Last post on Oct 13, 1998 at 8:16 PM
You are in the Pickups - Archived Discussions
#23 of 32 email@example.com
Aug 05, 1998 (4:21 am)
There are two ways that I know of to calibrate a speedometer.
In California, the local yokels are required to establish speed limits by a speed survey. This used to be done by having some one sit in a car with radar and clock everyone for a day or so. The rule was to throw out the top 15% as reckless and the bottom 15% as dangerous and average the remaining. Hence a new speed limit.
Today this is done with a trailer based machine. This system automatically records your speed AND it also displays your speed in 2 foot high numerals for you to see. When I find one of these set up, I go through it 4 or 5 times at different speeds.
The second method is the mile markers on the highways. They are not wide spread in CA but there are some. Usually there is a sign, "Speedometer Check Ahead." Then followed by Mile 0, Mile 1, etc. up to 5 miles. The trick here is to use cruise control and a stop watch. Just time yourself over the 5 mile course. Record your time at each mile. If you have a stop watch that calculates the speed for you, the answer is obvious. If not use the following formula:
Miles x 3600
Speed in MPH = -------------
Time in seconds
There will be variations in the speed control so the speed will vary, however it does give you a good idea.
I did both of these steps in my '92 F-250. The most amazing thing was that the speedometer read about 2.5 MPH fast over the range from 35 MPH to 80 MPH. I had done this many times at highway speeds before and after new tires. The old tires were maybe 2.25 and the new tires 2.5. There is one other interesting fact about this speedometer, that is the feed for the speedometer was pulses and not an analog feed from the transmission. The pulses were derived from the ABS sensors.
A trip to Vegas this weekend in the '99 SD. I'll be sure to bring the stop watch.
#24 of 32 lwf
Aug 05, 1998 (1:07 pm)
Thats really interesting. I've seen the police-owned signs that tell you what your current speed is, but I didn't realize they were used for speedometer-calibration puposes. But that only tells me how my speed can be measured external to any equipment in my vehicle. Now that I know my speedometer is in error, by let's say 2 1/2 percent, is there something in my ABS I can tweek and then go back and check it out using the readings on the cop's 2-foot-high speed sign? Regarding your statement "The pulses were derived from the ABS sensors." What movement is being sensed by the ABS? Also, I had assumed these pulses were source infomation for both the speed indicator and the odometer, which would mean if one is off, the other is off by the same percentage. Is that right or wrong?
I haven't driven in CA in many years, but I recently took a drive from NJ to Phoenix and back and used the highway mile markers in almost every state (Pennsylvania, Ohio, Indiana, Illinois, Missouri, Oklahoma, Texas, New Mexico and Arizona.) to check my odometer. Whoever made the mile-length measurements in those states did it the same in every one, because my odometer was inaccurate by the same amount in every one of those states. I had simply assumed that if the odometer is off, so is the speedometer by the same percentage.
#25 of 32 firstname.lastname@example.org
Aug 06, 1998 (5:41 am)
My odometer was reasonably accurate. Something like a third to half of a tenth in 5 miles. The odometer was LCD and it was a kind of guess. I don't remember checking for distance accuracy in Arizona where they have mile markers. (As I remember from my younger rallye days they were very consistent.) The only reason I know about the speedometer sensor being the ABS sensor is because it failed. The light ABS came on, the flash code said bad sensor, the speedometer was jumpy and the transmission was confused.
The shop manuals said that there was a feature in the '92 for auxiliary equipment that needed a speedometer input. This feed offered about 8000 pulses per mile going from 0 volts towards plus 16 volts.
#26 of 32 lwf
Aug 06, 1998 (1:48 pm)
That is prettty accurate. Extrapolating the numbers you gave gives somewhere between a .66 and 1 mile error in 100 miles which could easily be accounted for by tire wear.
I guess that blows my theory to hell.
Still, it seems surprising that a big engine such as yours won't push it to the century mark. For what it's worth, that isn't a problem with your truck's kid brother, the F150 with a V6 and a 3.08 rear end. I certainly agree with the others who have implied that driving at that speed is risky, but while I was on that trip I mentioned in the earlier post, there were lots of occasions with no other vehicles in sight (no moose or deer either), so I opened it up one time. It went to 100 even with a speedoemeter which seems to me to be reading low. There seemed to be lots of pedal left, and the tachometer was at only about 2600 RPM (I believe that engine's torque and HP peak at above 3000), so it might have gone faster. But I wouldn't have been able to tell how much with a speedometer that goes up to only 100, so I slowed down.
