195/65R15's on a 2008?

I agree, altho my experience isn't with a Prius, but with a 2013 Hyundai Elantra & a 2008 manual tranny Accent. With some used wheels, I switched the original 195x65x15 inch Elantra tires to the normal sized 175x65x14 inch Accent. The Accent dropped 300rpms on the highway, drives with much better handling & cornering..... & gains a trace of MPG, despite the wider, heavier wheels/tires. Also, the Elantra with used wheels & used 225x45x18 inch tires, its handling & cornering are superior.... & no loss of MPG. More compliments with both cars, too.

 

But the RPM drop is due to smaller circumference overall. 24.98 inch on original tire, versus 22.95 inch on replacement. When you do the math to fix the speedometer error you now have, your fuel economy is down. You have no gain.

 

I'm confused as to which tire is replacing which, in the above statement. I think he's going from smaller OD to larger OD, if rpms are dropping. And yeah I'd concur: once you start changing tire OD you're throwing the odometer off, which "complicates" the mpg calculation; you certainly don't want to ignore that.

 

No, you have it backwards. I did the odometer(not speedometer) corrections. My Accent MPG with the larger diameter tires did go up. As I said in other threads, 8 tanks of gas in a row were over 40MPG, 5 tanks in a row were over 45MPG, & two tanks were 47+MPG & 48+MPG. The big reason is the 2008 manual Accent, like several other small cars in that period, had the top 5th gear at highway speeds in an inefficient rpm rev range. Altho my reduced revs were still too high, they did drop 300 revs lower & did give better MPG.

 

When you correct for the odometer/speedometer error your tire change has created, you have no gain. Your new tire size has to rotate 10% more to go the same distance.

 

The odometer was accurate to begin with. But I expect you're going to explain otherwise.

Again, I think his new tires are larger OD, so they'd be rotating less for any given distance. Still, changing tire diameter (either direction) is a can of worms, with first outcome being a messed up odometer.

 

I'm goin' ta let Mendel & cnc97 figger et out all by themselves......its easy as pi or GPS or pacing the highway mile markers for 10 miles to get it accurate.

 

GPS is by design only accurate to with about 30 feet. 300 feet (30 feet times 10 miles) times 15000 miles a year is a level of inaccuracy that I personally don’t see any added benefit.

The vehicle manufacturer chose the OE tire size for a reason. You may see a need to change it, but your one car result versus the thousands or even millions in operation with the OE size says you are the outlier.

 

So in the first comparator, I had the 15 inch size listed first. I flipped it, since I may have listed it backwards, and this is the speedo discrepancy.

As I said in my other post (#28), a level of inaccuracy that I personally do not see any added benefit. However, it is your car, you do as you wish.

 

I like that.

 

It is good that cnc97 (or is it 79cnc?) admits it had its calculation backwards & it don't see his backward calculation as a benefit. As I've said, use pi or GPS or highway markers over 10 miles to reduce the errors. Use all the processes & more than once to reduce the errors, further. Yes, accuracy gets better by repeating the processes...... specially when yer backwards. Hey, we'll all agree, when we are all accurate.

 

And I found that owning up to my mistake makes things all that much better.