I bought my 2006 civic hybrid with the civic ex wheels on the car with stock size 205/55/16 non lrr tires. The seller also gave me the stock 15 inch hybrid wheels with stock size 195/65/15 tires that are also not lrr tires. I have put 15,000miles on my civic hybrid with the stock 16's and I average 40mpg. I have never driven on the stock hybrid 15 inch rims.

I just baught some light weight volks le37k 17x7.5 +40. I bought new 205/50/17 low rolling resistant contenental pure contacts tires. I have put 3,000 miles on this new setup and I'm geting 34-36 mpg. How the hell did I loose 5mpg going to a rim/tire combo that weights 5 pounds less plus I'm now rolling on lrr tires for the first time!!!

The stock 16's with tires weigh 40 pounds and 6oz. The stock hybrid rims and tires weigh 34 pounds and 7oz. My volks with lrr tires weigh 35 pounds and 5oz.

How the hell did I loose 5mpg going from the 16's which weigh 5 pounds more to my 17 inch volks also with lrr tires.

Some one make sense of this for me.

Tonight I just took my volks off and put the stock 15 inch hybrid rims on for the first time ever. Let's see what mph I get with the stock hybrid rims which don't even have lrr tires.

 

Your mileage may not really be going down! What's happening is, the larger diameter tire/wheel is messing up your odometer. (I believe your speed/odometer is based on tire rotation expecting the OEM size). So the car PCM computer 'thinks' you are driving less miles than you really are, but because you are driving further, the mileage calculator is off.

In many cases, the dealer can re-program the computer to the new tire size. It may be worth asking.

That's my guess. Since you can't rely on the odometer and speedometer with larger tires, you'd have to use a GPS to get your actual speed and determine mileage

 

The tires are all the same overall diameter. They are all 25 inch tire in terms of diameter. The speedo is not affected and is not off at all. I know the speedo is not off because I can check the cars speed with my harming navagation.

Trust me the car lost a huge significant 5 mpg.

 

OK I mis-understood. The sizes were both the same 205 to 205.

Could it be that you've been using your A/C more as the weather is warming up? That would reduce mileage by 5MPG

Also, try putting 40PSI in all the tires and see if that improves anything?

 

All my tires are always inflated to 40psi. Driving style and ac usage is the same.

 

Lighter tire / wheel assembly means there's less inertia which will impact you fuel efficiency a little bit. Can't imagine that much though
Increase your tire pressure, contact area with road will be less and therefore less rolling resistance, which also means you'll loose some wet and snowy road traction.
Time of year typically impacts mpg's too

 

any result? Maybe it has nothing to do with the tires and something else is happening with the car. A coincidence.