Too bad they couldn't find another 5 mph out the the top speed. For those of you who do not know, the top speed of the Grom is "just under 65 mph". Translation: top speed is an INDICATED 63 mph which is an actual 60 mph, slowing to 55 mph with any hills. In other words, not really for interstates and barely fast enough for some secondary roads. The power is all down low and in the middle. What they should have done is made 4th gear very tall...tall enough to get you up to an indicated 70 mph...perhaps something akin to the 6th gear in the Ninja 300 which takes the bike all the way to 104 mph or so and allows it to cruise at an indicated 75 mph. Smaller engine of course with the Grom, but they could still have increased the top end considerably with some gear ratios.
From what I've read, 4th gear is very "flat" on a Grom.. it isnt powering through it and banging the limiter the whole time you ride, while you can probably tach it out in a tuck or maybe down a hill, 60mph is on par with 125cc scooters with the same hp.. If you can't handle doing 60mph WOT sitting up then maybe the bike isn't for you?
Also if you're dissatisfied with the top end, resprocket it for higher mph.. Honda isn't dubbing these "mini highway monsters" they're made for urban areas (i.e. geared for short distance riding) but as Composimo said in comparing it to his Madass, the Grom would probably kill it down low but since the Madass was geared for top end it would eventually catch up and pass it
When being realistic about performance expectations, the Grom is right on point with power, mph and lack of aerodynamics
At 100 kmph my msx is at about 7300 RPM which is still shy of the 8100 red line. The rear sprocket has 32 Teeth. Just go to 30 if you want a bit more top speed, but you will be sacrificing some acceleration. I find it is a pretty good balance stock. If you really are disappointed by this maybe the Grom isn't the bike for you.
As it is, 8.50mph per 1000rpm is very tall gearing for this displacement...an impressive achievement in it's own right. Re-gearing probably won't make much, if any, difference in top speed, only change rpm. There's an acceptable range. But, Honda usually chooses a very good "compromise", so I wouldn't be looking for more than maybe a 300-500rpm decrease, at max cruising speed. I'll guarandamntee that one more tooth on the C/S sprocket will make the bike weak in top gear...and, from a standing start, you'll be riding the clutch like a rented mule.
70mph?...in a full tuck, descending a steep grade & with a strong tailwind...maybe.