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post #1 of 12 (permalink) Old 05-08-2004, 10:27 AM Thread Starter
 
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Question Speedometer problem

My speedometer is reading fast. At ~45 on the speedo i'm actually travelling at ~38 MPH and at 85 on the speedo my actual speed is about 70MPH.

I have recently installed a new Art Carr Street/Strip transmission to replace my old stocker and in the process I re-used the speed sensor from the old transmission. According to Art Carr the speedo gear in the tail shaft is the same as stock and I should not have any issues with the stock sensor.

I can't make sense out of the problem. It seems to me that if the speedo is off by 7 MPH at 45 then it should also be off by 7 at 70MPH. Am I wrong? Any suggestions?

Thank you!
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post #2 of 12 (permalink) Old 05-08-2004, 10:38 AM
 
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My speedometer does something similar, at 40 and below its on point, but at my cutout of 105mph, the speedo shows 115.

I'd like to know why too.

I know why its reading wrong, but dont understand why its different at 40 and 105


Thanks
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post #3 of 12 (permalink) Old 05-08-2004, 10:56 AM
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It s proportional because it a rpm thing. Probably tire size the speed is calibrated for one size tire the travels so far in one rpm. to make it easy lets say 1.25 feet so in 100 rpm you have traveled 12.50 feet. Now you put a lower profile tire on it and it only turns 1 foot in 1 rpm so now you only 10 feet in that 100 rpm That difference will show more as you go faster because it is a percentage. lets say 10% so at 45 mph you will be off by 4.5 mph and at 80 you will be off by 8 mph. Hope I explained this good enough so you can understand it and that it helps.

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post #4 of 12 (permalink) Old 05-08-2004, 10:35 PM
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The calibration range on the speedometer may be broader than one would expect. Here's the chart from the 1994 Service Manual:

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post #5 of 12 (permalink) Old 05-08-2004, 11:11 PM Thread Starter
 
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According to the chart I should be no more than 7 MPH at 60 mph. At 70 i'm 15 mph off which suggests that I have the wrong speed sensor gear installed.

What i'm stumped about is that Art Carr insists that they are using a stock drive gear in the transmission and I simply moved the sensor from the old to the new.

I wonder if the core that Art Carr used was a 4R70W from a year other than 94-95 and hence a different drive gear (8 tooth maybe) ???

If that is the case then all I would need to do is replace my old speed sensor with a 19 tooth sensor and I'm done.

Any thoughts?

Kent
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post #6 of 12 (permalink) Old 05-09-2004, 01:05 AM
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In my experiences in speaking with a couple of calibration/conversion shops, and a tech at Ford Racing, speedometer error can be either a constant or a percentage error (i.e., always the same mph off at all speeds, or the same percentage off at all speeds).

"Same as stock" doesn't necessarily answer the question "How many teeth does the speedometer drive gear you installed on the output shaft have?" Stock for 1994 is a 7-tooth drive gear. A more informative answer may be required from that company.

What other changes are there? Different rear-end gears and/or tire/wheel size changes can affect the driven gear requirement. Check this link: http://www.sccoa.com/faq/speedgr.html - it can assist in determining what's required....

Another formula for what you may need:

(Drive Gear Teeth x Axle Ratio x Tire Revolutions per Mile)/1000

Example (7 x 3.73 x 815)/1000 = 21.3 (driven gear teeth - a 21-tooth gear is required)

Example is from FRPP catalog.

Last edited by IronChopz; 05-09-2004 at 01:28 AM.
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post #7 of 12 (permalink) Old 06-15-2004, 09:30 PM
 
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Mine is pretty accurate. At about 110 I'm doing about 107MPH due to smaller tires. Or it could be my friends slightly bigger tires on his 245/45/ZR17s make his slower than what he's running. Everytime I get pulled over around above 70MPH what I was doing and what the trooper says it on the money. 95% I don't get a ticket though.
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post #8 of 12 (permalink) Old 06-15-2004, 11:53 PM Thread Starter
 
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Thanks for the info guys...

I'm pretty sure at this point the problem is the newer transmission has the 8 tooth gear and my original had a 7 tooth. All I need to do is find some time to install a different speed sensor gear.
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post #9 of 12 (permalink) Old 06-16-2004, 12:49 PM
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post #10 of 12 (permalink) Old 06-16-2004, 03:27 PM
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Mine's off by 5 at 30mph. Had a chance to test it with one of those side-of-the-road radar wagons that the cops setup. I hope they don't think that makes people slow down. If there is no cop car by the wagon, it makes people see how high they can get the numbers up to.

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post #11 of 12 (permalink) Old 06-16-2004, 05:17 PM
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Quote:
Originally posted by Al_Florida
If there is no cop car by the wagon, it makes people see how high they can get the numbers up to.
Yup. Uh, I mean no...I wouldn't do that...
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post #12 of 12 (permalink) Old 07-03-2004, 01:52 AM Thread Starter
 
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I finally got around to changing out the old speedo gear to a 19 tooth and it seems to have solved the problem. I'll know for sure once I get a chance to time a stretch of road.

Kent
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