Those of you that went with 205/50/16 tires...

[RABID_MP5]

You guys are absolutely right to figure in the larger diameter of the replacement tires. But there's a piece you might be forgetting.

Typically, if you're swapping tires, the tread on the old is pretty worn. Worst case, down to 2/32" at the wear bars. Vs. about 10/32 new. So 8/32 difference in radius means 16/32 or 1/2 inch (!) in diameter.

Since the diameter is about 24 inches, there's another 2% mileage "drop" accounted for by the tread depth (in the maximum-wear example).

I've also known some tires to deviate more than you'd think from their theoretical diameter (not e.g. one in four, some just run big or small) I usually run a tape measure around things to see what's going on. Then maybe calibrate the OD off mile markers.

So, rest easy. And hey, your mileage gets better every mile! (boom06)

 

[poison]

So which is better for gas mileage, diameter-wise? I would assume larger would give worse mileage from a start, as gearing is higher, but better at freeway speeds, as RPM's would be lowerat a given speed. Or am I mathematically challenged?

 

[gus5]

Really it has more to do with width/how sticky the tire is. You'll never really get anything diameter-wise that's so far from stock that will effect your mileage besides screwing up your speedometer/odometer.

To get the highest mileage possible, get something in the stock size or in a 205 that isn't very sticky. Which is pretty much any all season tire in 195 or 205. Over-inflating them to the max allowable on the tire sidewall will also help, though isn't recommended.

So which is better for gas mileage, diameter-wise? I would assume larger would give worse mileage from a start, as gearing is higher, but better at freeway speeds, as RPM's would be lowerat a given speed. Or am I mathematically challenged?

 

[yangsui]

So which is better for gas mileage, diameter-wise? I would assume larger would give worse mileage from a start, as gearing is higher, but better at freeway speeds, as RPM's would be lowerat a given speed. Or am I mathematically challenged?

Hard to say. Yes, cruising RPM will be slightly lower but the larger diameter tire needs more torque to drive so you'll need to step on the throttle just a little bit more to maintain cruising speed. That means the engine works a little bit harder, sucks in more air and fuel. I think the fuel economy hit due to size difference alone is so insignificantly small that it is not really worth looking at. There are so many other factors that have larger effect on fuel economy. Tire compound as previously mentioned, driving style, weather, traffic conditions etc.

 

[poison]

Ah, perfect, 'who cares' is the answer I secretly wanted. Because I'll never buy non-grippy tires. lol

 

[Hank3]

Hmmm...I guess I was doing it wrong.

 

[yangsui]

Just couldn't let this go, you need to multiply the displayed mileage by .983, or you could multiply by .017 and then subtract that amount from the displayed total. The displayed mileage is 1.7% greater than the actual mileage.

Um, no. He upsized so the odometer reading is LESS than actual distance traveled.

Hmmm...I guess I was doing it wrong.

No you weren't. Multiplying by 1.017 is correct.

 

[Tom03es]

I run some Goodyear Eagle F1 tires in the 205/50-16 size (really sticky in the dry, they seem to be wearing like iron and they are great in the rain- they completely suck in the one surprise snow storm I was caught in with them). On a recent road trip to Iowa I gassed up, zeroed the trip odometer, reset the "trip" on the GPS and headed off.

As others have stated, the larger tires will throw off the odometer and speedometer. For me, I would be showing an idicated speed of about 72-74 mph on the speedometer while the GPS showed 77 mph. Also, my first tank of gas lasted me 385 miles according to my trip odometer but 401 miles according to the GPS. Both of which are about 4% off of the GPS data.

On a previous set of wheels I had the MSP size 215/45-17 tires. On a road trip across Kansas I had the cruise set at an indicated 5 mph over (this was before I had the GPS) and forgot about the speedo error. A well hidden KS state trooper clocked me at 80 mph (5 over from what I was indicating). He jumped out behind me, I slowed down to an indicated dead on and apparently he was not impressed as I continued to do about 5 over even as he tailed me. I got a $100 reminder that my tires were throwing off my speedometer.

In the end- really don't worry about it. Just keep in mind when you intend on doing 5 mph over the limit, you might be doing closer to 10 mph over the limit.

And pretty much a wider tire, no matter what, will decrease MPGs as the wider contact patch will increase rolling resistance.