[Cyberats]

This may or may not be electrical, but hey it's in the dashboard.

Ever since I owned the vehicle, the speedometer is showing a % of speed higher than actual and I have measured this. The higher actual speed the more false (higher) the needle shows. Say at 25mph it maybe +5 and at 65mph it is +20, is it the gauge or the sensor or the transmission alignment or what ?
This is a stock car with Dealer rebuilt transmission, recently changed engine mounts.

[Rustydawg]

What year and model is this vehicle?

Do you know if the speedometer is electronic? If in doubt, drive in reverse - the speedometer should show speed regardless of direction.

The non-linearity that you describe is hard to figure, but non-factory height tires or a swapped speedo gear in the trans would make for error.

The electronics in the dash could have gone whacky but that's not the first place I would look.

[Cyberats]

1991 Mercury Grand Marquis LS, no problem showing speed or going up and down dial, just that the speed is not correct. Most likely electronic driven gauge. No change in wheels, dealer claimed he rebuilt transmission, other than that no changes from stock. I did find out it possibly may have head a door swap when I painted it, base color black when all others silver. Other than that it is a one wheel hopper so I doubt they messed with the gears.

What I wouldn't give to be able to strip it all down and rebuild mechanical parts only on everything (except the bare needed electrical components).

[75gmck25]

On my 2000 Ford Expedition the speedometer can be adjusted by changing tire size and axle ratio parameters in the Ford computer system. However, it is limited to a list of factory settings (no weird axle ratios or tire sizes). I'm not sure if this was also available in 1991, but the dealer should be able to tell you.

Bruce

[Cyberats]

Is that something I can hook up to software on a laptop ?
The unit maybe out of adjustment or fried.
Ultimately, I guess I can visit with a Mercury dealer see what they find.

My last known "good" mechanic did half-assed jobs twice, despite tips and a good relationship. I opted for not going back to him again, rather than have him attempt to fix what he messed up and "discover" other things bad.

[jimfulco]

They probably used the wrong speedo gear or the wrong output shaft on your transmission, or used a rebuilt trans that originally came in another car.

The Ford AOD uses an easily replaceable driven gear that's available in several tooth counts, but the drive "gear" is machined into the output shaft itself, and can't be changed without disassembling the transmission. There are three available tooth counts on the shafts.

http://tciauto.com/Products/TechInfo/speedo_gears.asp shows available driven gears and their color codes (scroll to the bottom). You need one that has more teeth than your current one has, so pull yours out and count the teeth. Count twice, because it's easy to get cross-eyed with those tiny little teeth. If you're lucky, it'll still have some color in it, and you won't have to count at all.

Figure out exactly what percentage your error is, using as high a speed as you can for better accuracy. If your speedo reads 20% fast, get a gear with 20% more teeth, etc. If you can't get it real close like this, the output shaft is wrong. My T-bird's like that, so I just do a little mental math to get my true speed.

The gears should be availabe at any decent-sized Ford dealer.

[Cyberats]

Thanks, if I ever get that far, it sounds like it's on the money.
Did I mention I'm not a mechanic with a lift or bay ?
If I ever get the cash to do that, I'd probably want a Positronic rear end and probably higher gears (nothing over 3.50:1), so that changes the configuration. What would I need then ?