what is usually found in other Mustangs. The only way to determine if your drive gear is a 6 or 7 tooth unit is to see its color (i.e.- 6 unit is black & 7 unit is yellow). Without taking the tail housing off, you may be able to see it with a mirror or a boroscope (sp?) through the driven gear hole. The drive gear is mounted to the output shaft, so whether or not the tail housing was replaced it should not make a ifference.

If you have stock sized tires (225/50R16's) and the proper rear gear (3.73's) your speedo should be within 5-10% @ 60mph, so your calculations show that you are within that range.

A quick calculation shows that the 6/19 combo = 0.316 and the 7/19 combo = 0.368. This is a 16.67% difference, which is more than the estimated 9% (60 vs. 66mph). Another thing to note is that the 7/19 combo will turn your speedo faster than actual, so you should be showing high, not low. This would suggest one or a combo of the following: 1) non-stock rear gear, 2) non-stock tire height and/or 3) an out of calibration speedo.

What brand/size tire are you running and what rear gear is in the car? With that I can calculate the driven gear that would best suit your combo. I needed to step up my driven gear when I switched to 245/50R16's in the back on my car, otherwise my speedo would be showing low.

[JDAB]

Speedometer Problem

Thanks for the reply. I have been busy with work and have had little time to sit down and log on.

I am running Kumho ECSTA Supra 225/50-16 tires. I think that I turned the wheels about two years ago (right after I bought it) and it appeared that the ratio of the drive shaft turns to the wheel turns was around 3.7:1. I haven't opened the rear end cover and counted the teeth on the ring and pinion.

If I get some time in the next week or two, I will attempt to see into the tailhousing to see if I can see the color of the tailshaft drive gear. If that turns out to be black (6 teeth), then I will open the rear end and count up the teeth.

If both the rear gears and the tailshaft gears are correct, I will need to fix the speedometer. With Corvettes you can buy new calibrated speedometer drive units from various vendors and install them yourself into your speedometer housing. Is there a source for speadometer guts for the SVO speedometer? If so, do you have an address?

Thanks,
John