I think the Hondas are meant to run at higher RPM. According to my owner’s manual, it recommends shifting at 53 MPH from 4th to 5th.
That speed in 5th is about 2300 RPM.
I would say 3500 or a bit higher is more dead center 
And even with me zipping around at 80MPH around here, I am still averaging 33MPG (really have to get around to correcting a slight rear misalignment though).
I just wanted to chime in with the results of my non-scientific experiment (N=1). At 3500 RPM my '94 Saturn SL with 5 speed (non-overdrive) is doing an indicated 90 MPH. Not bad for a 4 cylinder displacing less than 2 liters.
John
My Honda Accord Ex with V6 Vtec engine turns 2250 RPM at 70 MPH. So it should do 108 MPH at 3500 RPM.
This is the only flaw with my Integra, which is in all other respects a beautiful car. At 60 mph in 5th it’s doing 4000 rpm. On long journeys, the engine noise gets irritating, and I long for a 6th gear.
Is it possible to get the gear ratios changed?
My 1988 Fiero (Automatic, V6) Does 70 @ 3000.
My truck does about 115 at 3500RPM in 5th gear… Only about 2150RPM or so at 70.
SNEER I used to have a '74 Dodge Dart with a 360ci V8. It redlined at only 6500, but did a comfortable 140 mph in top gear at around 6000 rpm (never did take it any faster than that).
Sure, it only got about 18 mpg, but it was worth every penny.
@ 3500 rpm, my '83 mustang is doing 80 mph on the nose.
'97 Integra GS. 78mph @ 3500 while in 5th gear. My old '87 Civic didn’t have a tach (or a lighter or a passenger side mirror…it was the world’s most de-contented car), but I used to drive daily over a small mountain going 60mph, often wound out in 3rd, since it wouldn’t pull 4th on certain sections of the road. 63bhp if I recall correctly. Even though it faced this abuse for over five years, it’s still going strong with over a quarter-million miles on it, and it still burns no appreciable amount of oil between tune-ups. The two Hondas I’ve had may sound like methadrined sewing machines, but they’re so reliable, that I doubt that I’ll ever own any other make of car. And the Integra just eats up the corners on those mountain roads…
Well, I think you’ve already got your answers, but I thought about this thread on the way in to the office this morning. All I can report from my brief (two exits) freeway foray is 70 mph @ 2000 rpm. Assuming that was in top gear (auto tranny) and ignoring any slippage, a linear extrapolation gives a calculated speed of 122.5 @ 3500 rpm.