For the benefit of anyone else interested in the performance of the F150/4.2V6/3.08, there were some decent hill-climbing tests along the way (in and out of Wheeling W Va and the 7000-ft climb to Flagstaff from the south), and it took all of them easily. MPG figures were between 17 and 21.5 with 17-19 figures in the West where speed limits are 75 but the traffic moved at 85, and the higher numbers pertained to driving in the East where the limit is 65 and I stayed pretty close to it, because it appeared to be enforced when I went through states like Indiana, Ohio and Pennsylvania. The MPG figures are not as good as those of my previous Nissan PU, but they seem reasonbly high to me and the power, of course, is far more than what I had before. The only problem for me, however, is that I seldom drive on highways. Most of my driving is local where the MPG figure is typically only 16 to 17; whereas, it was more like 20 with the Nissan V6, but I have to keep telling myself that was a compact and this one is full size.
This one has an automatic which I never before had in a truck and don't really like, but I got it because the towing capacity is higher than what Ford provides with a stick. That fact seemed strange to me, but that's the way it is. I haven't towed very much with it yet, but I have done it a few times........about 2 tons about 50 miles each time. It did it easily and without very much perceived strain on the vehicle.
#27 of 32 email@example.com
Aug 10, 1998 (5:46 am)
re: Speedometer accuracy
First, the mile markers on I-15 (South bound) in southern Nevada are not placed accurately. Sometimes off by as much as a tenth.
Second, in the California desert, where they use aircraft to patrol the highway, there are mile markers, of sorts. These are placed every mile and usually within 50 feet of a call box.
Using a stop watch I clocked speeds 70, 75 and 80 MPH. The stop watch calculates MPH from the time. My speedometer is reading between one and one and a half miles fast at those speeds. Each test was done 5 or 6 times. (i.e. 5 or 6 one mile segments.)
This is what I expected based upon the experiences with my '92. As I can further calibrate my speedometer, I'll report other speeds.
#28 of 32 richflynn@Yahoo.com
Aug 10, 1998 (6:00 am)
The ABS sensor in the '92 was sort of like the tachometer sensor. The sensor is placed near a rotation gear. Pulses are derived from the proximity of the sensor and the ferric material in the gear teeth. Amplifiers then shape the signal into a square pulse. I don't think that you can adjust the pulse generation rate, however the shop manuals mentioned something else. Evidently there is an adjustment possible to the speedometer. There is a flash memory in the instrument cluster. The shop manuals mentioned something about setting it for either a tire size change or a rear end ratio change. The manuals STRONGLY cautioned that this was a three shot deal. You could make the change three times and the flash memory was done for ever. It soulds like it was really an EEPROM and had 4 total locations for the calibration code. I don't remember much else and the shop manuals went with the old truck.
Actually, I would prefer the speedometer to read a mile of two fast. Sort of set the cruise at a comfortable ten plus and I'm only doing eight or nine plus. There's a lot of security in that.
#29 of 32 firstname.lastname@example.org
Sep 11, 1998 (11:56 pm)
Speedometer accuracy update,
I've driven through a radar based speed indicator sign. (Owned and operated by the Westminister, CA PD.)
At an indicated 35, the sign displayed 34.
At an indicated just under 30, the sign bounced between 27 & 28.
Based upon these two tests and the stop watch timings, it looks like the speedometer is reasonably consistent and indicates about 1 mph faster than you're actually travelling.
I guess that if you know at what point the bears are writing tickets, just set your cruise control to that number and you should be safe.
Oct 11, 1998 (4:20 pm)
Easiest way to check speedo accuracy is to take the GPS unit from your boat and carry it in the truck for a day or two. Modern GPS units update speed over ground every few seconds. Forget the odometer or mile posts... they are not accurate enough.
Oct 12, 1998 (3:37 pm)
My V10 is speed-limited to 94mph also. IMO, that's a bit low -- the Banks kit is supposed to get rid of the govener though.
#32 of 32 richflynn@Yahoo.com
Oct 13, 1998 (8:16 pm)
If you get the Banks chip, let us know about it.
I'm still waiting for a call back from Banks for when they start producting the diesel version.