Turns out my tires did it....

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I purchased new tires for the Lifeliner and the speedometer is now off. A PCS buddy suggested I use my GPS to check my speed and while I had it out yesterday did so. Turns out the Lifeliner reads 6 mph faster than my real speed. Not sure what it would take to get the speedometer recalibrated but may need to check into that.

Richard
 
all tires are not...

All tires are not created equal. A 235x75x15 for example from maker A may have a different circumference than a tire from maker B.
There may be an adapter to put in at the speedo drive on the trans. A trans shop may be able to help here. Or you can just do the math required and keep in mind whether you are driving too fast or too slow.
A lot of tire sellers tell me that my original size is not availible, 'but a 235 will work'. It seems to be ok. I did do a measured mile and was just a bit slow. Not enough to make a big difference. My mile was only short by about 100 feet from marker to marker at 60 mph.
The stuff we have to know to keep these things going right for us.
FUN, FUN, FUN!!!
 
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All speedometers have an inherent error built into them. My Ford Explorer and the Mercury Mountaineer both read 3 MPH high when compared to the GPS. You might be able to correct the error by changing the speedometer gear in the transmission, or you can take the speedometer head out, and have it calibrated by giving the shop your actual speed, and the reading on the dial. Personally, I don't think it is worth the expense or effort. If you were actually going 70 MPH and the car read 65 MPH, then that might be a problem. Even modern day police cars with certified speedometers are not accurate after a year on the road.
 
you can change the gear in the transmission. you need to go to the next size bigger gear to make the spedo go slower. you went from 32 in diameter tires to 29s. I told you so. now then if you had a 75 you would be right on. they used the j78x15 on them.
 
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