Speedometer Way Off!! Need Help

Discussion in '2005 - 2009 Specific Tech' started by Brian Clevenger, Jul 3, 2013.


  1. Brian Clevenger

    Brian Clevenger New Member

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    I just bought a 2008 mustang gt and the previous owner lowered it, and tuned it and cold air intake and bigger wheels and tires.. 255/35/20 front and 275/35/20 rear. It came with a diablo predator programmer part number diablo-u7145. The speedometer is 5-10 mph fast and have no idea how to diagnose what it could be and dont wanna use the programmer unless I know what im doing. any help would be great!
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  2. Brian Clevenger

    Brian Clevenger New Member

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    I'm assuming he flashed back stock and may have aftermarket gears?
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  3. F. Jason Garriott

    F. Jason Garriott Member

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    Hi, Brian:
    There's an outside chance it may be the guage cluster. I don't think bigger rear tires would cause that big of an error. My '05 GT had a failed speedo when I bought it and the other guages failed one by one. It's a common problem on 05s. Go on ebay, there're companies that take your existing guage cluster and solder in new stepper motors. It's an easy job to remove and send the cluster. It's about $80. The car will run without the guages. Or try [email protected]. Good luck.
    Jason
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  4. 805mustang

    805mustang Member

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    Taller tires will throw the speedo off, you need to tell the comp what size tires your running. Some aftermarket tuners have this option or the dealer. If the problem started after u put the tires on I would suspect that is your issue. Only other possible causes is speed sensor in or instrument cluster! If you can view live data on your tuner or if u have a scan tool compair them, if there is a difference your cluster is going out. If there equal its your tires. You can put the old wheels/tires on and see if it goes back to normal. Let me know what you find
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  5. Owner1

    Owner1 Member

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    It's your rear tires. Your stock tires had 10" of total side wall height (actually called "section Height"), and your new tires have 7.6" total sidewall height. If the car came with 18's and you now have 20's, that means you gained 2" in tire diameter from the wheel change, and then lost 2.4" in tire diameter from the lower profile tire. So your rolling diameter is now smaller, so your speedo would be reading fast unless it was corrected with your tuner. If the car came with 17's then your problem is even worse.

    My SCT tuner can correct for tire size, but I don't know about Diablo.

    Owner1
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  6. 805mustang

    805mustang Member

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    Yep....you nailed it, although rim size is not the issue it's the overall diameter of the wheel. You can go up or down in rim size and still keep the overall wheel diameter the same.
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  7. Brian Clevenger

    Brian Clevenger New Member

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    got it back from the dealer and they are idiots and said they had no idea what it was.. But i plugged the tuner in and tuned for 4.11 gears and the speedo is now correct. used a gps app and reset the trip and drove a mile and it was correct. the diff says 3.31, can you put 4.11 in the stock diff size? or is the correction for the gears the same difference i would have with the tires?
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  8. 805mustang

    805mustang Member

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    It sounds to me like you do not know if the gears in your rear diff are actually 4.11 or 3:31 and that your diablo tuner does not have an adjustment for tires .....just rear end gearing. So you tuned your car to the 4:11 gears and now the speedo is close to accurate. Am I correct on all of that? If I am correct I would not tune it to the 4:11 gear setting unless you have a manual trans, even then I would not do it but can't really see any harm being done. It would be better to live with the inaccurate speedo than to tune it to the 4:11 setting. I can say that from what your telling me its all in the tires.....that said you need to have the PCM updated with the new tire size. The dealership where you brought your car seems like they don't know their elbow from their *******.....I would find a different dealership or shop that he's the capability to program the PCM with the new tire size! If you cannot find a different shop call and ask how much it would cost to program in a different tire size. Don't let them tell you they need to look at the car or sell you some snake oil, just be firm and say you have put a smaller diameter tires on your car and need the computer updated with the new tire size information. Be firm and ask to speak to a manager if they give you the run around. It's a simple procedure.......all they want is for u to bring it in so they can find something wrong, again stay firm and tell them this is all I want done, they don't need to look at your car! Might want to speak with a manager anyways and show him the invoice you got saying they did not find anything wrong and tell him about the tires.....there is no reason to miss such an easy diagnosis, if it was me I would have them program the PCM for free on account they probably charged you for nothing! Any questions or problems email me mikericotta at yahoo dot com..........(ase certified tech. Here......9 years in the field 5 as lead tech)
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  9. Brian Clevenger

    Brian Clevenger New Member

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    took it out on the highway today and at 70mph in 5th im at right around 2400 rpm so I'm gonna venture to say its gears and not the tires.. I've changed tire sizes on my truck before and it never changed the speed that much. But the way it drives points to gears... really short first gear and high rpms on the highway. Based off of all the info i've read on mustangs with the 4.10 gears vs stock
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  10. 805mustang

