My 2003 Yukon has 110000 miles. I know the plugs and wires need replaced. What else should I do.

Do you have after market tires that are larger than stock ? This will affect your speedometer.Another thing could be the speedometer gear which is easy and cheap. Its at the end of your speedometer cable inside of transfer case.

Remove the wire from the top of the spark plug by twisting and pulling on the boot part. Then you will need a 14 milimeter plug scocket wrench, turn the wrench counter-clockwise to remove. You install them by doing the steps in reverse. A note of caution: the tightening force is 15 foot pounds of torque whitch isn't a lot of force.
The original spark pulgs are good for at least 100,000 miles unless it's had a problem to cause them to fail. Also installing a different type plug then original equipment has lead to poor performance at times.

Fuel filter about every 30,000 miles. Transmission filter about 50,000 miles.

Ever 3,000 miles 1 Engine oil and filter 2 Lube suspension and u joint
Every 10,000 miles 1 Rotate wheels 2 Inspect for worn steering parts 3 Replace fuel filter 4 Replace air filter
Every 40,000 miles 1 Transmission oil, filter and boost valve 2 Transfer case 3 Front and Rear axle oil changed
Every 100,000 miles 1 Service ignition system. Spark plugs/ wires ECT 2 Service cooling system, Thermostat and coolant flush
Thank you for using fixya and replace any worn parts though out inspection during servicing

There is no speedometer cable on the 2003 Yukon, if there is no transmission issues like shifting problem. Your instrument cluster has gone bad, to pinpoint. Your stepper motor to your speedometer gauge has failed and needs to be replaced. You can do a search on Ebay for the stepper motor for around $10.00 which is better then the $400.00 cost of another instrument cluster.
There was a TSB on the replacement of the instrument cluster for your Yukon free of charge. The only cost to you is the labor charge unless your willing to do the job your self. Just go to the Gm Dealer and give them this TSB Number: # 07187C: Special Coverage Adjustment - Instrument Panel Cluster Gauge Needle Function - (Mar 20, 2009)
Subject:
07187C -- SPECIAL COVERAGE ADJUSTMENT - INSTRUMENT PANEL CLUSTER GAUGE NEEDLE FUNCTION
Models:
2003-2004 CADILLAC ESCALADE, ESCALADE ESV, ESCALADE EXT
2005 CADILLAC ESCALADE, ESCALADE ESV, ESCALADE EXT (U.S. ONLY)
2003-2004 CHEVROLET AVALANCHE, SILVERADO, SUBURBAN, TAHOE
2005 CHEVROLET AVALANCHE, SILVERADO, SUBURBAN, TAHOE (U.S.ONLY)
2003-2004 GMC SIERRA, YUKON, YUKON XL
2005 GMC SIERRA, YUKON, YUKON XL (U.S. ONLY)
Some customers of all 2003-2004 model year and certain U.S. 2005 model year Cadillac Escalade, Escalade ESV, and Escalade EXT; Chevrolet Avalanche, Silverado, Suburban, and Tahoe; and GMC Sierra, Yukon, and Yukon XL vehicles have reported that one or more of the instrument panel (IP) cluster gauges stick, flutter, or become inoperative. This may cause inaccurate readings, including the speedometer and fuel gauge. Special Coverage Adjustment This special coverage covers the condition described above for a period of 7 years or 70,000 miles (110,000 km), whichever occurs first, from the date the vehicle was originally placed in service, regardless of ownership.
Dealers are to replace the instrument panel cluster after the condition has been verified. The repairs will be made at no charge to the customer.

For verified failures between 70,001 miles (110,001 km) and 80,000 miles (130,000 km) on U.S. vehicles, dealers are to provide the customer with an instrument panel cluster (IPC) only. Any additional costs, including labor to replace the instrument panel cluster, will be the responsibility of the customer. Dealers are responsible for the return of the customer's IPC back to the Electronic Service Center

Thank you for using fixya and good luck.

Yes and 80,000 miles and change the water pump the same time too

My brother had a similar issue with his Ford F-150, he needed the fuel injectors replaced. Good Luck!

you don have to do much to that gmc as long the coil is good, just replace all 8 spark plugs, air filter and basic oil and filter change; use good brands of oil, such as castro, mobile, shells ect. if your the original owner and have high miles and never have replace thermostat, I would go ahead and have it done, I would not wait until it fail and over heat on you. if you have done lot of pulling with your vehicle and have 75-100k mile, have your transmission flush with new filter, it will give the tanny life last a bit longer.

how to take spark plug wires off 2003 yukon xl

new piston rings