Ont924s

The speedo on my 1987 924s does not provide a very accurate indication of my speed. For example when I am doing 100 km/h (62 mph)according to the speedo my GPS says that I am actually doing 90 km/h (55mph). The difference is the same, ie showing approximately 10% more than I am actually going, at other speeds. The car has 205/55 R 16 tires. I'm just wondering if this is common with 924s's and also if this would affect the odometer readings? Thanks

PaulD_944S2

924S's originally came with 15" rims and 185/195/65/60 tires, not the 205/55 R 16 tires you currently have mounted. Possibly the source of you errors.

There are several tire manufacturer guides that will tell you the diameter of a mounted tire. You could use that to determine if the error is from tire size mismatch, or a problem in the speedometer.

FYI, the 924S speedometer is cable driven from the left (driver's) side front wheel hub.

Ont924s

Paul. Thanks for the reply. I took your advice and checked between the 195/65 15 and the 205/55 16 recommended in the owners manual. And from what I found the difference in circumference between the 2 is 0.3". Also the 15 inch tires travel 832 revs per mile and the 16 inch tires are 835 revs per mile. So given that I don't think the tires are the issue. Any other likely causes? And do you think this issue would affect the odometer readings? Thanks

PaulD_944S2

832/835 = .9964% (or .0036%) and is not enough to make any real difference where 55/62 = .8871% (or 11.3%).

How much is the speedo off at half the speed? It might be the needle was taken off to replace the odometer gear and not installed correctly.

Otherwise, the speedometer head needs to be recalibrated.
You might try a used speedo if you can find one to try.

Ont924s

The speedo seems to be off by about 7-12 kph throughout the range. So would it be worth trying to re-set the needle to see if that helps? Or is re-calibrating the speedo the only way to go?

PaulD_944S2

Go for the needle reset.

In my opinion, if you sent it somewhere that's one of the things that a speedo shop would do. Give it a try, you really have nothing to lose.

But before you pull the needle off, read up on how to remove it. Look for articles that explain how to replace the odometer gears in a late 944 speedo/odo. The needle MUST be lifted STRAIGHT up off the stem otherwise the stem will break or the needle will get bent. There are a couple of good tricks to making it easy. ***** DO NOT use your fingers! *****

Also, cover the face of the speedo with thin plastic or cardboard so that you don't scratch the surface when lifting the needle.

Swenny

​​​​I checked out a tire comparison site and got this:Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
195/65-15 5.0in 12.5in 25.0in 78.5in 807
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
205/55-16 4.4in 12.4in 24.9in 78.2in 811 0.0%

807-811 revs/mile,that's about 4% difference or 2.5 mph @ 60 mph. That would match up with my Chevy S-10 that was changed from 15" to 16" rims and comparable tire sizes. The faster that you go,the greater the speedo error. That's if I did my math correctly.
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PaulD_944S2

Please check your math.

The difference is:

807 divided by 811 is .9950678 - 1.00 = .00493; times 100 is .49 %, or little less than a half percent. I think you had the right math but misplaced a decimal.

Ont924s

I had the car out yesterday on a bigger highway. Got the speedo up to 120 km/h but the GPS read only 105 km/h. I am certainly going to have to try to re-orient the needle as soon as I can.

Swenny

Lower profile tires will make the speedo indicate a higher speed than actual. On a car with with a differential you would need to change the gear ratio. On our cars I'm not sure if there is a gear to change at the wheel or not. Go to Pelican's 944 forum and check out mikepellegrini, he has one type of fix for the same issues that your having.

PaulD_944S2

He has a 924S which has a mechanical speedo with a cable from the LF wheel hub.

Swenny

Yes he does,doesn't he. 924,924S,944 up to early 1985 should have the same set-up. If anyone is curious,Clarks Garage has a "tire size calculator" go to-index-speedometer-tire size calculator.

Ont924s

Swenny I didn't have any luck finding the post you mentioned from Mike Pellegrini. I tried contacting him directly but no luck so far. Any suggestions on how to find the post???

Swenny

Here is his page:
http://forums.pelicanparts.com/members/48110.html
His post reminded me of what you're going tthough . I'll continue to look for the post.
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Holytin

Not sure what to you are trying to accomplish here. The drive cable for the Speedo goes through the left wheel hub and from there through a square opening in the grease cap, which turns the cable when secured to the hub. A cotter pin secures the cable to the grease cap. You simply cannot change the gear ratio because there is no gear. Have you tried another Speedo?