Replacement Speedometer

[Howard] built his own replacement speedometer for his truck after the original speedometer cable broke. He’s using surface mount components and produced a two-board design that is quite nice. When he tipped us off he mentioned that this is Arduino powered and uses a hall effect sensor. There’s not talk of this in his writeup but we gather that he’s just using the bootloader on an AVR chip and that he hall effect sensor measures the rotation of one of the wheels. When the vehicle isn’t moving the board alternates between max speed and trip distance. Once he’s on the roll it shows current speed.

36 thoughts on “Replacement Speedometer

  1. Seeing as most vehicles require the speedometer cable for the odometer to work, I’d suspect this to be illegal in the States. Also, I wonder how he calibrated it. He references his SatNav. But, is that really accurate? I know in all the cars I’ve owned, my speedometers were always dead on with the radar things they use around town to warn you that you’re going too fast, so his claim is invalid.

  2. I’d venture a guess that he ran the sensor off the driveshaft rather than a wheel. More accurate and more accessible.
    Also, speedometer error is quite common. Look through new car and motorcycle reviews, almost all will list a slightly inaccurate speedo.

  3. We have a yearly MOT (Ministry of Transport) test that checks the safety, roadworthiness and emissions etc.
    Working speedo is not checked on the MOT, however, other regulations say that if its fitted it has to be in good order… as well as display the speed in MPH and kilometer per hour.

    I think so long as there is a way of judging the speed you should be ok

  4. @Josh: I’ve never had a car with an accurate speedo, but some are worse than others. Usually the error gets worse as you go faster. Mine is off by only 1-2mph at 40, but off by 5 at 75mph (actual speed is 75 when the speedo reads 80).

  5. He should do this for the fuel gauge. Fuel gauges in the states are extremely nonlinear. For example, the gauge on the 2001 Ford stays stationary at Full until the fuel tank drops below 90%, then it stays still at half-full from 60-40%, and then gets stuck near empty for a while too. It’s completely irritating and I’ve seen it on every car I’ve driven. I heard an interview with an auto-exec justifying that practice because consumers will like the car better if the gauge stays at full for much longer after they fill up the tank.

  6. >I heard an interview with an auto-exec justifying that practice because consumers will like the car better if the gauge stays at full for much longer after they fill up the tank.

    I’m not sure how true this is. My understanding is that it’s because fuel tanks have unusual shapes and the fuel gauge works due to a float connected to a rod in the tank. This provides variable resistance. The actual amount of remaining gas is not consistent because of the tank’s shape. Also, the gas sloshes around as the vehicle moves and the float moves with it, so what you’re seeing on the fuel gauge is actually an average estimated amount of gasoline in the tank. Lots cars attempt to correct for this in the car’s computer nowadays and quite a few do a very good job of it.

    Here’s a page that describes how they work and talks about exactly those caveats.

  7. I’ve never seen vehicles not pass inspection with broken speedometers. of course, I’ve never had a vehicle that had to be connected to a computer to be emission tested either and one of mine is 3 years away from being only safety tested. Classics are only safety tested in most states and odometer readings become exempt after a certain number of years/miles as well I think (all my titles say exempt for mileage – newest vehicle is a 93). Since his original speedometer was cable driven, it should be even older than my 93 with it’s electric speedometer. So legally, there should be no issue in the states (except California and other “it’s been modified, I can’t inspect it” areas).

  8. judging by the picture of the inside of his truck, he probably has a ton of miles on the odometer anyway. I have no idea about the UK, but in the states, once you’ve cleared 100k miles, no one seems to really care anymore if the vehicle has 101k or a million. Some older vehicles also only have 5-digit odometers (like my old VW beetle), and this person’s land rover seems like it might be of similar vintage.

  9. This would be great to reverse the digits and make your own Speedo Heads Up Display. I have the HUD in my Grand Prix GTP and would love to add it to my other cars as well. I don’t know how I went without it. Though the reflection in htose wind shields might have slight double vision it would be nice.

  10. @Alan
    I like the idea of using a motor to drive the factory speedo.

    It might actually be cost effective for me too. Cables for my car are over $100 and break every-other year.

  11. Last time I bought a speedo cable it was only about $15… though I really hate cable based speedos since they’re noisy and the needle bobs up and down.

    As for the Fuel gauge… it shouldn’t be too difficult to estimate consumption, we know full open throughput of the injectors, how many injectors are the in the engine, and the duty cycle specified by the computer at all times… it shouldn’t be too difficult to build something that captures that data and estimates fuel consumption LIVE and then based on total tank capacity gives you a proper idea of how much is left.

    Heck if you don’t know what you’ve got for injectors you could even just tap the injector pulse outputs from the ECU, fill up, record the results and the next time you filled up determine how much fuel you had to put back in based on the sum of the open time for the injectors… and use that as a basis to estimate consumption

  12. ^another method I just thought of would be to monitor the Air fuel ratio and apply it against the current engine RPM and overall engine size… it’d be a lot more lose estimate but easier to implement still miles ahead of the stupid tank sensors they currently use.

