I have an 04' Lariat and went from 18" to 20" wheels. I noticed that my mpg went from 13 to 11. I mainly drive city, approx 800 miles per month.

The question is: Would my city mpg increase if I goto 3.73's from 3.55's? If so, is worth $$$ have them swapped out?

 

City MPG should go up, figure $500 per axle. Also, your speedo will be off because of the change in tire size...have you factored this into your MPG?

 

What is the overall tire size that you had before and now have? It all has to do with "effective ratio".
http://www.4lo.com/calc/geartire.htm
If it were me and I was going to pay all that money to get new gears installed I would go with 4.10's I dont see much point in going from 3.55 to 3.73 IMO.

 

x2
Why bother changing to a 5% shorter ratio when you won't even feel it?

Go with 4.10s, then you're tallking about a 15% gear reduction that will get your power back, plus a bit more. I run 4.10s on street-sized tires. They work great

 

Yeah, it has changed; it' was 265's 60/18. The new wheels with tires are 275's 55/20.

 

Too small of a change. If you want better mileage, start with a good chip tuner.

 

265-60-18 = 30.5 tall tire RPM @60 2409 (4sp auto)

275-55-20 = 31.9 tall tire RPM @60 2304 (4sp auto)

Unless there is something else going on here your MPG should be better with your new 20'' package.

Also going from 3.55 to 3.73 will cause higher RPMs at the same speed and result in a loss of MPG.

 

Playing with the numbers, the 3.73 should put you close to the same rpm as the stock tires. Tire diameters vary so just close until you can get a good measurement.

The 4.10 will give you about another 400-500 rpm at cruising speed. If your towing, that's a good thing.

If you don't plan to tow, I'd go with the 3.73. It will help around town.

 

But he wants better gas MPG. The lower the gears (higher numerical value) the worse his MPG is going to get.