Doug Posted February 3, 2019 Report Share Posted February 3, 2019 Hello, I have a '85 720 PU that the speedometer reads 10 MPH to fast - says 45 but I'm only going 35, has been checked by GPS and radar. I could understand a slower speed issue being a dragging speedo cable but I don't know why it would read faster than normal - any advice would be greatly appreciated. Thanks, 1 Quote Link to comment
Crashtd420 Posted February 3, 2019 Report Share Posted February 3, 2019 At some point something was swapped... tire size, rear end gear, or even if it's a different transmission it may have the wrong speedo gear In it... Do one more check... mileage... Use the odometer and highway markers... I was told to drive 10 miles.... And compare the odometer reading.... I was able to correct my odometer but I'm still about 5mph off on my speedo so that part would need to be recalibrated... The odometer can be corrected by changing the speedo gear... So you'll need to remove that to see what gear you have to determine what you'll need.... 2 Quote Link to comment
G-Duax Posted February 3, 2019 Report Share Posted February 3, 2019 (edited) Dang ! If it's off 10 at 35, how far off is it at 75? If it is due to ratio, or tire change, it will be linier, that is off 30% at 35, then it will also be 30% at 75. Most likely just old oil inside the speedo it's self, as the oil gets thicker from drying up over the years, it creates more drag making it read faster than you are actually going. Most of the time, if it's due to things gumming up, at higher speeds, it will level out. My 620 does that. At 50-65 it's dead on, at 75 on up it reads faster than I'm actually going, and below it reads slower. I've driven buy enough radar signs that the DOT places along side the road to know how far it's off through the entire range, and adjust my driving accordingly. The only way to fix it is to take the speedo apart, de-gum it, and lube with a light oil meant for watches, or just try to find a new one (next to impossible). Edited February 3, 2019 by G-Duax 1 Quote Link to comment
DanielC Posted February 3, 2019 Report Share Posted February 3, 2019 Check the mileage first. The mileage is a solid geared connection between the rear wheels, rear axle, driveshaft, speedometer pinion in the transmission, a cable, and more gears in the odometer. The speedometer is magnet in an aluminium cup, and as the magnet spins, it induces a magnetism in the aluminium cup, and drags the cup in the direction the magnet spins against a spring. If dirt, crud, or a small bug crawls between the cup and magnet, that can make the speedometer read high. If somebody swapped a car transmission in the truck, the faster driveline speed of the truck will make the car transmission speedometer pinion spin too fast. 2 Quote Link to comment
datzenmike Posted February 3, 2019 Report Share Posted February 3, 2019 The cable is a tightly wound spring. If it was being slowed down by rubbing or thick oil it would eventually have to twist off. A sticky cable causes the speedometer to jump as it releases the stored turns. 35 at 40 is about 28.6% out. That won't be entirely caused by too small a tire. Stock tire is 25.5" diameter. It would have to be under 20" diameter to get what you have. Pretty much running on the rims without rubber. The speedometer drive gear in the transmission will be around 19 or 20 tooth so to be 30% faster you would need a 15 tooth drive. I don't know of any 720 transmission that goes that low. As mentioned, drive 10 miles on a marked highway and track your odometer. It will tell the truth. 5% or 10% can be from a transmission change. The speedometer gear must ALWAYS remain with the differential and be swapped into any new transmission. 1 Quote Link to comment
G-Duax Posted February 4, 2019 Report Share Posted February 4, 2019 (edited) The cable 'spring' is 100 times stronger than the spring in the speedo Mike. Just a little drag will effect speedo readings, and no where near what it would take to twist off a 3mm cable. 3 hours ago, DanielC said: If....... a small bug crawls between the cup and magnet, that can make the speedometer read high. LoL, never thought about a bug, but yeah....:) Edited February 4, 2019 by G-Duax 1 Quote Link to comment
giantcone Posted March 25, 2019 Report Share Posted March 25, 2019 Just wanted to reply to Doug, that I'm encountering the same issue on my '85 RWD 720 (I may have mentioned this issue in the past, and datzenmike likely answered). My speedo "over-reports" the speed, and I found that the discrepancy increases with increasing speed. Now I'm running the 195/75R14 tires and the speedo cable is the original, and I haven't done anything to whatever speedo gearing comes out of the transmission. Would the speedometer over-reporting also lead to an over-reporting by the odometer? Quote Link to comment
Crashtd420 Posted March 25, 2019 Report Share Posted March 25, 2019 21 minutes ago, giantcone said: Just wanted to reply to Doug, that I'm encountering the same issue on my '85 RWD 720 (I may have mentioned this issue in the past, and datzenmike likely answered). My speedo "over-reports" the speed, and I found that the discrepancy increases with increasing speed. Now I'm running the 195/75R14 tires and the speedo cable is the original, and I haven't done anything to whatever speedo gearing comes out of the transmission. Would the speedometer over-reporting also lead to an over-reporting by the odometer? If its increasing with speed its probably the speedo gear.... And yes the odometer is tracking whatever mileage the speedometer is receiving.... Do the 10 mile check ... If the 10 mile check is wrong you need a new pinion gear..... if it's right the speedometer is out of calibration.... I would get the odometer reading properly then see how the speedometer reads..... If your lucky the new gear will fix both... I'm off a steady 5 mph at any speed but my odometer tracks correctly.... 1 Quote Link to comment
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