BMW i3 REx Goes Mountain Climbing


Loveland Pass, Continental Divide, Colorado. Elevation 11,990 ft

Loveland Pass, Continental Divide, Colorado. Elevation 11,990 ft

A few weeks ago Don Parsons of Denver, Colorado took his i3 REx on a 128 mile road trip from his house to Loveland Pass (Continental Divide, Colorado). On his way up to the 11,990 ft elevation of Loveland Pass he stopped at Beau Jo’s Pizza for lunch and to charge on their public ChargePoint EVSE where his i3 REx accepted 8.9kWh’s of juice to help with the rest of the climb up the mountain.

The car showed 18 miles remaining at the top of Loveland Pass, and he nearly made the trip entirely on electricity when 62 miles later the range extender kicked on and he was only 2 miles from his home.

The trip summary:

  • 64 miles each way
  • 8,960 feet of climbing, 2,329 feet of descending on way out
  • 2,329 feet of climbing, 8,960 feet of descending on way back

Having heard about this Continental Divide conquest, I asked Don if he wanted to write a guest blog post about the trip, but he offered to do one better. His next challenge was to take his i3 REx up to the summit of Mt Evans which is the highest elevation paved road in the US. The trip would take him over 14,000 feet above sea level and would most certainly push the range extender beyond its limit. This was not the kind of road trip BMW envisioned people taking the car on when they designed the REx, but nonetheless they have to expect some people like Don would do just that. So what happened? Read Don’s words below to find out:

*Editor’s Note: This post appears on Tom’s “The Electric BMW i3” blog. Check it out here.

BMW i3 REx

BMW i3 REx

I’ve had my BMW i3 REx for almost two months now and haven’t really used or tested the REx engine. Before today, I’d driven about 1750 miles total with only about 10 miles using the engine. I decided to drive from my house in Denver to the top of Mt. Evans and return without stopping for gas or topping off the charge. Using the REx engine in the mountains can be tough because the output of the small engine can’t really put out enough power to go both highway speeds and climb uphill. However, the road to the top of Mt. Evans is pretty narrow, has steep drop-offs, no guardrails and a lot of cyclists sharing the road so you really don’t want to go much faster than 35 mph. For this reason, I thought the small engine could hopefully handle the climb.

Soaking Up The View

Soaking Up The View

For those that don’t know, Mt. Evans is the highest paved road in North America with an elevation of 14,130 feet above sea level! I live in Denver which is at 5,280 feet above sea level. It’s 63.5 miles from my house to the top Mt. Evans. With some up and down hill climbing, the total ascent is 12,446 feet and the total descent is 3,686 getting to the summit.

BMW i3 REx

BMW i3 REx

I set the car into Eco Pro+ and set out on city streets of Denver, then I-70 west up the mountains to Idaho Springs. Mt. Evans highway winds its way south from Idaho Springs to the Mt. Evans summit in 28 miles. When I originally entered the destination into the navigation system, the guess-o-meter said 61 miles of range. I tried to keep the cruise control set to about 5 miles over the speed limit.

BMW i3 REx

BMW i3 REx

I was surprised to find that the Rex engine didn’t turn on until about 52 miles into the drive (about 10 miles from the summit) with a total trip average mi/kWh of 2.8 when the REx kicked on. However, about 1 minute after the REx turned on I got a Brake error message that ended up making the brake pedal feel stiff and pretty much unusable. Fortunately, I was still heading uphill and the regen seemed to be working normally.

I could hear the engine speed up during the straight parts of the switchbacks and as I slowed down for the sharp curves, the engine almost immediately slowed down as well. I never really wanted to travel faster than 35 mph so I didn’t notice any performance hit until near the summit. On the last few switchbacks, I put my foot to the floor and couldn’t get the car to travel faster than 26 mph. At close to 14,000 feet of elevation, the engine was probably severely limited from its usual output at sea-level. I’ve heard people say that an ICE reduces power output by 5% for each 1,000 feet of elevation. In any event no other cars were traveling any faster than 25 or 30 mph so I didn’t feel unsafe.

