TheSamba.com :: View topic - I feel really dumb for asking, .... but...

Mon May 09, 2005 11:43 pm

I'm trying to get a handle on how to read my speedo. My original owners manual is glued shut from a leaky window, so that's no help. I've got the 10,30,50,70,100 model. So is the small mark directly above the number relate to it, or is it one of the larger marks to either side?

Not to mention that the tires are taller than stock which is throwing it off. I recorded a .25 mile discrepency over a 5 mile test zone along the highway today. Where's one of those photo radar rigs when you need one? :lol:

Tue May 10, 2005 12:44 am

The "10's" should be the fatter marks, inline with the numbers you see. the smaller marks are the "5s" - is that what you were asking about?

So a ratio of 5.25 : 5 means you have an odometer variance of 5%. That's actually not too bad, tire heights vary between manufacturers and wear will affect them too, so +/- 5% isn't uncommon. Not great but I've seen worse. Do you have a non-stock tire size or is it just off by that much?

From the gallery, you mean you have this speedometer right?

-andy

Tue May 10, 2005 5:49 am

I have 165r15's and the 110 MPH speedo, every time I speed past one of those "slow down, your speeding radar box trailer thingys" the police like to park on an open stretch of road people like to speed on, it clocks me at 5 MPH slower than my speedo. :( and I thought I was fast. :oops:

Tue May 10, 2005 8:56 am

As far as accuracy goes, speedometer variance and odometer variance are two different issues. The odometer is tied directly to the amount of wheel RPM's, whereas the speedometer uses a rotating magnet to pull the needle up - so if the spring tension on the needle is too strong or weak, the speed might read way off, but the odometer may be right on.

andy

Tue May 10, 2005 11:51 pm

Well, now I'm still kornfoozed. My guage has the "fat" marks on either side of the number. But if the mark directly above the number corresponds to that speed, then I guess it doesn't matter if it's wide or slim. :?

As for the speedo variance, I was told by the PO that the tires (all 5) were new and "oversized, to give better ride and a little taller gear ratio". (They are new too! The spare still has all the little ribber spikes on the tread! :D) While this isn't the first time I had taller tires on a VW, I dunno how they measure up to a "stock tire". They are Jupiter SR-802's in a 165R15 size.

At any rate, I guess I'll have to remember that I am going faster than I am. Or would it be slower? I guess I need to go hunting for a speed trap. :D Or talk to my local speedo shop.

Wed May 11, 2005 7:49 am

Bugman_Jeff wrote: Well, now I'm still kornfoozed.....

At any rate, I guess I'll have to remember that I am going faster than I am. Or would it be slower? I guess I need to go hunting for a speed trap. :D Or talk to my local speedo shop.

Here is a handy little calculator for figuring this stuff out.

http://www.miata.net/garage/tirecalc.html

Just select the stock tire size....looks like they only have 80 series as an option, but it will be close......and the new tire size and it will calculate the error in the speedo reading.

Smaller diameter then stock indicates higher speed on the speedo.
Larger than stock indicates lower speed on the speedo....I know this one for sure since I received a speeding ticket in my brother's GMC Jimmy once for doing 78 in a 65 with an indicated 60. :oops: Way oversized tires on that vehicle.

Wed May 11, 2005 9:05 am

165R15 is the stock size for tires on a T-3. or for bias ply, it would be 6.00 x 15.

Wed May 11, 2005 11:23 am

Using one of those speedometer variance calculators is only a rough estimate... why? Two reasons basically:

1. Like I mentioned before, the spring tension of the needle in the speedometer sets up the potential accuracy of the gauge. Remember how old these cars are now! That spring could be worn out. Plus who knows if someone's been inside the speedometer before.

2. Tire heights vary between manfuacturers, also. If you go to Coker Tire's website and look at the tire height ratings of their various offerings, you'll find ratings on the 155R15 and 165R15 tires between 25.1 and 25.4 inches. Bias ply tires are taller: 560x15s range 25.79-25.88, and 600x15 are 25.96 (they also have one that's a huge 26.7 tall!) And tire wear will alter your variance too.

The only way for sure to know how far yours is off, other than looking at those roadside radar speed indicators or having a cop pull you over, is to time yourself against mile markers and compute it yourself. [ for one mile: 3600 ÷ time(in seconds) = actual speed ]

andy