What is going to change when I put 20's on a 04 Lightning?

RPM when cruising??
MPG???
Speed change??

 

Depends on the size of tire, if you end up with the same diameter as the 18s, with less tire then nothing will change, but if you fill up the wheel wells with a larger overall diameter then you should see a slight drop in RPMs overall as the larger the diameter the les turns to take to get down the road.

 

Your truck will slow down if the tire diameter is larger.

 

Will that change the gas milage?

 

You're final gear ratio will drop with larger diameter tires.

Gear Calculators

 

Larger tires will probably help gas mileage as it lowers RPM.

 

and, if the combo of wheel and tire is larger than what came on the stock L, your speedo will not reflect the correct speed that you are traveling at.

 

Thanks Guys

 

You won't like it unless you get your computer recalibrated.

I had 2 inch taller tires on the back for a while and it changed the shift points and it lost a lot of pep.

Of course that was the 99 which had the 3:55 RP to start with.

Might not be as noticeable with a 3:73

As far as gas mileage goes. Taller gears don't always = better mileage even though your RPM drops.

If your not on the camshaft sweetspot your foot is in it deeper to make the same speed and your efficiency isn't there. If you have accurate gauges you might even see a temperature rise on the engine for the same reason.

The factory did a great job in choosing ratio's to suite this truck and engine.