View Full Version : Speedometer reading high

04-17-2007, 11:21 PM
I rarely drive my 89 SC 5spd. It was my fathers and after he died, I bought it from my stepmom. Anyhow, the weather was great today so I took it out and cruised on the highway. I set the cruise at 65, but it felt slow. I was getting passed like crazy by semi's and little old ladies. So, I bumped it up just to maintain with traffic. I drove home and grabbed the GPS and went back out. I noticed a 2 mph difference in my speedometer at 40mph (gps 38). I made it back to the highway and locked it in at a speedometer reading of 70 mph, but the gps said 65 mph. up to 75... gps 69. The ratio only increased with speed.

I thought the SC was stock, but could this be due to gears or maybe tires? What else would cause this large difference in the actual speed?

04-18-2007, 12:24 AM
Gearing effect speedo vs actual mph most time...tire size can throw it off, but not that much as far as I know.

04-18-2007, 12:43 AM
at any point was the speedometer replaced on the dash?

04-18-2007, 12:56 AM
What is the tire size? I got a speeding ticket because my tires were not stcok size and threw out the speedo.

04-18-2007, 07:58 AM
I am not sure of the car's history at all. It was garaged the past 2 years, then I bought it. So, if the speedo was replaced I don't have knowledge of it. Dad never mentioned it when he was alive.

Also, the speedo is indicating that I am going faster than what I actually am. Is that more indicative of gears than tires?

04-18-2007, 08:44 AM
I think this is fairly common with SC's. I have the same thing in my white car, and I KNOW it's never been messed with. Reads about 5mph low at highway speeds, however the odometer is right on the money. I posted this up two years ago, but never got an acceptable answer beyond "that's just the way they are". Since I don't really drive it, I don't care all that much.


04-18-2007, 08:55 AM
I have an 89 5-speed and I notice the same thing. As far as I know everything is stock in the way of gears. I know the tires were stock size. I don't have a GPS to prove this but I know the speedo reads higher then the car is moving. My buddy's 89 feels the same way to me, I think the calibration is just a tad bit off which multiplys the faster you go.

Since somebody else has noticed this, I have my 17x9 cobra rims and I wander what slightly taller tires would do for this. Right now I have 245/45's which is the same height tire as stock.

I wander where the error is??


04-18-2007, 09:12 AM
My 89 does the same thing and I have never repalced the speedometer and I am running on 225 60 16's

I just checked it when I passed one of those speed limit trailers the police set up on the side of the road.....

I was doing 50 and the speed limit trailer said I was doing 45.....

04-18-2007, 02:38 PM
This is a little frustrating to me.... I want the speedometer to read properly. Is anyone aware of a fix for this?

04-18-2007, 02:46 PM
Are you running the stock tire size of 225/60/16?

Kurt K
04-18-2007, 02:54 PM
I can't take credit for digging up this C&D article, but it explains almost everything you need to know.


04-18-2007, 07:18 PM
I think that 5 mph at 70 mph is not that much. Especially from Ford's perspective. Why would they bother to spend extra money on calibrating it to the extreme? Notice that the Ford police cars come with a "Certified Calibrated" speedometer, and a label to prove it. They also have to check them every so often or it can negate speeding tickets that they issue. Also, from a manufacturer's perspective, it is better to err on the high side so no driver can complain to them that they got a ticket or crashed because the speedometer read too low.

Now, from your perspective, it could be annoying. There are a couple of ways to adjust the speedo reading. One is to swap the driven gear on the VSS, but I think you are close enough that you won't be able to improve it that way. the second is to install a variable resistor on the speed sensor output. There are products on the market that do just that, but you could also rig one up yourself. I am actually editing an article about it for the club. (Sorry, GLDIII - I know I am way late with that thing! I will get back on it.)

04-18-2007, 08:22 PM
Just remember that changing gears, tire sizes, or speedo gears will effect the odometer accuracy too. As near as I can tell, the odometer is right on, but the speedometer is annoyingly inaccurate. We pretty much have to live with it.


04-18-2007, 08:26 PM
The inner works of these speedometers are controlled by two little motors. One works the speedo needle and the other works the odmeter. If the odmeter reads correct (matched to the mile markers along side most state roads & highways) then I would suspect the motor controling the needle is going bad. If the odmeter is off a couple of tenths per mile, then either the tire size is wrong or your Dad might have changed the gear in the rear end.

Do a search of the yellow pages on line for your area "speedometer reapir" and see if you have a place close by. Chances are it will cost 75-100 bucks to do the repairs if you pull it and bring it to them. If they pull the cluster, then figure 150.00.
You could always look in the local salvage yard and take your chances on one of those.
Or post in the "wanted" fourm for one.

04-18-2007, 08:32 PM
Hmmm. I didn't think of checking the odometer too. Thanks for the tip on that. I will get her back on the road tomorrow and check that also.

04-18-2007, 10:00 PM
Just drive 5 MPH over the posted speed limit......

We ALL drive the posted speed limit.......