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?
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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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)
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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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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