More Soaking In Of The View

More Soaking In Of The View

I finally made it to the top at 14,130 feet! The temperature had gone from 68 degrees in Denver to 35 degrees and quite windy. This road usually shuts down for the winter sometime in September so they will be expecting snow to start accumulating up there pretty soon!

At The Top

At The Top

As I was getting ready to head down, I was worried about whether I would have any use of the brakes since I had turned off the car and walked around the summit for about 5 minutes hoping the error would reset but it didn’t and I still had a stiff brake pedal. Since it was 35 degrees up there, I was pretty cold and didn’t feel like waiting any longer so I thought I would start to head down and see how well the regen worked to keep the car in control. You can imagine how happy I was to have such a high regen rate because I felt like I was in complete control all the way back home.

During the descent from the summit down to Idaho Springs I was excited to see that the regen had built up a full 25% of the battery SOC and the guess-o-meter said as high as 28 miles of range on the battery. I drove home significantly on battery but the REx kicked in a few times where there was some climbing. It also stayed on once I got out of the mountains but I was easily able to maintain 75 mph on the highway leading east back into the city.

Here's a screenshot of the elevation and speed of the whole trip

Here’s a screenshot of the elevation and speed of the whole trip

BMW i3 REx

BMW i3 REx Trip Info

Some stats on the whole trip. I traveled 127.6 miles and averaged 4.9 mi/kWh and average speed was a total of 39.6 mph. I used a little over a quarter of the rex tank which I think is pretty minimal for traveling almost 130 miles! As you can see from the picture, the brake error was still in place when I arrived home. However, after being on my EVSE for a little over an hour, everything was cleared out and a quick trip to grab lunch showed that everything was back to normal.

I should point out that I haven’t gotten any software updates yet as I haven’t been able to set aside the time. Until this trip, the only error I’ve seen from the car is the Check Engine Light, which remains illuminated. There is a software update that will eliminate this waiting for me at my dealer, so I guess it’s time to get the car into the shop for the updates. Hopefully the brake error is related to the 12v battery issues that others have discussed and will also be fixed with the latest software version I’ll be getting.

BMW i3 REx

BMW i3 REx

When I thought about getting the i3 Rex, I figured that I would use the battery over 95% of my driving miles. So far, it seems like I’m using the battery over 97% of my miles. That said, I’m still happy to have the Rex as it completely takes away any range anxiety when I’m traveling in the flats of the front range of Colorado.

What about the mountains? I was concerned about using the i3 REx in the mountains and still believe that having a REx hold mode similar to the European version of the i3 or the Chevy Volt would make this an even more enjoyable car in Colorado. That said, a couple CCS fast chargers placed strategically off I-70 would go a long way to helping the issue. Locations in Idaho Springs, Silverthorne (where the 8 Tesla Superchargers are located) Copper Mountain, and Vail would be ideal spots to get a quick top-off and be truly useful in the mountains. I’ve also kept my 335xi for long distance ski trips since it has all wheel drive and is still a great car. However, even without the fast chargers I was able to make it to the summit and home without a problem.

I’m very happy with the the i3. The performance, handling, smoothness, and quietness all contribute to a great experience. Like others have said, it’s hard to go back to a regular internal combustion engine after experiencing electric!

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18 Comments on "BMW i3 REx Goes Mountain Climbing"

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Dr. Kenneth Noisewater

This leads me to think that a better option for the range extender would be to have VVT and DI, with an electric supercharger and exhaust-driven generator to recapture exhaust energy. The VVT cam profiles would be for Atkinson and Miller cycles, switching between the two depending on load, altitude, etc.

Almost all BMW CAR engines now have valvetronic, turbos and direct injection. But this engine was from a motorcycle, so I’m not sure what it has..

I’m surprised the BMW isn’t turbo and direct inject. Don’t they have mountains in Germany?

Only the Alps 😉

So you guys are suggesting they put a bunch of money into making the scooter motor more efficient at two mile altitude, so the twenty people who will ever climb up there can run 5 mph faster?

Dr. Kenneth Noisewater

No, I’m suggesting improvements in technology and engineering that would make the REx motor not be an inefficient joke. The damn thing is less efficient than the Volt’s POS iron lump, at half the power.