  13. Isn’t this just for the purpose of using an Arduino ? As I see it, what he did could have been done without programming, using only a cheapo bike computer. Usually they can do up to over 100 km/h anyway, and have odometer, trip, current speed etc etc. All you would need is to install sensor and calibrate wheel size. Plus maybe add some ambient light for readability.

  14. I like the idea, but isn’t there a more time/cost effective way to do this? I have a feeling there is an over the counter unit that does exactly this.

    And @Andrew. The same can be said for digital fuel gauges. They run about $80-$150.

  15. My Nissan’s spedometer is broke but the odometer and tachometer work perfectly. I’ve been too lazy figure out why and I know it’s a known issue with Nissan. This looks more worthy of my time than just fixing the plain ol stock speedometer. Also, there are no inspections where I live.

  16. sega said: “So when a cop pulls him over for a speeding ticket he’ll just look at the “max speed” display?”

    HAHAHAHA i spose he could either take the ticket, or argue the speedo is faulty… whichever is less.

  17. Holy hell, I guess he gets props for putting this much work into designing something that has been designed over and over again, but dang, why not just get another speedometer cable???

    To the claims that speedometers are off by ~5mph when traveling at higher speeds: Yes, older, cable-driven vehicles were this innacurate, but any newer (read: mid 90’s) vehicle does not have this problem. Speedometers that are driven by a sensor instead of cable are incredibly accurate, and in my experience as a technician are generally within 1-2mph of actual speed, even at highway speeds. This error is introduced by slight variations in tire diameter, and by variations in the speed markings on the speedo face itself.

    Basically, if you don’t have a broke-ass 80’s car, and have not drastically changed the outer diameter of your tires, your speedometer is quite accurate.

    Also, DanS, there is no reason for your speedo cable to break every other year. A good cable will last the lifetime of the vehicle. Sounds like you are not properly lubing the replacement cables, or are going below the minimum bend radius that the cable is rated for.

  18. “I know in all the cars I’ve owned, my speedometers were always dead on with the radar things they use around town to warn you that you’re going too fast, so his claim is invalid.”

    I realize I am incredibly late to this show, but just because your speedos are accurate doesnt mean everyone’s are.

    The speedometer in every car I have ever driven has been off by 1-2 mph or more. This is far more accurate. The differences between new tires and bald ones can cause 1-2 mph variations in reported speed.

  19. Agreed, the speedo in my 98 Accord and my mom’s 2001 (?) Nissan Altima are off by 5 mph at highway speed. Heck, on my motorcycle, I just use my old GPS as my speedo. Works much better than the stock one.

  20. I applaud the build. Anyone who makes something to suit a purpose has to be admired. Personally, the more Heath Robinson, the better.

    As for the legality of this as a suitable replacement? I can only add this…

    In the UK, the speedometer and odometer are not checked at the annual Ministry of Transport test (MOT).

    However, in an effort to reduce instances of ‘clocking’, vehicle odometers are checked periodically to ensure consistency and reduce fraud. Without a functioning odometer, this is impossible.

    Also, despite rigorous calibration, the speed recorded by a police radar will always win in court. Their argument being that a DIY replacement speedo cannot be accurate.

    The only place where this build could be considered legal would be on private land where the rules of the road do not apply.

    I don’t know where the HE sensor is placed, but if it came off and the speedo was unable to accurately report the car’s speed, it’s an offence. I know someone who was done by the cops many years ago because his regular speedo was faulty and that was the basis of his defence. He was fined and got points.

  21. Whew! I forgot Hackaday featured my little project. Thanks for all the comments, guys :)

    In answer to a few questions/points:

    1) a new speedo cable would have been cheaper and simpler, but not accurate on this truck as the wheels are a different diameter from the originals, so the speed was always incorrect. Also, it’s difficult on this truck to thread the speedo cable through the various bulkhead holes without half diassembling the thing first.

    2) Was this just an excuse to fit an arduino? Damn right it was – I did it cos it was fun. Also, I like lights and displays and things even if they’re not completely necessary

    3) Not sure about the UK law regarding speedos, but since the existing one was inaccurate to the point of being useless I can’t see anyone penalising me for using a more accurate home-made one

    4) This has been incredibly reliable and consistent – the HE sensor is (as someone guessed) fixed near the driveshaft, and there’s a little neodymium magnet glued to the shaft itself. Haven’t had any problems yet.

    5) Odometer / clocking: doesn’t apply on this vehicle. Too damn old. (it’s a 1969 vehicle, exempt from tax, seatbelts etc)

    6) If I get stopped for speeding I’m certainly not gonna blame the speedo :)

    7) I call it a truck – you can call it a car if you like

    Thanks for the feedback, peeps, nice to know it was worth taking pics and doing the write-up. I’ve learnt such a lot from other projects on this site it’s nice to be able to return the favour :)


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