Speedometer 3 mph Off

[Newps]

Originally Posted by Spidergawd

Glad to hear it’s not just me.

Forscan sounds pretty interesting. I’m going to look it up. Thanks for the recommendation.

The first post explains everything.




https://www.f150forum.com/f118/forsc...-truck-348987/

[530ktm]

Strange, all my vehicles, motorcycles, jeeps, pickups and my coach all read pretty much spot on. The one thing that is never accurate is the fuel mileage range and mpg I am getting. It is always very optimistic.

[US1]

Error codes explained when correcting tire dia/MPH:

https://forscan.org/forum/viewtopic.php?t=15994

[Fiesta48]

Just buy larger diameter tires. That will correct the speedo error. And get better mpgs.

[AbdRahim]

Mine is reads a few mph higher than the gps. All of these things have some margin of error.

[Vibeman]

I have some time on my hands this morning so FWIW:
Accuracy and precision of any gauge are two different things. Accuracy refers to how close the actual reading is to a known standard. Precision refers to the calibration level of a gauge. I have a Thor Hurricane on a Ford F-53 chassis. The speedometer on my dash is calibrated in 5 mph increments. The markings give my speedometer a precision level of +/- 5 mph, no more. Any reading I take when the needle is between the markings is a estimation on my part and not an actual reading because it is not calibrated to that level. I can guess where 57 mph might be by looking at the position of the needle relative to the nearest calibration mark, but my speedometer does not read 57 mph, only 55 or 60 so it is technically accurate, saying I am somewhere between 55 and 60, but not very precise. My GPS, on the other hand reads in 1 mph increments and therefore can tell me when I am travelling at 57 mph, but not at 57.5 mph.

In the case of an analog speedometer dial, the viewing angle makes a big difference too because of the distance between the needle and the face of the dial. When my wife looks over at the speedo when I am driving and tells me to slow down, she will see a different reading than I do because of her viewing angle. When I am in the drivers seat my viewing angle is directly in front of the speedometer but looking down at it so that will affect what I see more at some speeds than others. 50 mph is at the top of the gauge so is probably not affected very much by my downward viewing angle, but 20 mph is over around the 9 o'clock position so a down viewing angle will affect that more.

If you want to really know the accuracy of your speedometer relative to your GPS (GPS readings have an accuracy +/- rating too) Set your cruise control so your speedometer needle is stable and as close as you can get it to an actual speed mark near the middle of the range when viewed from directly in front and looking straight on. If you are as precisely on the 60 mph mark as you can get it and the GPS is reading 63 mph then you have a 3 mph error in your speedometer, but ONLY relative to the GPS and ONLY at 60 mph because the error can change depending on where your reading was taken within the range of the speedometer.

[Crasher]

All of our vehicles are within 1 mph or less of actual. It's easy enough to check by timing between the mile markers on the interstate. 3 miles in 180 seconds is as close to 60 mph as you can get.

[tap4154]

Originally Posted by FIRE UP

The problem with having a speedo read incorrectly on the HIGH side, as in it reads faster than you're actually traveling is, you rack up miles that you AREN'T actually driving. If you're cruising on down the road at ACTUALLY 70 MPH but, your speedo reads 73, in that one hour, your odometer says you went three miles farther than you actually did.



The average owner, for the most part, could care less about such trivial differences. Well, that's ok if you have a vehicle that is OUT OF WARRANTY. But, if your vehicle has a year AND mileage warranty, like many or most do, what can happen is, you can easily arrive at the end of your warranty, mileage wise, long before you should.



Case in point. Quite a while ago, Honda car owners were having this exact problem. And, it was not only nation wide but, world wide. The problem began when folks were attempting to get their Hondas repaired for various issues, and thought for sure, they still had warranty. Honda of course, would deny warranty due to mileage.



Well, as one would expect concerning something like this, the problem grew, across the nation and folks got together and yep, formed a class action lawsuit against Honda. Guess what, Honda lost. After that, Honda was ordered to make their speedo's considerably more accurate. And, it was the same kind of situation with their Goldwing motorcycles too. After 2009, the speedo's on those were quite a bit more accurate.



So, do any of you care about this, most likely not. It's just info. But, if I were to purchase a car/truck/motorhome/motorcycle brand new today, and I hadn't signed the dotted line yet, and took any of those type vehicles out for a test drive and found the speedo to be off, NOT IN MY FAVOR, I'd make darn sure that the dealer, and if necessary, the manufacturer, would be acutely aware of it reading more miles than I travel and, I'd be discussing additional warranty AT NO COST TO ME. Now, would I get what I'd be asking for? Who knows. But, you get my point.

Scott

My old Ford F-150 read about 3 miles an hour fast, and I always figured they did that so I would lose the factory warranty quicker. My motorhome reads pretty accurately, it's a Ford E450, and my Ram pickup does as well.

[Steve70-5]

How do you know the GPS is right? I would like to see different GPS side by side and see if they agree. I have a older GPS and it matched the speedo, my new GPS does not match.

[TandW]

Doesn't anyone ever consider wheel slip upon the pavement when powering that big box down the road at ever higher speeds?
My View will comport with GPS pretty accurately up to about 50 MPH. After that the speedo would need to read 78 while GPS reads 75. On level roads.

[US1]

Wheel slip on on RV? can't say thats ever been a consideration

[TXiceman]

All speedometers are off a bit. The only sure way to check is find mileage markers on the highway designated as a speedometer check point. Check the speedometer over several mile and time it for 60 MPH or one-mile per minute.

65 mph on my GPS is 63 mph on the speedometer. Checking it over a measured miles, I am actually doing 64.5 mph.

Ken

[TandW]

Originally Posted by US1

Wheel slip on on RV? can't say thats ever been a consideration

Traction loss and resultant tire wire are compounded by high speeds.
Actual speedometer error is governed in the US and Japan by SAE J1226, while the Europeans adhere to UN Regulation 39. Hope this helps!
Odometer error is handled differently.

[twinboat]

Originally Posted by TandW

Traction loss and resultant tire wire are compounded by high speeds.
Actual speedometer error is governed in the US and Japan by SAE J1226, while the Europeans adhere to UN Regulation 39. Hope this helps!
Odometer error is handled differently.

So if I'm going 70 MPH into a headwind, my tires are skidding and wearing faster ?

Wouldn't that make the RV loose control ?

I'm not buying it.