Probably has more to do with small packaging and light weight that the motorcycle engine has. Do think the Volt engine would fit behind the axle of an i3! And the Volt engine does directly couple at times to give it even more of an advantage. But bottom line is the i3 EPA rating, even on fuel is rated slightly better than the Volt

edit: I don’t think the Volt engine would fit back there. And as it has been said before, since the i3 can go twice as far as the Volt in EV mode, it will rely even less on petrol to begin with. Not to mention the i3 will run circles around a Volt in acceleration and handling. Probably braking too.

The Volt is great at handeling and braking, with 50-50 weight distrubution and low Cg.

Acceleration is “adequate,” but just barely. 0-60 mph in 9.0 sec. Better than a Prius, but not as good as the i3.


It would appear that for speeds up to 70 mph that is not the case.

In this informal test the Rex was actually 14% more efficient than the Volt.

Above that speed the ICE in the Volt probably goes into driving the wheels directly?

A thermoacoustic generator is independant of altitude. They could make a combined airco, heat pump, generator thermoacoustic system.

Hats off to Dan Parsons, doing BMW’s mountain testing…in the car he purchased for $50,000!

Sounds like a successful trip: 26mph at the summit, no brakes ( stiff pedal to the floor ).

Dan, do you make a regular habit of purchasing a $20,000-higher-than-average-price-paide-by-Americans-for-a-new-car, then thrashing it to within inches of it’s capabilities? LOL!

I mean – I suppose you did a service to the 15 to 20 people out there who will ever attempt that drive, or one in the Himalayas and were wondering if this is possible…. but….WHY?!

Dan’s possible answer: “BECAUSE IT’S THERE!”…


Well, Dan, my hat’s off to you – I surely wouldn’t buy a new vehicle and attempt to see if I could break it. This would be like if I bought my new Volt and took it to the local SCCA track day just to see if it could handle it!

Dan said it is only 62.5 miles from his Denver home, so that is not a strange thing to do. It is also interesting for people living near mountains in other places like around Vancouvert, in the Alps, the Andes or even in Nepal.

I want to see a side by side test against the Volt on a 5% grade. Run the 2 cars to PPR mode and see what the steady state speed is and the miles duration before PPR reduced.

I did that test for the Volt and the results are here.


Don’t any of you BMW guys have an instrumented car?


Talked to an i3 loaner driver down on the front range, and he loved the car, but was not impressed by the REX setup. He drives from Denver to Colorado Springs regularly, which is about 60+ miles at highway speed. Ten miles outside the Springs is a significant hill which the REX was not able to maintain speed on, dropping the driver to 40mph on the highways.
This was mainly due to the low start point for the rex in terms of battery charge, which limited the buffer of power available in the battery forcing the rex to provide all the power (which it could not going up a long incline). He hoped that BMW would allow drivers to customize the start point of the rex which would remove this issue.

I think there are two things that constrain the i3 Rex performance in mountains. First is the lack of a maintain mode so we can turn on the Rex before 6.5% SOC, the second is the reluctance to run the Rex ax max RPM at low speed. Below 44 MPH or so the Rex won’t run at full speed regardless of charge. Don noted this as he slowed down for curves. If the Rex would run at full power when the SOC is really low regardless of the speed then Don might not have had any problem near the summit. Sure the Rex would have sounded a little worse going around those corners at low speed, but I think that’s a reasonable tradeoff for maintaining SOC in trying conditions. It is also something BWM could modify without running afoul of CARB.

Why isn’t anyone, most of all, the author himself worried about the brake error that came up as soon as the REX kicked in?

My BEV’s are not used for record settings, it is used to haul my family and I would be most upset if ANY brand new car had a brake problem so early. Brakes must function properly all the time.

In the author’s particular case, the attitude was more like “oh it’s just an error light, the brake is still stiff”. I on the other hand, would have driven straight to the dealer and demanded a fix or a rental.

Thanks for the story. Personally, I like seeing cars pushed to the limit. It helps realize their potential and/or help us to avoid catastrophe.