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I have a '67 with a four speed trans. The speedometer reads ten miles an hour slower than what I am really going. My question is, the teeth on the original gear is 23. I purchased a 25 tooth,
this did not change the speedo at all. Has anyone on the Forum ever experienced this and do I go to the other end of the spectrum and try an 18 tooth gear. This is driving me nuts.
Thanks for any input you might have.
Tom
Tom - Pretty sure you want to go 'down' on the tooth count, not up. I recall each tooth adding or subtracting 'around' 5mph so if it's already reading slower than it should by about 10 mph, try a 20 tooth Blue driven gear or maybe a Green 22 tooth gear.
Mike T - Prescott AZ
Oh so sorry Officer, I need to explain that I was using you, to calibrate my Speedo!
Maybe have one reading too fast, could be a good thing. I have a dual sport motorcycle like that. At 70, I am only doing like 60, but it feels like 70, until someone honks at me!
Last edited by TCracingCA; Mar 19, 2021 at 10:22 PM.
If your true speed is 60 mph but your gauge is reading 50 mph, you will need a driven gear with fewer teeth. Fewer teeth spin the speedometer cable faster which spins the speedometer internal magnet faster which increases the "magnetic pulling force" which results in increasing needle movement And, the opposite situation of your true speed reading 50 mph but your gauge reading 60 mph, you will need a driven gear with more teeth to slow cable, magnetic and needle down. I am sure that you are also aware that changing tire size and rear end ratios also have an effect on speedometer readings.
A correctly calibrated speedometer should read pretty consistently at true speeds of 15, 30, 60 and 90 mph. This calibration is a function of the internal magnet having the right magnetic strength. The magnet can lose strength overtime for various reasons but it can be recharged. The effect of a weak magnet will be needle readings that are low and an over-charged magnetic will be needle readings that are high. My point is that if changing the driven gear is yielding inconsistent results, your gauge might also require professional calibration.
there is a driven gear calculator somewhere on the net where you put in you variables like tire size, rear end ratio etc and it will tell you what size gear you need. Also if the trans is being rebuilt, they should ask what you have in the car so that they can install the correct gear, my guy did.
Interesting, glad somebody brought this subject up. My '67' runs exactly 5mph faster than true speed. Example: At 40mph on speedo, actual speed is 35mph. This is based on a couple of those laser speed reminders around town. I know those so called laser speed reminders are dead on as my truck and Acura are dead nuts on when passing by. So, based on comments above, looks like I need a gear with one more tooth which should bring me closer to actual speed. Of course I could play it safe and leave it so I'd always be less then the posted speed limit.
Not sure if you have any interest in pulling the cluster to recalibrate the gauge, but the starting point for fixing the whole speedometer system is getting the cable speed correct first. If your trip odometer works, take the car on a highway with mile markers. Drive for 20 miles or so and note the difference between the marker mileage and what the trip odometer reads. Thats the error in need of correction with a different speedo driven gear. Its a straight up proportional correction to the number of gear teeth.
Once that's fixed, you can directly measure the speedometer error with a reference like GPS. Good idea to do that on an open road at speed, say 70 if the law allows. If the error bothers you, then pull the gauge and have it recalibrated. You may find that correcting the cable speed also fixes the gauge speed, but at least you won't be compounding one error with another one.
FYI - I had to replace my speedo due to a partially broken off needle and other issue but it was reading 15 mph over actual GPS speed. After new OER speedo installed it is within +/5 mph accuracy compared to GPS.
Interesting, glad somebody brought this subject up. My '67' runs exactly 5mph faster than true speed. Example: At 40mph on speedo, actual speed is 35mph. This is based on a couple of those laser speed reminders around town. I know those so called laser speed reminders are dead on as my truck and Acura are dead nuts on when passing by. So, based on comments above, looks like I need a gear with one more tooth which should bring me closer to actual speed. Of course I could play it safe and leave it so I'd always be less then the posted speed limit.
Apparently the highway patrols in Germany think they are still **** and speeding is taken VERY seriously. As a result, all BMWs show about 5 mph FASTER than actual speed.
The previous owner of my '65 had the speedo out for repair back in 1988.
It reads 2 miles off(slower) from those "your speed is" radar billboards.
Not enough to worry about.
May be the 215 tires too.
Marshal
The previous owner of my '65 had the speedo out for repair back in 1988.
It reads 2 miles off(slower) from those "your speed is" radar billboards.
Not enough to worry about.
May be the 215 tires too.
Marshal
I'm not certain my reading is accurate in absolute terms, but the portable municipal signs are all over the map. When compared to my speedometer reading, they are not consistent one sign to another.
speedometer reading wrong
