Why is it not faster?

[JOHNNYMAXXIMA]

I know guys, most of my questions always have to do with speed.
That's just the horsepower junkie in me.:D (I know you can appreciate that coolhand).

So I got my new 9L in on my Maxx today and took it out for a few hot laps up and down the street. Everything went fine, ran smooth, lots of power but it just wasn't as fast as I thought it was going to be. Not as fast as my dual 9L anyway.

You see, I thought that by going a single setup with allot less weight was going to give me similar performance.
Here's the specs for both, RTR.

Dual 9L w/28 cells = 14lbs 10.9oz. geared 66/20
Single 9L w/14cells = 8lbs. 11.3oz. geared 66/19

Yes boys, that is a 6 pound difference!

So I thought that maybe it was the tire difference. I had buggy tires on the single and revo tires on the dual, so I put regular maxx tires on the single and took it back out. Maybe a couple mph difference but still way off the mark. I didn't bring out the radar gun but I guesstimate maybe 35-40mph with the single compared to about 50mph for the dual setup.

So what's the problem? Could it be something wrong with the motor. I bought it used on ebay and the guy said he only had 2 runs on it? (yah right)
Thanks.