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.

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 :)

 

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?