Inaccurate speedometer. May 21, 2009, 10:10:15 PM My speedometer seems to be off by some exponential function. I'm not sure what the reason would be. I believe that It was off before I did my trans swap. When I swapped the tranny I didn't swap the sensors. Mainly because the one from the Crown Vic was melted on the exhaust. I got two tickets: The first one is fuzzy. I remember watching my speed climb to 40 in a 25. I remember getting the ticket and the officer telling me I did 40. Then I get into court and somehow the speed was increased.The second one My speed got to 51 in a 40 and I get clocked at 60. I believe I have the right size tires. Any ideas? Quote Selected
Inaccurate speedometer. Reply #1 – May 21, 2009, 10:32:05 PM If you're unsure, any decent transmission shop should be able to get you the right speedometer gear for your combination and install it for very cheap. Stay away from the large chain stores as they won't work with you much.I believe speedometer gears are setup so they are as accurate as possible at 60mph on the gauge, or something. Due to the signal being electronic, it's certainly possible that there is a wiring issue somewhere. I'm sure others will give a more specific answer but those are the two things I'd look at. Quote Selected
Inaccurate speedometer. Reply #2 – May 21, 2009, 10:35:21 PM On a cable driven speedometer there is a gear on the tranny end. This gear matches the rear end ratio. If you swapped a tranny from another car with a different rear end ratio, your speed will read wrong. I assume this is the same for VSS (non cable) speedometers. You might need to find the correct sensor.In the mean time, have a friend with an accurate speedometer drive in front of you and talk on the cellphone. Have him go 25, see what your speed says, then 35, 55, etc. CoogarXR Quote Selected
Inaccurate speedometer. Reply #3 – May 22, 2009, 12:59:41 AM The crown vic probably has different teeth counts on the speedo gear. I'll bet if you put the gear from your stock transmissions it would work fine. Quote Selected
Inaccurate speedometer. Reply #4 – May 25, 2009, 01:22:49 PM Quote from: Haystack;273639The crown vic probably has different teeth counts on the speedo gear. I'll bet if you put the gear from your stock transmissions it would work fine.I believe I used the "speedo grear" from the original transmission. Not sure if were talking about the same thing. Some sort of a plastic sensor with a gear at the end of it that goes into the tranny. Quote Selected
Inaccurate speedometer. Reply #5 – May 25, 2009, 03:46:03 PM IIRC, those speedo gears are color coded for different gear ratios. Whatever color your gear was with the rear end you are using, is the one you want. That said, one way to verify if something is wrong, is to drive with a GPS unit. The small handheld variety works just fine. Quote Selected
Inaccurate speedometer. Reply #6 – May 26, 2009, 04:10:07 PM Not all AODs use the same drive gear. The "speedo gear" is often referred to as the "driven gear". There is a metal gear mounted to the AOD output shaft that has 6, 7 or 8 teeth I believe. Try this calculator.http://www.sccoa.com/faq/speedgr.html Use a GPS to figure out what your real speed is. AOD Final drive is 1:1 (in 4th) I believe, so you can look at your tach, speed, figure in your rear end gears and you should have your correction needed. Pull the driven gear from your trans and check the teeth count. Use the calculator to figure out what you should be running.-dz Quote Selected
Inaccurate speedometer. Reply #7 – June 12, 2009, 01:51:59 AM Thanks Guys! That calculator will come in handy. I'll look into these things when I get some time. For now as long as I have a rough idea what speed I'm going. I'll definitely borrow someone's GPS. Quote Selected
Inaccurate speedometer. Reply #8 – June 12, 2009, 02:16:55 AM Quote from: Haystack;273639The crown vic probably has different teeth counts on the speedo gear. I'll bet if you put the gear from your stock transmissions it would work fine.x2 Quote Selected
Inaccurate speedometer. Reply #9 – June 12, 2009, 02:20:53 AM MISC SPEEDO GEAR INFO---thunderjet302For those of you who have installed 3.73 gears with your stock AOD you know how hard it is to find a speedo driven gear to make your speedo close to accurate. In reality a 24 tooth gear would put it dead on but a 23 tooth gear makes it with in 1-3 mph of being correct. The only problem was that there was no 23 tooth auto trans driven gear avaliable from Ford. You either had to use a 21 tooth gear like I did and accept the 6-7 mph difference between what the speedo said and your actual speed or use a 23 tooth gear for a T-5 which would wear out fast. There is a solution though. I found a 23 tooth AOD driven gear at Mustangs unlimited. I ordered it and it looks just like my 21 tooth gear with 2 more teeth. I installed it and vola my speedo is almost right on. Here's the link if anyone is interested. ---thunderjet302--turbocoupe50Cool... But I have the later super coupe AOD with a 7 tooth output. A 21 works fine for me.--turbocoupe50 Quote Selected
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