I put a K &N filter on my HHR, not much change that I can tell on the mileage.
Then I put a set of Solstice wheels and tires on the Princess. She looks better and rides soo much better. The wheels are taller and I lowered the car so I was hoping for lower RPM and less wind resistance on the highway.
With the taller tires the was supposed to be a 11.64% speedometer error. It is the same (No change!) verified by my GPS!!!! How can this be? I am pretty sure the speedometer was right with the old shorter tires but I know it is right now with the taller tires? The tires went from 205 (23.25")-45-16 to 225 (25.97")-45-18.
At all speeds the RPM is the same? I did not have speedometer recalibrated or anything. With the taller tires RPM should be lower at the same speed and the speedometer should read about 10 MPH slow at 80 MPH.
Why is the RPM, Miles per hour and mileage the same?
RPM should be lower, MPH should be higher and mileage should be better due to.
I did notice for sure the car is much more smoother ride, more stable, does not blow around in the wind as much and there is less wind noise. Not sure how much of it is because of the lowering springs and how much is because of the wheels and tires but it is a whole new car and much nicer. The car can corner harder now too but that is not what I use it for. I may put the old tires back on to see if it is the tires or the lowering that improved the ride so much.

The car looks better now to. http://i413.photobucket.com/albums/pp217/airetexbruce/P9191290.jpg

So I am very pleased with the modifications but not exactly as I expected. Better ride and looks nicer but the RPM and mileage did not change?

I put the old wheels and tires back on (front only)for some testing today.
With the taller Solstice wheels 225-45-18 there was no speedometer error at 60 MPH = 0 error
70 MPH = 1-2 MPH faster
80 MPH = 2 MPH faster

Put the stock wheels 215-45-16 and tested it. My original post above I incorrectly said 205 on the tire size.
60 MPH = 58 MPH actual
70 MPH = 68 MPH actual
80 MPH = 78 MPH actual

Both read 40 MPH obvserved and actual.

There should be a 10% speedometer difference (and mileage difference!)yet I only got maybe 5% difference. Very Strange.

The car rides and corners better with the new wheels tires and lowering. With stock wheels back on it after lowered it still rides better than stock but did not corner quite as well. So the wheels, tires and lowering were a big improvement in every way. Just do not understand what is the problem with the speedometer error??? That really makes me wonder about my mileage and even if it helps at all. Curly

Not that I have anything usefull to add, but it is strange.

Here is what should have happened:

The circumference of the P215/45 x 16 original tire is 74.1984 inches

The circumference of the P225/45 x 18 aftermarket tire is 81.5947 inches

Your speedometer reading is 90.9% of your true speed.

The ratio of your old vs. new tires is 0.90.

The current differential ratio is:
To retain the same overall gear ratio, you will need a differential with a 0 ratio.

--------------------------------------------------------------------------------
Without correction, your speedometer error will be:

Indicated speed: 10 mph. Actual Speed: 10.9 mph.
Indicated speed: 15 mph. Actual Speed: 16.4 mph.
Indicated speed: 20 mph. Actual Speed: 21.9 mph.
Indicated speed: 25 mph. Actual Speed: 27.4 mph.
Indicated speed: 30 mph. Actual Speed: 32.9 mph.
Indicated speed: 35 mph. Actual Speed: 38.4 mph.
Indicated speed: 40 mph. Actual Speed: 43.9 mph.
Indicated speed: 45 mph. Actual Speed: 49.4 mph.
Indicated speed: 50 mph. Actual Speed: 54.9 mph.
Indicated speed: 55 mph. Actual Speed: 60.4 mph.
Indicated speed: 60 mph. Actual Speed: 65.9 mph.
Indicated speed: 65 mph. Actual Speed: 71.4 mph.
Indicated speed: 70 mph. Actual Speed: 76.9 mph.
Indicated speed: 75 mph. Actual Speed: 82.4 mph.
Indicated speed: 80 mph. Actual Speed: 87.9 mph.
Indicated speed: 85 mph. Actual Speed: 93.4 mph.
Indicated speed: 90 mph. Actual Speed: 98.9 mph.
Indicated speed: 95 mph. Actual Speed: 104. mph.
Indicated speed: 100 mph. Actual Speed: 109. mph.

