Speedo reading high
 

My speedometer reads about 15 mph high when at highway speeds. I have 185/65 15 Pirelli tires on factory alloy rims. Should this non-stock tire size make that much of an error in the speedo reading?

I'm running 205/60-15's and my speedo runs anywhere between 10-15 mph high, depending on time of day/year, planetary alignments, proximity to radar, etc. Various tire heights and profiles will throw it off some, but most of its error is through plain-old age.

As our beloved 914's grow younger with each passing year, stuff like speedos and clocks are going to be "off" a tad, but I admit that I'm going to have to get my MPH-o-meter recalibrated. There are places like North Hollywood Speedometer & Clock Co. ( www.nhspeedometer.com ) that will recalibrate and customize your gauges, a route I'm considering.

You have to give that clock some praise, though. Mine is dead-on, gonna-punch-in-late accurate, with hair on it. Not bad for a 26-year-old.

My estimate of +15 mph is taking the speedo's indicated speed and subtracting what I know to be normal traffic speed for that road & time of day. In other words, the speedo will show 80 mph indicated when the traffic is only moving at 65 mph.

I found a website recently that would be of interest to those in on this discussion.

Palo Alto Speedometer - search for it on yahoo - they have a free calculator program online where you tell it stock and proposed tire/wheel sizes and how much the speedo will be off at any given speed.

My car with 195/60/15's reads only 0.3 mph faster than one with 165/65/15's. Did I use the right stock tire size?

[This message has been edited by bowlsby (edited 11-17-1999).]

Sounds like you might want to download the GEARS program I wrote and it is on this site. It will let you enter each gear ratio, final drive, tire info (dia, revs/mile, circ) and will tell you RPMs for various speeds. I recently found several Porsche shops are using it since customers are asking what different tires will do the speedo indication. A BETTER way is to find a set of Mile Markers, time yourself and see what it takes for 1 mile. Traffic flow is not very accurate, since I routinely get passed by socker moms in their minivans when I am doing 3000 RPM in 4th (88mph) and I have had the speedo in our 1987 Turbo calibrated. (Must be an ego thing for them???)

The one on PA Speedo's site http://www.paspeedo.com is OK if you don't know some of the info, like the final drive, etc.

Stock tire for the 914 is 165/78-15. The 195/60s are (I recall) about 8% lower, while 195/65s are more like 2 or 3% low.

--DD

If there is one thing I learned about speedos, you cannot trust that other drivers are traveling at the speed limit. I remember it like it was yesterday. I was out testing my speedo in a differant car, it read just over 90. I was getting passed on the express way, and I was passing a couple of people, but mostly keeping pace. Thats when the cop pulled me over for doing just over 90. To make a longer story shorter, I explained about testing the speedo and got off.

To test speedo, have another car follow you. If your within 5mph, Id say thats about as good as you can get testing this way. You really need to get to a shop that can calibrate the whole car. I beleive any dealership can do this.

I have some old road test from road and track and they reported spedo eror was 5 MPH high at 70mph -- right out of the box.My spedo doesnt work in both of my 914,s I use the tach. And did you know that the early 914 was Motor Trend Import car of the year, dont remember which year.The correct speed in a 914 is just fast enough to stay out of the way of those pickup trucks.

Best way I have found to test a speedo is with one of these new handheld GPS units. If you don't have one, borrow one from a friend.

Either take the targa off or put it on your dash so the antenna can get a clear view of the sky and go drive on a reasonably straight stretch of road.

My Garmin 45 takes about 2 seconds to update the information after a speed or direction change. It is also increadibly accurate.

The suggestion to use a handheld GPS unit is a great one! I have one and will try it this weekend. Thanks, Ascaris!

Just remember to set it to the right units (miles - not knots or km) and that it lags what you do by a second or two.

It is actually kind of scary to see how acurate the GPS is rather than the other way around.

I have a '76 914 just purchased last month and my speedo also reads about 15 mph faster than it should (checked against the speedo of my other car driving along side) It has 195/60/15's which should not account for that much of a discrepancy, right?