I hope I didnt just make a big mistake. 2002 Dodge Ram 1500 quad with 4wd,
100,500 miles. I just today replaced the Goodyear tires 275/60/20 with
Michelin Cross Terrain 305/50/20. Now my worry is that since these tires are
about an inch shorter (lowered my truck an inch) how far off is my speedo
going to be, am I losing fuel economy and will I damage the rear or tranny?
Please let me know something.
Locate the tire diameter specs on the manufacturer's site for both tires
then do the math. Always divide the new tire size by the old tire. You may
get a number over 100%, but that means the tire is turning that much faster
than before and is the percentage to DIVIDE the indicated speed by.
For example: if your speedometer indicates 75 mph and your tire is turning
115% of the old tire's rpm, then you actual speed is 65.21 mph. Odometer
mileage will be affected the same way.
The PCM can be re-programmed, by the dealer, to accomodate the new tire
size. He'll need to tell the techs the new 'revs per mile' of the tire,
usually available from the tire manufacturer. Alternately, it can be
calculated based on the advertised size of the tire. In this case, it works
out to 630 revolutions per mile.
I went up a tire size on my 99 ram and the speedometer is now about 4% off on
the low side... When the GPS says we're doing 65 mph, the speedo says 62...
The dealer wanted $60 to flash the computer and said that it would "get it
close"... Knowing that we're going 3 or 4 mph faster than indicated is close
enough for me...
I had thought that since we went up to a tire size that was an option in 99,
that they could get it as "close" as stock, but I guess not..
Go here: http://www.miata.net/garage/tirecalcold.html
With the new tires, your speedometer will read 3% faster.
Now, that assumes an ideal world where all things are exactly as they are
be. Some tire companies vary the diameter from published specs. And some
speedometers are 2-3% off anyway. Who know, now you could be closer to your
speed than you were before.
Go out on the freeway and clock out 10-20 miles with the mile markers. See how
your odometer matches up. In some places you'll find a radar trailer or radar
signpost on the highway, that's usually pretty accurate. And the new GPS hand
are pretty accurate, too.
If you have to change the speedometer, I'm not really sure. I think you have
sensor on the differential. The only way to change it that I know of is to
the computer with something like a Jet tuner. And you would want to do that
to make the truck run better.
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