    805mustang Member

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    I'm going to disagree but it's your car. You bought the car used I'm guessing and was never told about any gear swap in the rear end correct? There is two easy ways to find out.....ask the previous owner or put the original tires back on! The tires you have on now compared to the oem tired are not as tall, this would have the exact same effect as swapping gears, no way to tell the difference unless you verify it for yourself! You already know tires will cause this issue, now add the 4:11 gears on top of the shorter tires and the difference would be quite large! Also if your tuning to 4:11 gears and the speedo is correct the tires would have to be the stock oem size and we know they are not! From what your telling me there is no way your gears are 4:11s....sorry to disappoint! You can always drop the diff cover and count the teeth to verify! Is what I'm saying making sense to you?
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  11. Brian Clevenger

    Brian Clevenger New Member

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    I'll tune for tire revs per mile. The 275/35-20 is 731 rev per mile. So we will see what that does. But I know the tires aren't making the engine run at high rpms and the car was built to race but we will see. I value your opinion so I will check
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  12. 805mustang

    805mustang Member

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    I apologize for the earlier post.....as they are correct ......I just realized you have 20 inch rims and not the stock 17 or 18s! Due to this new info (new to me, sorry) I can say either your instroment cluster gauge stepper motor is off or your vss (in trans) is going bad or (in your case) the gears in the rear diff. Are not stock I.e. 4.11 gears. If you do the math 275/35r20 wheels/tires are actually taller than the o.e.m. 17 or 18 inch wheels! The way it works is, and this is just an example using your wheels. 275 is the tread width in mm, 35 is the height of the side wall in percentage of tread width, in your case this would be 275 x .35 equals 96.25mm or 3.789in.....multiply that by 2 equals 7.578in plus rim size, in your case 20 gives u a total of 27.578 which is actually taller than the stock 17 or 18s. This would mean your speedometer would actually read low! I would be sure on your tune and either ask the previous owner or take the cover off the rear diff and count the teeth on the ring gear and divide that by the pinion gear, this also gives u a chance to change the fluid which will prove very beneficial as I believe these cars have clutch packs and the fluid deteriorates quick causing chatter and minor slippage. Good luck, sorry for the inconvenience.
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  13. Husky44

    Husky44 Active Member

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    Before you go pulling diff covers, jack the back end up, (put the car on jack stands!), put the car in neutral, and make an index mark on the driveshaft. While a helper rotates the rear tire EXACTLY one turn, count the number of turns the driveshaft makes.

    Slightly more than 4 rotations? You have 4:10s (4:11 isn't a Ford Racing ratio for the 8.8). Between 3 and 3.5 rotations, you have 3.31 or 3.55s.
    You say the speedo is 5-10 mph fast, but you don't say at what speed. Assuming your issue is either a gear ratio issue, or a tire diameter issue, the error will vary directly with speed, meaning as you go faster, the error will get bigger. Tire diameter can make a 5 mph difference easily at highway speeds. A gear ratio swap from 3.31-4.10 will make a MUCH bigger difference than 5-10 mph at highway speeds. For instance, on my car, 80 mph indicated represented ~66 mph actual, when I swapped from 3.31 to 3.73. Tire diameter will have some impact on that as well, but my main point is that gears would most likely have a bigger impact.

    If you have the ability to adjust for tire revolutions per mile with your tuner, make that adjustment based on the tire manufacturer's published revs/mile (should be on their website). If your ratio is different than stock (based on the shade tree method described above), set that ratio in your tuner. Then, if you're still off, I'd start checking speedo issues.
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  14. Brian Clevenger

    Brian Clevenger New Member

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    Yeah I corrected myself.. Idk where I pulled 4.11 from my bad
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  15. 805mustang

    805mustang Member

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    Husky44 are you still viewing this thread? You seem like you know your way around these ponies. I have an issue with my variable cam timing that no one seems to be able to help me with.....I've posted 2 different threads and both have over 50 views but I can't seem to get any straight answers.
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  16. Husky44

    Husky44 Active Member

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    I know a lot about a few things... Variable cam timing isn't one of them. Sorry. Thanks for the got of confidence though! :)
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  17. 805mustang

    805mustang Member

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    No prob.....can I ask if you have a scan tool that views manufacture specific live data? I'm getting 2 completely different readings from my var cam time but I do not have a reference value to go off so I cannot determine which one to focus on.
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  18. Steve491

    Steve491 Member

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    Man what a long thread on a speedometer issue. :bang: His tires were not that much taller than stock 2005 tires, less than half an inch. It doesn't matter if the tires were different, the gears were different, or both; the solution is changing a setting with a tuner. If you can program the ECM to think it had 4:11 in the rear end and the speedo is correct then you're done!
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  19. Brian Clevenger

    Brian Clevenger New Member

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    I'm always asking questions before I just take the time to figure it out myself... But I do appreciate all the replies!!! that was makes a good forum! Glad I joined
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