I have an 04 Lesabre and the speedometer is off a bit. It would be convienient to be able to calibrate it myself especially when switching between tires.
Is there a tool that will do this and will it cost a fortune?

How do you know its off?

You need something like this to calibrate your speedometer:

http://www.summitracing.com/parts/dbl-i1000

i would agree, need some sort of tuning software to be able to change it yourself, its not real cheap

Like Mike says, how do you know it is off?

If all of a sudden, it is way off, check your DIC to make sure you didn't switch from English to Metric.......

All it takes to check your speedometer, is a watch with a second hand or stop watch on your phone, and mile markers on the highway.......

Find a stretch of highway, that is relatively flat......drive at a constant speed of 60 miles per hour.....it should take EXACTLY 60 seconds to travel a mile.....

So when you are at a constant 60 mph(by your speedometer), note where the second hand is as you pass a mile marker........as you pass the next mile marker, the second hand should have moved 60 seconds, if the speedometer is spot on.....if the time is more than 60 seconds, tan your speedometer is reading slower than actual.....if less than 60seconds between markers, then you speedometer is actually faster than the reading actually is......

For example, take the time between the markers, divide that time into 3600.......so if it was 57 seconds between markers, 3600/57=63.16....so you were going 63 MPH, instead of the constant 60 you were driving....if the time between markers was 65 seconds, 3600/65=55.38, so your speedometer says 60 but you are doing about 55 mph....

As a quick estimation with no math, if the difference in time is 10 seconds or less, each second represents 1 MPH...above 60 seconds, you subtract.....below, you add.....so 64 seconds would be 60-4, or 56 mph....

I think his comment about switching between tires may explain why his speedometer might be off.

I know....he should be using the same size tires that'* listed on his "B" pillar...this quick test will tell him the difference between what his speedo says, and what he is actually doing.....if it'* just a few MPH, why bother programming, and then, you can drive your car accordingly?

I'd like to know what size tires his car calls for, what size he has switched to, and what the difference in speedo accuracy it creates...

A satnav will normally tell you how fast you are travelling. Easier and safer than watching a sweep second hand on a watch.

Roger.

Some people can't walk and chew gum at the same time....just kidding....

I'd rather use a stop watch. Its far more accurate than GPS. GPS can be up to 100ft off and +/- 5 MPH.