Hi, i've got an 87 samurai with the 1.3. It has the tach dash and i see that it redlines at 6500. This thing goes slowwwwww up hills. It always seems to want to run at about 3500rpm but today i kept it in 3rd gear up a hill and pushed it to about 4700rpms for about 3 miles up the hill. It got me up the hill at about 48mph which is faster than it usually does!   

I don't know much about engines and i was wondering if pushing it that far for that long could do any harm to the engine? It feels like it is winding out pretty hard.  It is obviously an old engine. I was curious if the "red line" area drops with age of an engine as parts wear?

Also on flat ground 4th gear pedal to the floor takes me up to about 55mph at 3500rpm, it won't rev any higher. Is this normal?
5th gear on flat ground will keep me at 55 at 2500rpm and won't rev any higher, and if just the tiniest bit of a knoll comes up it'll drop my speed to 30 within seconds and i have to down shift to 4th and it'll VERY slowly work its way back up to about 45 until the road levels out or starts down hill then i can get back up to about 55. Is this normal also?

Sorry for all of the questions, i know sammys are slow, but i'm just wondering if i can rev it that high and if that sounds normal or if my engine is just getting tired?  I also have 235 75 r15 tires on it so i know that slows me down a bit as well.  Thanks for any info.

The G13 engines thrive on revs, most Suzuki engines do, it's a part of their motor cycle heritage - they make peak power relatively high in the power band (compared to american iron), and will run all day like that.

You'll find they run out of breath in the 5000~5500 rpm range, well before red line, so unless you're unlikely to do any harm.

As you stated, your tires are larger than stock, but you still have stock gearing.  The Samurai was never a powerhouse to begin with, but the tires are the reason your highway performance is what it is.  A 4:1 t-case gearset would an easy way to get better highway performance and great gearing for offroad.

I also would not rev an old Samurai engine above 5,000 rpm.  Most of these engines have not been well cared for (zero oil changes) and seriously abused over the years.

Your engine sounds tired. Your flat ground speed is low. On flat ground mine would go way faster with the 235's it just took awhile to get up there. I have the 4:16 t-case gears now and way more power. I think that running under 3000 rpm puts more strain on the 1.3 than running 5500. I live in the mountains and have to run 3rd gear on parts of it to keep it over 40.  And in 4th on the freeway and open roads, I run 65 yo 75 at 5500+ all the time. I drove to Tuscon once (450 mi.) with my foot on the floor. Around 70 or so. They love it.l

Yes, you really have to keep these engines in their power band.  They really do not start making much power until around 3000rpms.  IMHO, the sweet spot is from 3000-4500 rpm.  It is producing a good amount of torque and HP at those rpms.

The shift point is at 4500 RPM.  And Tim Hardy said it himself.   

4500 is just a nice cruising speed for me I usually shift about 5500 and maybe 6000 or so when I'm in a hurry(which is almost always) I still think your engine sounds a little tired or maybe needs a tuneup. I think with 235's you need some re gearing. But it should be a little faster and have more power with stock.