Yes, you are correct about the speedo difference too. It can be quite a bit. When my stock speedo says I am running 70, I am really about 79 or 80 (CTS2 says 82, but I think its not perfectly accurate). Also, keep in mind that the faster you go, the more it is off. It is not a linear relationship. (Old Tire Size / New Tire Size) x (Speed on Stock Speedo) = True MPH

I know folks will argue about mpg with bigger tires, but with stock gearing on 37"s and traveling at interstate speeds, fuel economy can be just as good as stock because the engine does not have to work too much harder to push them, but you are traveling further per each revolution of the tire. I have not hand calculated my mileage ever because I am not that worried about it, but taking a roughly 300 mile trip on the interstate and only using about 18 gallons comes out to about 16.67 mpg. Not too bad for a 19 year old truck with a lift on 37s and 300,000 if you ask me.

 

...Deleted...

 

if you have access to AE you can change tire size to match your tires and get your Speedo and odo back to reading correctly

 

 

I don’t care what anyone says.......Big tires and wheels kill fuel mileage. Also on thing I think people may overlook from time to time is the output of your hpop. As the miles start rack up on these pickups the hpop oil starts to come into play. A healthy pump can help your mpg outcome. Might be worth checking out

 

Just to clarify, I do not think bigger tires help mpg, just that interstate driving at the speed limit, on a full tank, and in my case AL and TN driving so no altitude of big hills, the mileage for that trip can be around stock. Now, driving around town... well that is a whole other story haha.

May seem like I have my head in a pillowcase, but I did cut a whole at the other end.

 

The best things I did to improve mileage was new up-pipes to fix a boost leak, a new EBPS, and some may snicker but my fiberglass topper helped. Also if I drive 60-65 in warm weather I can get very high teens but push it to 75-80 it drops to the low teens. I bought the AIS but didn't see any improvement from that.

 

And clean the tube too.

I did a test last spring. I drove the same around town but set my cruise control at a maximum speed for an entire tank. Since I commute and have to deal with traffic on a regular basis I only drove the truck to and from work to match driving conditions as closely as possible for each tank. The first tank I drove at 65mph on the freeway, the second tank I drove 70mph, and the third tank I drove 75mph on the freeway. The first tank I averaged 15mpg, the second tank I averaged 14mpg, and the third tank I averaged 13mpg.

Granted that I was not able to keep conditions exact but I also don't have access to a test track so I made due with what I had. I feel it's close enough to say that every 5mph above 65mph costs me about 1mpg. I could go slower and save even more but at 65mph on the freeways in SoCal I get passed by everything like I'm standing still. Any slower and I'd be going backwards.

 

I get 23 empty and 17 towing 12,000#. Truck in signature.

 

um, not even close....but it's fun to say it out loud

 

And I was lookin in your sig for a Dodge.....

 

 

Live on a high hill, visit friends in low places, walk home.

But seriously, looks like the guys got it covered