Chevy Mgr, your chart is close to the tire size web site that I got the information off of yet both are considerably less than my actual speed as verified by my GPS.
This just does not make any sense to me and I do not know what to think about my mileage now? My mileage appears to be almost the same and if I am going 6.9 miles further @ 70 with the same mileage according to my DIC then I am actually getting 9.1% or so better gas mileage? But as far as I can tell by my speedometer and GPS which is totally independant it has not changed that much. RPM has stayed almost the same???? And I also put the original tires back on to verify that I am not crazy (did no good, LOL!)
That is one of the reasons I went with a little taller tire is to lower highway RPM some and hopefully help mileage. Thanks for your help and ideas. Curly

Smoke & Mirrors Department:If you're looking for a mileage improvement with the taller tires,it's only going to be a paper increase.Your vehicle still travels the same distance over the road,burns the same amount of gas at the same true road speed and ENGINE r.p.m.(not tire r.p.m.).

With taller tires or gear ratio the engine turns at a lower RPM for the same speed. Now if you do not have to put your foot into it more to maintain that same speed then your mileage will increase the same as the RPM drop.

That's not exactly correct.... curly .....OR ztony8. There are a few factors that determine mileage. The key is EFFICIENCY. You need to know what factors increase the efficency of your specific vehicle and engine to determine what will increase mileage.

I'm not going to start spouting off what I think (I've learned that even if I am 100% correct about something, dosent mean I should force everyone to believe it also).

Your doing good, and you'll figure it out with a little more research.

If I listend to everything I heard on here I would think that 94 octane is some holy grail.

Nah, silly rabbit, tricks are for kids :)

I put a K &N filter on my HHR, not much change that I can tell on the mileage.


Did I not tell you the airbox/filter would be a waste of money if you are after MPG gains? The barely-open-at-cruise-RPM throttle blades are a far greater restriction than the airbox.

This isn't going anywhere either?

Not to sure what the differance in RPM will be , but with my 2.4 5 speed i get better mpg at 50 mph than at 70 mph best is about 60 or so . i know there is a wind sheer factor/drag but if you are still in the torque band the lower RPM should help with the mpg ? yes or no? i have though that GM should have put a little less gear in the car to keep the rpm down .

PS car looks great no matter what the change in milage . did ya have spacers behind the wheels , might have find a set of these .

PS car looks great no matter what the change in milage . did ya have spacers behind the wheels , might have find a set of these .

The car does look great :). Big wheels and a little lowering is almost always a plus.

I have 7/32 (.218") Mr. Gasket spacers on all four wheels. I do not like the spacers and looking for better quality ones for the car. They do not cover all of the seating area on the wheels.
I do believe the combination of taller tires, lower RPM and the engine mods I am doing will increase mileage but I am not where I want it quite yet.
The car does look better in person, soon I will take some good pictures to post. With the wheels and lowering the car drives better and corners better than stock.
There is a Solstice Forum and sometimes they have those wheels on there. I bought mine from EBay.

Stock the car got its best mileage at about 60 MPH and I hope to move that up to about 70. Curly

I put a K &N filter on my HHR, not much change that I can tell on the mileage.
Then I put a set of Solstice wheels and tires on the Princess. She looks better and rides soo much better. The wheels are taller and I lowered the car so I was hoping for lower RPM and less wind resistance on the highway.
With the taller tires the was supposed to be a 11.64% speedometer error. It is the same (No change!) verified by my GPS!!!! How can this be? I am pretty sure the speedometer was right with the old shorter tires but I know it is right now with the taller tires? The tires went from 205 (23.25")-45-16 to 225 (25.97")-45-18.
At all speeds the RPM is the same? I did not have speedometer recalibrated or anything. With the taller tires RPM should be lower at the same speed and the speedometer should read about 10 MPH slow at 80 MPH.
Why is the RPM, Miles per hour and mileage the same?
RPM should be lower, MPH should be higher and mileage should be better due to.
I did notice for sure the car is much more smoother ride, more stable, does not blow around in the wind as much and there is less wind noise. Not sure how much of it is because of the lowering springs and how much is because of the wheels and tires but it is a whole new car and much nicer. The car can corner harder now too but that is not what I use it for. I may put the old tires back on to see if it is the tires or the lowering that improved the ride so much.

