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How to Recalibrate/adjust your speedometer

MrMarty51

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FWIW, I probably rearranged the VSSB 3-4 times before I got a speedo reading that matched my GPS. Either the speed calculator is a bit off or my GPS is?
Thats when it gets to sort of like splitting hairs.
For the DOT, they started installing distance meters for setting work zone signs. Too many Tort claims because signs was off by feet or yards.
The vehicles that got the distance meters, the meters had to be calibrated. The engineer dept went out on the interstate and marked off a stretch that was two miles. Once we got the meter installed then we got to go out and drive the shoulder of the interstate that distance and recalibrate, with dip switches, until the meter read correct to the distance. It was usually off a foot or two but with the dip switch that was as close as we could get them.
If the sign said men working 1,000 feet then that distance had to be accurate within XX amount of feet in order for the state to keep from getting sued if there was an accident within that distancez
 

MrMarty51

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Years ago we had mile marker lines on the highways, they are no longer there.
Yup, we still have mile posts. For the most part they are all pretty close to being right on the money.
Drive with speedo on 60 and it should take a minute to go from one post to the next, average it out over three or four miles and it should be mighty close to that many minutes. 90 about 45 seconds, 120 about 30 seconds and so on and so forth.
Old 1974 T-150 Triumph Trident, about 20 seconds over five miles, between mile posts, still had some good acceleration to go. I guess they dont call them a T-150 for no reason. 😹😹😹🤪🦅
 

dbrannon79

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yesterday when I tracked my miles, I set the GPS to record using the stride mileage app when I left the front entrance at work. got home and looked at the difference between the counter on the dash and the app. the counter showed 5.9 miles and my app showed 5.14. it's fairly close but it was a short trip, longer trips can skew mpg numbers for sure.

I can try one or two settings off from where it's at on the chart and see if the counter gets closer.
 

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FRANKENBURBAN
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You're lucky if your speedo is +/-4 mph of actual across it's sweep. These clusters use air cores, and the slightest rfi change will change there reading. You can be going at a steady speed set on your cruise, shift from 4th down to 3rd, and your speedo will go up some due to the tach reading faster(hence how if you rev your engine setting still, your speedo will move from the signal going to the tach and rfi cross talk). My 99 is dead on perfect doing 30 when it shows 30, but when it reads 70 I'm only doing 67(and I went through a few air cores to get one that was this close). My 95 will read 34 when I'm doing 30, but 70 is dead on, but bump up to 75 on the speedo, and I'm doing close to 78 actual. I always set them so the odometer reads correctly, set the cluster up up on the bench with the tach at 2k rpm's and the speedo reading correct when straight up(50 mph for the 100 mph cluster, 60 for the 120 mph cluster), and call it good.
 

dbrannon79

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You're lucky if your speedo is +/-4 mph of actual across it's sweep. These clusters use air cores, and the slightest rfi change will change there reading. You can be going at a steady speed set on your cruise, shift from 4th down to 3rd, and your speedo will go up some due to the tach reading faster(hence how if you rev your engine setting still, your speedo will move from the signal going to the tach and rfi cross talk). My 99 is dead on perfect doing 30 when it shows 30, but when it reads 70 I'm only doing 67(and I went through a few air cores to get one that was this close). My 95 will read 34 when I'm doing 30, but 70 is dead on, but bump up to 75 on the speedo, and I'm doing close to 78 actual. I always set them so the odometer reads correctly, set the cluster up up on the bench with the tach at 2k rpm's and the speedo reading correct when straight up(50 mph for the 100 mph cluster, 60 for the 120 mph cluster), and call it good.

How does the PCM read speed on my 95? I haven't checked what it sees with the scanner yet, but since calibrating with the 3.73 gears, the truck's TCC in 4th engages at 50mph now where before it would go in at 45.
 

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FRANKENBURBAN
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How does the PCM read speed on my 95? I haven't checked what it sees with the scanner yet, but since calibrating with the 3.73 gears, the truck's TCC in 4th engages at 50mph now where before it would go in at 45.
The PCM could care less what your speedo sees, it reads everything from the vssb. Lockup at 50 with 3.73's is about where it should be(3.42's don't lockup until almost 55). The vssb reads the 40 tooth output reluctor wheel, it then converts the 40 tooth per driveshaft revolution AC signal into a 5 volt DC square wave signal to the PCM, it also will convert it over to a 2k pulse per mile signal to the PCM for speed input, and it does a 4k ppm output for your speedo & such. I would never go by the speedo when calibrating the vssb, scanner only.
 

dbrannon79

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That makes sense to calibrate by the scanner. I never thought of that. back when I got the truck I found out by working on the dash that someone before me had been inside the cluster and apparently played with all the needles on the gauges. I've had to reset them one by one by verifying sensors on the scanner and other means while pulling the needles off the gauges and placing them where they should be. sounds like I might need to do the speedo in the same mannor. re-calibrate using the scanner and a GPS, then work with the speedo gauge. :)
 
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