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General Tech Do we really care what our LBC's speedometer reads? Should we?


I posted this question in another forum and am interested in the variation (if any!) in answers here...

Of course we want our speedometers to read correctly - that's how we avoid speeding tickets. But an incorrect speedometer reading also directly affects the odometer reading. And THAT directly affects the value of our cars.

You might not even realize your speedometer is off, but simple changes in tire size or gear ratios can make it inaccurate. Being a purist, I want to keep the original mechanical speedometer in my classic cars. I'm trying to see if anyone has either found or would be interested in a way to guarantee an accurate speed reading while keeping the original speedometer AND without removing the speedometer. Would love your feedback!



Obi Wan
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Well, a couple of things. First, in ordinary mechanical speedometers, the mechanisms that indicate speed and the parts that measure distance are entirely independent. So, an incorrect speed reading does not necessarily mean that the mileage is wrong, as well. Second, I think it's silly that people are considering mileage when they buy or sell classic cars. In a 50-year-old car, the engine has probably been rebuilt or replaced a couple times, ditto the transmission, and ordinary replacement parts like brakes and tires have been replaced many times. For that matter, it's highly likely that any LBC's speedometer has been replaced, perhaps a couple of times. It's also likely that the cable has broken and been left unrepaired for some time. In short "mileage" tells you little about a classic car. Note that the valuation sites like Hagerty and NADA don't consider it.

Back in the old days, all car speedometers were inaccurate; they always read high. Modern cars, today, do the same, but are a bit closer to reality. I just accept it and use a GPS if I think I need more accuracy. The greatest advantage to having some kind of variable gearset, which I think you're hinting at, would be to compensate for modified rear-axle ratios; I think that would be of value. But even if you could get the gears so the mileage was dead on, you still might have to recal the speed by moving the cup a bit, if you want good speed accuracy.


Jedi Hopeful
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For several years, our speedo never read above 42 mph. Annoying, but endurable by using the tach - 4th gear, 18 mph per 1000 rpm. After getting a T-9 5-speed, I thought it would be nice to have an accurate speedometer, and Nisonger obliged. It now reads within 1 mph +/-, and the odometer is spot on. I still use the tach sometimes, but it is nice having an accurate speedo. Not absolutely necessary, but I like it.


Great Pumpkin
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Oklahoma exempts antique and older classic cars for mileage. What that odometer reads doesn't mean a thing and even though it's exempt in some states, it's illegal to change it. PJ


Obi Wan
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Here in CA, if you're selling a car less than 10 years old, you have to verify that the mileage is correct or explain why it isn't. It's a signature on the title, when you transfer it, so you're legally responsible for it. After the 10 years, I don't know what the law is, but in practice no one cares. My TD didn't even have a speedometer for 30 years, so I guess technically it wasn't even street legal. I installed a correct speedometer in the restoration, zeroed it, and now it has only 350 "original" miles, heh, heh. I think that's the only "mileage" that has any meaning.


Great Pumpkin
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If my spedo fails, I'll just set my speed where everybody passes me, reason, my spedo works now, but everybody still passes me! 3000 RPMs, 59 MPH! :highly_amused: PJ


The original question was about keeping the Smiths/Jaeger speedometer and having an accurate speed indication. While professional rebuilding is an option, it can be expensive, particularly if the odometer requires recalibration to read correctly. The odometer requires gearing changes while the speedometer can be tweaked by changing the magnetism of the internal rotating parts.

Assuming the speedometer and odometer match each other and you are addressing wheel, tire, differential, or gearbox changes, another option is to have a ratio box made to go inline with the speedometer cable. It will convert the incoming turns-per-mile from the gearbox to the turns-per-mile the gauge expects. Places like APT (gaugeguys.com) can make these for you.

Another option is to leave the Smiths/Jaeger speedometer alone. You can install a Sigma bicycle computer in your car to give you a very accurate speedometer and odometer. The only issue with the concept is that the bike computers do not have a permanently "on" light for night driving. Some have a momentary light pushbutton but that's not practical in a car. This means adding a small light focused on the computer's display if you want to drive at night.

Lastly... I am probably one of the few people left who does not have a cell phone. For those of you who do have them, most will support some form of GPS function that will allow speed displays. Of course, most stand-alone GPS units will do the same. The downside of these options is that you would not have an odometer associated with the speed display.


Country flag
I'm trying to see if anyone has either found or would be interested in a way to guarantee an accurate speed reading while keeping the original speedometer AND without removing the speedometer. Would love your feedback!

Do you mean making the speedo accurate without removing it from the car ever? I don't know how you would fix/adjust it without removing it, they come apart reasonably easily, but you can't do it with the gauge on the car, except I suppose you could get a ratio adapter custom made (basically a gearbox that goes inline somewhere between the gearbox and the gauge on the cable) and then you could adjust it all. However, if the speedo is off and you still have original gearing in the car, such a fix would make your odometer read incorrectly.

Other options if you are allowed to remove the speedo to fix, magnetize or demagnetize the mechanism as needed to adjust the speed; or add small hight strength magnets if it reads low; establish actual 60 MPH by GPS, timing with mile markers on the road, or going by one of those radar readouts when no other traffic is near, note the speed on the speedo, say it reads 55 MPH, take the speedo mechanism out of the case, manually turn the needle/indicator to 55 MPH, hold the internal mechanism carefully but firmly so it won't turn, and turn the needle indicator to 60 MPH.

I agree with others about odometer mileage, unless of course it is an original, low miles, unmolested car. I ran LBC's as daily drivers/used cars in the 70s and 80s, and you did indeed run them with the cable or speedo broke, and you swapped out the gauge when one broke. I suppose if you were rich you could get the gauge properly repaired right away, but if you were rich you probably weren't driving a 10 year old LBC! In our state on the title mileage becomes "exempt" (from being stated on the title at transfer) after the car reaches a certain age, don't remember exactly how old, all MGs, Triumphs and Healeys are well past the exempt date.
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