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Back in 5/15 I posted this: "Just completed my 1366 mile round trip, Asheville to Chicago. I kept the Volt on Mountain Mode (MM) almost the entire trip. I left with a full battery but after that didn't plug in. Average mpg: 43. (1366 miles, 1272 gas miles, 54.5 electric, 31.73 gallons of gas used, 16.4 kWh used.). A very nice highway-driving car....(etc.)"

I've just returned from another round trip to Chicago, this time totaling 1399 miles, using 34.6 gallons of gas, achieving only 40.3 mpg. Conditions were the same: left with a full battery but did not charge thereafter, same route, and kept it on Mountain Mode much of the time. The first trip, I'd had the Volt about a year and had perhaps 9,000 or 10,000 miles on the OEM tires. This time I had brand-new Continental PureContacts. Both trips I had the tires at 42 cold pressure.

Given the similarity of the other conditions, I can only conclude that the reason for the reduction in miles-per-gallon from 43 to 40.3 is the change in the tires. The OEM tires really are more "efficient" in road-rolling, it seems. I should mention that in this recent trip we encountered a lot of very heavy rain, but I can't imagine that this made a big difference in mileage. The OEM tires didn't last very long, though: 38,000 miles doesn't seem like a lot to me. I hope the Continentals last a lot longer.
 

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Given the similarity of the other conditions, I can only conclude that the reason for the reduction in miles-per-gallon from 43 to 40.3 is the change in the tires. The OEM tires really are more "efficient" in road-rolling, it seems. I should mention that in this recent trip we encountered a lot of very heavy rain, but I can't imagine that this made a big difference in mileage. The OEM tires didn't last very long, though: 38,000 miles doesn't seem like a lot to me. I hope the Continentals last a lot longer.
It takes new tire a while to "break in". You'll probably see at least half of that drop come back in a few months.
 
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LRR tires are 5 to 7% more efficient, so we should expect to see a reduction in both mpg and in EV range when we install non LRR tires

It's really hard to make a true A to B comparison using the odometer, because it's directly affected by the amount of tread on the tires. Worn tires make it look like you went further than you actually did, so the numbers 'look' better than they really are and no two trips are exactly the same with speed, weather, traffic and so forth

You can save a few $$$ buying cheaper, non LRR tires or you can pay more and get higher mpg and better EV range. Maybe it all comes out the same in the long run . . . . maybe not

Don
 
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In my experience, "very heavy rain" can make a large difference in MPG.
I think it's too early for you to compare efficiency of tires...
 

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Sorry, but MPG is a meaningless number for a car that can move down the road without using any gas at all. Had you recharged the battery even once during either trip, the total distance would have remained the same, but the gas consumption, and thus the MPG for that trip, would have been different. And, as long as you’re driving on grid power only, rain or cold weather or brand new tires or driving uphill or fast or in crosswinds have no impact on MPG at all...

The trips were not really comparable: the 2015 trip was 1,366 miles, the recent trip was 33 miles further. If those were gas miles, then "different tires" wasn’t the only reason the recent trip used 2.7 more gallons of gas than the 2015 trip, the trip included more gas miles. The 2015 trip was described as a 1,366 mile round trip that included 1,272 gas miles and 54.5 electric miles. It’s unclear what fuel was used to drive the remaining 39.5 miles.

You need to compare fuel mileages before and after the change in tires. How did the 2018 MPGcs compare to the 2015 MPGcs? How did the 2018 miles/kWh compare to the 2015 miles/kWh?
 
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