In just five months, more than 3,100 BMW i3 electric cars have been delivered to U.S. buyers.

That's a fast pace out of the box, and it indicates that BMW is serious about plug-in electric cars in a way that many other major automakers aren't.

But to keep that pace going may require what car dealers call "cash on the hood," otherwise known as incentives.

DON'T MISS: 2014 BMW i3 Lease Deals Appear: $369/Month For Electric Car

Last month, more reasonable lease deals started to emerge at rates like $369 a month for base-level BMW i3 models--against initial monthly lease rates of more than twice that number.

While it's a BMW, the technically advanced i3 battery-electric version is rated at just 81 miles of range. That puts it in the company of many lower-priced competitors, most notably the Nissan Leaf.

2014 BMW i3 (German-market version), Amsterdam, Oct 2013

And that may explain why it appears a majority of BMW i3 buyers in the U.S. are choosing the range-extended i3 REx version.

That adds an optional two-cylinder gasoline engine under the rear deck to provide another 78 miles using gasoline, on top of the i3 REx's rated electric range of 72 miles.

MORE: 2014 BMW i3: Is Range Extended Version Outselling Battery Only?

For the month of October, BMW has added some additional incentives on top of existing program that increased residual values to reduce the monthly lease payments.

But the new "innovation credit" incentives are twice as good for the battery-electric version as for the REx.

2014 BMW i3 electric cars waiting at East Coast shipping port for distribution, May 2014

Specifically, BMW is offering an additional $2,000 credit for the 2014 BMW i3 battery model, and $1,000 for the range-extended i3 REx.

New Jersey BMW salesman Manny Antunes, who's sold multiple i3 models to local and regional customers, suggests that these incentives are "as aggressive as they're going to get" through the end of 2014.

MORE: 2014 BMW i3 REx Vs Chevy Volt: Range-Extended Electric Cars Compared

But why the difference in amounts?

"At the end of the day, the regular i3 is another 80-ish-mile electric car," says noted electric-car advocate Chelsea Sexton.

"It'll command a premium of $100 a month or so, because it's a BMW, but nothing like the leases we see at $600 or more--which were a huge improvement over the earliest quotes of more than $1,000 a month."

The 2015 Nissan Leaf is larger, offers four fully usable side doors, and comes with a rated range of 84 miles, and lease prices run from $250 to $390 on average.

But the BMW i3 REx is a different kettle of fish, with a total rated range of 150 miles.

On technology, it competes with the Chevrolet Volt range-extended compact hatchback, which will be redesigned for 2016--and the BMW name clearly has more cachet than Chevy (and Nissan too, for that matter).

ALSO SEE: 2014 BMW i3 Electric Car: Why California Set Range Requirements, Engine Limits

If BMW planned its production around range-extended i3 models being only a minority of sales, it may be having more trouble shifting the i3 battery-electric model.

Hence, higher incentives.

In other words, if you want the battery-only version of the 2014 BMW i3, this may be your time to shop.

[hat tip: Manny Antunes]

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