Tire Diameter VS MPG'S

I think your first statement is true, but your conclusions are not correct. Would a solid rubber tire have more rolling resistance than a bald ordinary tire? The internal friction causes heat which can destroy a tire. Higher speed rated tires have less internal friction because the thicker plies do not flex as much and therefore have less rolling resistance. Of course, external friction of the tires gripping the road is another matter. I think I would want that in a new tire.

 

Barry, the writer of those tire tech pages, has been "employed as an engineer by a major manufacturer of tires." Those are his conclusions.
Barry's Tire Tech

 

Somewhere TireRack has an old article explaining why new tires (same type) will be lower "apparent" MPG due to a number of factors, but the diameter due to less wear is bigger for starters. Posted here some years ago.

 

The increased rolling resistance from more material in the tread is likely from tread squirm. The raised ridges and knobs of the tread wiggle some as the tire rolls down the road. The deeper the tread, the more wiggling can occur. This saps energy in the same way running on deep plush carpet does versus on hard floor. The article used off road and snow tires as examples, both have deep treads.

They are also an example that there are other factors are at play. Tread pattern can help or hinder rolling resistance. The extra material also increases tire weight, which is the worse place to add weight to a car for efficiency.

I found this interesting from the article, "It ought to be obvious that as you add more inflation pressure, its effect on the deflection decreases - meaning that adding 1 psi over the placard inflation pressure has much more effect than the effect between 5 psi to 6 psi over the placard pressure." So keeping tires just slightly above the recommended pressures could be enough for visible improvements to fuel economy.

Which is why EPA testing rules limit the amount of wear on the tires. Around 4000 miles worth is the limit.

 

If a smaller diameter tire covers less distance per revolution, how does the car think you've gone further?

 

The Prius(*) will go the about the same number of real surveyed miles on a gallon, regardless of reasonable tire size, but the extra tire revolutions will produce a larger miles number on the odometer to put into the MPG equation.

Plus, the car will be moving slower than you think for a given speedometer reading, and the slower speed boosts real MPG.

With different tire diameters, both of these factors need to be calibrated out.


(*) The situation on old traditional manual transmission cars not geared for efficiency, may be different. Different tire diameters will change the engine RPM operating point, which may either help or hurt the true fuel efficiency. A Prius with eCVT should be much less susceptible to this effect.

 

If the tire is smaller, the engine will be spinning faster than necessary to maintain speed, thus wasting power and fuel.

 

This depends very much on the engine and gearing, and is quite true of older-era cars geared for performance but not efficiency.

But it shouldn't apply to a well set up CVT or eCVT, maybe not even to some of the newer gazillion-speed automatics.

 

My speedometer is off by 3 MPH at 75MPH displayed. I’m actually going 72.
65 displayed is really 62-63.Stock everything. Drives me crazy.

 

Speedometers generally read one or two mph higher than you are really going, to keep you from speeding too much. Or at least that's what I learned as a kid, and I've never had any reason to doubt it--it has usually felt that way to me. Odometers, on the other hand, are legally supposed to be as accurate as reasonably possible.
Does anyone have evidence to refute this claim about speed?

 

That is very normal, and a standard industry practice, dating far back into the inaccurate analog gauge era.

In Europe, the difference is typically larger.

Note that odometers do NOT have a similar offset or error. The class action product liability / defect lawyers made sure of that some time age, collecting settlements for alleged warranty fraud from over-reading odometers.

 

Usually, speedos have a progressive percentage of underrating speed. On motorcycles, it is not unusual to see a 10% difference at top speed and a much lower percentage at low speeds. I use a radar detector that displays GPS speed and the prime is usually 1 to 3 mph off. Three mph at high speed (80+).

 

My new-to-me car's speedometer is perfectly in line with GPS speed at all speeds that I've looked at. It's a 2003 Subaru Legacy L Special Edition wagon with 5 speed manual. I just drove across the US in it the past 5-6 days. It's a real pleasure to have an accurate speedometer again!

It also has the most accurate gas level gauge I've ever experienced. Light comes on about 1.9 gallons from empty, and the needle goes down to the E line in a perfectly linear way. I've filled up at 15.5 gallons on a 15.9 gallon tank a couple times on the cross country trip with the needle just about hitting the E line. I was able to get three 500+ mile tanks that way in the past week, not that far from the range of my Prius. Maybe a record for the vehicle? I've got the highest gas mileage (with accurate data) 2003 Subaru Legacy on Fuelly.

 

That is a big improvement over my 1997 Legacy, also with MT. The top half of its tank was short, the bottom half longer, and a very large amount of usable range was below the 'E' mark. That below-E portion was necessary for me to get to the stations with competitive prices on my usual long route to see the parents.

My 2014 Forester's gauge is vastly improved.