Tire size questions

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cuda65vpt

cuda65vpt

cuda65vpt
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I'm confused. What affect is larger tires supposed to have on the speedometer?

I went to 14" SBP rallys on my 66 Barracuda and then later fixed the speedometer with a new cable. I expected the speedometer to be off because of the tire change and expected it to read slower than I am actually going. Bigger tires cover more distance with the same rpms.

It is actually reading about 15-17 percent off in the other direction. 70 mph on the speedometer is right at 60 mph with the GPS.

When I bought the car the 904 tranny wasn't installed. I had it checked out by a tranny shop. It seems to be the right tranny with the shifter mounts and all. I guess it could have come from a floor-shift B-body or something.:read2:
 
I thought of that too, but would have to be geared for 16" or 17" tires to create the error I am getting if my way of thinking is right.

I guess I could pop the 13" tires back on for a test run.
 
You can look on the YEAR ONE site to look at the chart to see what speedo gear is needed. Its the tire OD and gear in the rear end that matters not tire ID.
 
A tire diameter change will change the speedo reading also. It doesn't depend on rim size....I've seen 13" tires that were bigger in diameter than some low profile 14" tires. You may have to change the speedo gear to compensate.
 
a taller tire will slow down the speedo. maybe it just had the wrong speedo gear in it when you got it like others stated.
 
LETSRACE said:
You can look on the YEAR ONE site to look at the chart to see what speedo gear is needed. Its the tire OD and gear in the rear end that matters not tire ID.
Click to expand...
Couldn't find that chart but I did find a tire size calculator which confirms that my way of thinking is correct.
180-75-13 original to 225-75-14 should make the speedometer read slow by about 10 percent. I.E. reading 60 mph should be actual speed of 66 mph.

I think this makes it obvious I have the wrong gear in the tranny. Anybody know which gear I would need and where to get one?
 
cuda65vpt said:
Couldn't find that chart but I did find a tire size calculator which confirms that my way of thinking is correct.
180-75-13 original to 225-75-14 should make the speedometer read slow by about 10 percent. I.E. reading 60 mph should be actual speed of 66 mph.

I think this makes it obvious I have the wrong gear in the tranny. Anybody know which gear I would need and where to get one?
Click to expand...


Try this link for the gear chart, good luck.
http://www.yearone.com/yodnn/tech/T...Q/MoparSpeedoGearChart/tabid/415/Default.aspx
 
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