140 P.I. speedo question

[ironstealth]

What were these speedo's "calibrated" for exactly? gears,tire size's? I've noticed that my speedo is not exact--the faster a I go,the more off the speedo reading is. Before i changed tire sizes,i was only 1 or 2 mph off,no big deal. But at 80mph on the speedometer,I'm actually doing a little over 70mph. The tires are only .8 of an inch smaller. Could it be possible that these speedo's are calibrated for 3.08 gear rather than my stock 2.73?

[torquelover]

I haven't seen any 20 year old "certified calibration" speedos read accurately. My 2.73 geared 235-70-15 tired speedo was way low, about 10-12% off. Switching to slightly shorter 235-60-16s and a 3.27 gear got it somewhat in the ballpark. By somewhat, I am saying +/- 10 MPH at 70 MPH.

[trwp72]

Depends on what the rear end ratio was on the car it came off of, and what yours is.
Police speedos are marked in 2 MPH increments, unlike civilian speedos, that are usually 5MPH increments, so when they "pace clock" someone, it can be matched closed to the correct speed, since most states require a 5 MPH grace. A 5 MPH speedo wouldn't be much good. If it is that far off, go to a good speedo shop and have it re-calibrated. They can find out if it is off, and how much, then change the gearing at the transmission to get it back on track. Another thing it could be is that the speedo is dragging against something inside the case... So it may be in the case itself. (Mine did this when the mount for the sppedo needle assembly came apart, causing the needle to "stick" every once and a while. I fixed it by gluing the mount back together. Also, if it is pre 1990, you may have a speedo cable dragging, and needs lubricated.

The calibration for the speedos is actually done by speedo shops, they put the car on a roller arrangement like a drive on dyno, and then run at certain speeds, usuallu 25 - 35 - 55- 55- 65- 70- and 80-85, and see if they match the needle on the speedo machine. Then they are "certified" by the shop with a speedo certification certificate. The state of Florida requires that the cars that are used for speed enforcement, that "pace clock" cars, be certified every 6 months.

[gadget73]

Its calibrated in the car for the entire package, rear gears, tires, etc. If any of that is not the original size stuff, the speedo is off. They also just get out of whack from age. The one in my Towncar is 2mph slow at 65 mph. Its dead on at 85 mph. Only explanation I have for that is the speedo is worn out and needs to be re-calibrated.

[GM_Guy]

What were these speedo's "calibrated" for exactly?

Accuracy. =-)
Most speedos are 1000 revolutions at 60mph actual road speed. The calibration just means that the speedo will actually read 60mph under those conditions. Change tire diameter, rear gears, have the wrong speedo gear in the sender, your speedo will not read accurately.

When you "adjust your speedo" your changing the driven gear in the trans sender in an attempt to bring the speed the cable spins at as close to that magic 1000 revolutions at 60mph in order to get as close to an actual road speed reading as possible.

The important question is, whats your real road speed when the speedo shows 60mph? Use a GPS. You can NOT rely on other vehicles as most civilian cars read slow. Roadside radar boards should be accurate as well if they have those around your area.

Alex.

[torquelover]

The important question is, whats your real road speed when the speedo shows 60mph? Use a GPS. You can NOT rely on other vehicles as most civilian cars read slow. Roadside radar boards should be accurate as well if they have those around your area.

Alex.

Interesting older article here:

http://www.caranddriver.com/features...andal!-feature

[gadget73]

Changing the gearing will fix it to a percentage of accuracy. If you're say 10% out, you'll see the error get bigger as you go faster. It will be 1 mph for every 10. Tires and gear changes will make that sort of error. If you're not consistently inaccurate, like my car is, thats a problem with the speedometer itself. Theres also a limit to how accurate you can make it by changing those speedometer gears. The gross adjustments are usually done with gears, really finite adjustments have to be done within the speedo itself.