Help - Mod Problems - Tires and Wheels killing MPG?

[JFR1]

Situation:
'Have a 2011 LS with 3K miles. Added Flowmaster exhaust and Air Raid CAI within a week of first putting on the road. Factory Heritage wheels 245/55r18. About 35lbs per wheel. Spirited driving, 20% on interstate. When cruising, try to keep RPMS less than 2000 equals about 66mph and 33mpg on computer. Overall MPG calculated (not computer) after 2500 miles at 25-26. Computer showing 24mpg.

Local dealership had a set of 2LT wheels and brand new tires, $1100 for the set. Wheels have been triple plated chromed - look great. 245/50R19. 31lbs per wheel.

Problem:
MPG has fallen to 18-19mpg. I havent changed driving habits. I can feel the difference in the new Pirelli 19's vs the BR Goodrich 18's. Checked spedometer with GPS - right on. Both calculated and computer MPG same.

Could this small tire/wheel change make my MPG fall (25%)? The 19" wheels and tires are factory 2LT. Window sticker MPG is same on an LS and 2LT. Local dealership service department does not have a clue.
I wonder if GM has engine programmed differently for LS vs 2LT???
Dealership wants $150 to diagnose on computer. I know they will tell me the changes I made on exhaust and CAI is the problem. But I was getting Better MPG after adding exhaust and CAI. Only other change is wheel/tires.

Appreciate any ideas and Help. Merry Christmas!

[PlantCityPete]

Put bigger (taller and wider) tires on two previous trucks, both lost about 3 miles to the gallon on each. i have 1lt with 20 inch wheels, exhaust and cai , get average 20mpg. And thats mostly 75 to 80 on interstate. (80% highway)

[PlantCityPete]

By the way make sure you are running correct air pressure for that tire.

[CamaroDreams07]

Makes no sense at all. The change in diameter is only .1 of an inch by going from the 245/55/18 (28.6") to the 245/50/19 (28.7"). If anything you should be getting BETTER mileage with the lighter wheels. If you've got a good dealer who's not going to dick you around, something doesn't seem right. But you're correct, if they are shady, they could blame it on any number of things you did. I'd take it in, explain to them the order of things (CAI/Exhaust before the mileage drop) and see what they say. But I gotta tell you, I've never gotten that "before" mileage on my car! 18-20 mostly city, but still, you must have a light foot!

Might wanna post a link to this up in the wheels/tires section, may get more responses.

[CamaroDreams07]

Originally Posted by PlantCityPete

By the way make sure you are running correct air pressure for that tire.

Absolutely great point that I completely overlooked. That makes a big difference. Good catch Pete!

[scrming]

Originally Posted by JFR1

Situation:
'Have a 2011 LS with 3K miles. Added Flowmaster exhaust and Air Raid CAI within a week of first putting on the road. Factory Heritage wheels 245/55r18. About 35lbs per wheel. Spirited driving, 20% on interstate. When cruising, try to keep RPMS less than 2000 equals about 66mph and 33mpg on computer. Overall MPG calculated (not computer) after 2500 miles at 25-26. Computer showing 24mpg.

Local dealership had a set of 2LT wheels and brand new tires, $1100 for the set. Wheels have been triple plated chromed - look great. 245/50R19. 31lbs per wheel.

Problem:
MPG has fallen to 18-19mpg. I havent changed driving habits. I can feel the difference in the new Pirelli 19's vs the BR Goodrich 18's. Checked spedometer with GPS - right on. Both calculated and computer MPG same.

Could this small tire/wheel change make my MPG fall (25%)? The 19" wheels and tires are factory 2LT. Window sticker MPG is same on an LS and 2LT. Local dealership service department does not have a clue.
I wonder if GM has engine programmed differently for LS vs 2LT???
Dealership wants $150 to diagnose on computer. I know they will tell me the changes I made on exhaust and CAI is the problem. But I was getting Better MPG after adding exhaust and CAI. Only other change is wheel/tires.

Appreciate any ideas and Help. Merry Christmas!

Are you in the "COLD" part of Florida? I know you guys are having a bit of a cold snap down there... MPG goes down when it gets cold...

Do you use the same gas station?

[VASCAR2]

I can easily effect my MPG by 2 or 3 miles per gallon by how aggressively I drive on acceleration. I also notice my MPG went down in the winter with the different blend in winter gasoline by at least 2 MPG. My 1LT auto is stock and has 19" polished aluminum wheels with Pirelli all season tires at 35 PSI. I have about 50/50 city/highway driving and average anywhere from 20 to 24 in summer depending on how lead footed I am. I've gotten 29 MPG on interstate going 70 to 75 MPH for a 300 mile trip. Last winter the best mileage I got was about 21 maybe 22 MPG in my normal local driving. The more highway miles the more my mileage goes up. The more stops and starts the lower my mileage is.

[verboten]

The thing about wheels is you have to spin them, and that takes much more effort than it does to move an object that isn't spinning. You're dealing with the polar moment of inertia.

In fact, just for fun I looked up how much straight-line performance you'd lose by switching from 16" wheels to 20" wheels if the wheels weighed the same. It'd be about like adding 200 static weight to the car, which would take an additional 11ft-lbs of torque (roughly 15hp @ the rev limiter) to compensate and get the same acceleration as a stock car. That's bad enough.

In addition, that extra 200lbs of equivelant weight that you're adding to the vehicle? You're going to have to slow it down, too.

[JFR1]

Thanks all for the input and advice. Seems that my mpg at 19 is about what others are getting. Just cant understand the drop.
Does anyone know if the factory 'tune' is different for a 1LS auto vs a 2LT auto? I may just get Ted at Jannety racing to re-tune the computer. Thanks.

[GretchenGotGrowl]

Originally Posted by JFR1

Thanks all for the input and advice. Seems that my mpg at 19 is about what others are getting. Just cant understand the drop.
Does anyone know if the factory 'tune' is different for a 1LS auto vs a 2LT auto? I may just get Ted at Jannety racing to re-tune the computer. Thanks.

I'm pretty sure the tunes are the same. Like CamaroDreams07 pointed out, the difference in overall diameter of the wheels is only 0.1 inch, so that will have almost no affect on the speedomter calibration. It is mostly like what Scrming suggested -- colder weather means worse MPG. Mine has gone down about 1 MPG around town. If they only put ethanol in your gas during the winter, then that may explain why you saw a bigger drop (we have it year around here). Also, I don't discount that you may be driving a little more aggresively because these cars love the cold weather.

[a red maro]

i have a 2lt and haven't changed the tire size or anything and the computer on my car tells me that I'm doing about 24 mpg mostly in town some highway miles. that's with exhaust and CAI. never looked at the mpg before the mods, but in know that both of the mods that i have are supposed to improve mpg am i correct?

[JFR1]

Originally Posted by a red maro

i have a 2lt and haven't changed the tire size or anything and the computer on my car tells me that I'm doing about 24 mpg mostly in town some highway miles. that's with exhaust and CAI. never looked at the mpg before the mods, but in know that both of the mods that i have are supposed to improve mpg am i correct?

Yes - performance exhaust and CAI are supposed to increase MPG. Thats one of the reasons my case is so perplexing. However, it appears that 19-20 is not unusual.

[ljones]

I have the polished 20s, and I'm at 18.4. The number has been creeping up slowly, and I haven't changed my driving habits. Only difference, I switched from premium to regular at Costco. I'd love to see 24, given I'm on the interstate about 70% of the time.

[heRS]

Not about the Camaro but when Dennis put the skinny snow tires on his pickup and took his fatties off he commented on that he was surprised to find he was getting WAY better gas mileage.