Gearing Commander vs. Reality?

[JtheB]

Hey kids,
I've noticed a substantial discrepancy between the Gearing Commander and real world data for my machine.
'89 GSX 1100F
Gearing Commander has this bike in their database so all stock specs can be auto-filled.
I've confirmed that all the data actually matches my machine:
Tire sizes (and with minimal wear) - check
Final drive gearing 15/52 - check
The transmission driver/driven gear ratios filled in by the software has been checked against stock spec.

Discrepancy:
Gearing Commander claims these mphs @ rpms:
5000 rpm @ 79
5500 rpm @ 86
6000 rpm @ 94
My GPS claims this:
5000 rpm @ 70 - (smooth engine)
5500 rpm @ 76
6000 rpm @ 82 - (noticeable engine vibrations)

My end goal is to smooth out the engine at approx 80 mph (my highway cruise speed) - this would require dropping about 1000 rpms!
I'd have to use at least a 16/45 final drive to hit that 19% rpm decrease.
I'd think that first gear from a stop would be pretty painful to the clutch considering the weight of this pig.

Something is WAY off here.
If Gearing Commander can be trusted then is it possible that a former owner messed with the gear ratios in the transmission?

Questions:
1) Why such a big discrepancy?
2) Assuming the above cannot be solved is a 1000 rpm drop too tall an order on this never-gonna-race-the-Isle-of-Man land yacht?
3) Anyone else wish they'd thrown in a 6th gear?

Keep'in it Suzi-fresh in New England

J

[92xjunker]

Don't use the kat speedo for your checks. They are off from day 1. Use a GPS app, more accurate. Gearing commander is usually spot on by the math. If the tire size or trans was changed, then it's all out the window.

[shpielers]

You can put a 6 speed transmission from a Kat 750/600 in it. Not easy and requires a machine shop to do some work, but it is possible.
Also, the bike shouldn't vibrate a whole bunch - you might have something else going on. Ie carbs out of sync, or the bike running too lean.

[JtheB]

I appreciate the responses. Maybe I should slim down the data-dump.

According to my GPS readings my engine seems to be running at about +800 rpms over what the Gearing Commander says it should be at highway speed.
My tire sizes and final drive are stock specs.

What could be causing this?
Did a former owner possibly change drive/driven gears in the transmission or is there another possible cause?

Thanks

[92xjunker]

There may be 2 different size rear sprockets available for you bike as spec. 45/47 I know the 600 can have either.

[JtheB]

That's good to know 92xjunker.

When the bike model/year is auto-loaded into Gearing Commander it lists the final drive as 15/52 which is exactly what's on the bike.

Calculating for 45/47 sprockets would exacerbate the discrepancy issue.

It seems in lieu of changing out the transmission to a Kat 750/600 version I'd have to change the final drive to something that seems crazy tall just to get the rpm's back to oem spec (as claimed by Gear-Comm).

I'm at a complete loss on this one.

Thanks again for the help my friends.

J