The car looks better now to. http://i413.photobucket.com/albums/pp217/airetexbruce/P9191290.jpg

So I am very pleased with the modifications but not exactly as I expected. Better ride and looks nicer but the RPM and mileage did not change?
I believe your original wheel tire set-up was your problem. Aren't the stock wheel/tires 195/65 R16? So, you had smaller than stock tires and that is where your MPH errors were occurring. Hence your mpg's didn't change as you expected when you went with the bigger 18" wheels. Just my 2 cents.

There is a visable difference in tire size and should have a noticeable difference in speedometer reading. In fact there should be a 10% speedometer error now but there is not and I do not understand why.
So I suspect I may be getting better mileage than my DIC says because mileage, RPM and speed is reading the same yet the car is actually travelling further (maybe 10%) than it says. Yet my GPS did not show that much difference.
If that is true and it says I am getting 38 MPG on the highway and correct for the taller tires 10% you are looking at 41.8%. I have not been able to verify that with an independant source. Regardless I am pleased if there is a gain or not I like the way it looks and handles.

So the K & N filter may be helping some? Who knows?

Pappa Smurf, Do you have a picture of your car? The Aqua blue with the AA5 option sounds like it would look pretty nice.

I see the error. the stock tires are 215/55-16 NOT 215/45-16. The diameter of the stock versus the 18's is minimal (25.31 vs 25.97), so the speedo is still within normal specs.

BTW, I found that H&R makes a 5mm (0.200 in) spacer in our bolt pattern. kind of pricey at $50 bucks a set, but they are billet and have the proper bore (65 mm).

My Solstice wheels should be arriving next week and I look forward to seeing them on my vehicle.

I may be wrong, But the gain you are hoping to get with the tires might be lost with the wind drag at 70 MPG as compared to 60 MPG.

Just FYI, for better accuracy I would use the tire circumference not tire diameter.

Contrary to normal thinking,increasing tire size to lower rpm for milage purposes doesn't work on the newer vehicles.Heres how,,,the factory already has the car set to run the lowest rpm possible and maintain required epa and milage standards.Lowering the rpm further will only make the eng run harder thus use more gas.It lugs!I found this true on several cars Ive owned.If anything,raising the rpms slightly will probably gain better results as while the rpms are higher,the load on the eng is lower.Another thing to look at,these cars are highly controlled bythe OS,if the parameters don't match the system will try to correct it.Thats why when changing wheel/tire sizes its necessary to recal the speedo so the OS reads everything correctly.If the system does not correspond within the set parameters,you will not get the a correctly running moter.One change Ive made to another vehicle was a torque converter,went from an 1800 to a 2500 and it made all the difference in the world.(in reality it was 2200 due to the actual eng power) This enabled me to run 75 with AC and 2 bikes loaded in back and get 24 mpg in a 4800 lb(plus bikes) truck with a 5.7. Eng load dictatesn fuel usage,too low a rpm and you lug it.Looking at other 4 cyl motors that run at 2300 to 2800 rpm at speed,,the HHR runs low at 2000-2500.Gotta have torque to run at those rpms and hwy speeds and 4 cyls just don't have it.

I agree with much of what you are saying about larger tire lugging the motor and possibly hurting mileage.
My hope is when I complete all of the performance mods it will have more than enough torque to pull it at a lower RPM with out hurting mileage.
I know the modern motors have to waste so much energy to overcome the intake and exhaust restrictions from the factory. My motor should be much more effecient than stock. (I hope!) In fact when I am done with most of the changes I plan on putting factory tires back on to do some testing and verify the results. Good, bad or indifferent I will post the results for everyone to see.