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Incorrect Speed On Speedometer

sir_veza

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This year's winter / spring project is now behind me, transmission & rear end upgrade etc (special thanks to Chryco for all your advice and tips)!
- Had the Challenger out for the first time yesterday and there's an issue with the speedometer. As I changed the rear end from 3:91 to 3:55 I ordered a new speedo gear. (I believe the previous speedo gear when the car had the 3:91 was a 37 or 38 tooth and it registered slightly over 5 - 7 MPH).
The front wheels are 225 x 65 x 15 (26.5" wheel diameter). Rear wheels are 255 x 60 x 15 (27" wheel diameter). Purchased a 34 tooth speedo gear and it's registering around 10 - 13 MPH above the actual speed I'm going. Could it be just a defective gear? Very odd.
Any thoughts?
Thanks
 

moparleo

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How are you measuring your actual speed ? GPS phone app ? And is it off at all speeds ? Use a tape measure and verify your tires diameter. The tire charts are averages. Some are taller and some are shorter.
 

sir_veza

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How are you measuring your actual speed ? GPS phone app ? And is it off at all speeds ? Use a tape measure and verify your tires diameter. The tire charts are averages. Some are taller and some are shorter.
Measured the speed using my GPS. Good point - I'll take somebody with me to check completely as I was looking only when safe to do so in traffic and on a highway etc. Will also measure the tire diameter with a tape measure. Thanks
 

azmoparboy

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Hope this helps

1619737763247.png
 

sir_veza

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How are you measuring your actual speed ? GPS phone app ? And is it off at all speeds ? Use a tape measure and verify your tires diameter. The tire charts are averages. Some are taller and some are shorter.
The actual diameter of the rear tire is physically almost 26" therefore a 36 tooth speedo is probably needed.
Also I had someone record the actual speed with their phone this past weekend:

Speedo 30 mph = Phone 26 mph
40 mph = 34 mph
50 mph = 43 mph
60 mph = 52 mph
70 mph = 60 mph
80 mph = 70 mph
90 mph = 77 mph

(To me) it's odd the speed increments aren't the same?
 

moparleo

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Yeah, unfortunately the difference will not be linear.. Your speedometer probably needs calibration. What is most important are the speeds between 40 and 70 mph as those are the speeds you will most likely be driving.
 

sir_veza

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Yeah, unfortunately the difference will not be linear.. Your speedometer probably needs calibration. What is most important are the speeds between 40 and 70 mph as those are the speeds you will most likely be driving.
A friend texted me yesterday evening that back in the day, until digital technology took over, speedometer's were calibrated to 50 mph and all other speeds could be a little off - and this was considered normal. Not sure how accurate that statement is?
 
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