I have a 1999 Mazda 626 5 speed and my rpms seem to always be high weather I'm driving n town, 4th gear 30 mph 2100 rpm, or on the highway 5th gear 70 mph and rpms at 3100. This I'm sure is killing my gas mileage not to mention ******* my engine, any solutions?

https://www.youcanic.com/nissan-transmission-problems/

there is not enough specif details available to be able to be close to exact But I will give an example that you can compare
in a manual transmission with a diff ratio of 3.909 and in 5th gear the rpms for 79 mph is 3200 rpms
the difference between gears is around 250 rpms so if you try to correlate the differences if it was in 4th gear the rpms would be around 3750 rpms so I suspect that the over drive section of the transmission is not engaging ( assuming that it is auto transmission)
so to answer your question , I am saying that it is too high in rpms for that speed and I suspect that the od is not being engaged as the rpms at 80mph I would be looking at around 3200-3500 rpms depending on the diff ratio

You don't provide much information to go on (type of vehicle, type of engine, 2wd or 4wd, etc.) RPM is always going to be based on the speed you're traveling with a standard transmission. My 4 cyl 5 speed Nissan pickup in 5th gear runs at 3000 RPM at about 75mph.

I, also, thought this was high at first. After some research, I found that this car runs in that range at about 75 mph. So, yes. That is normal.

You're 5th gear syncro has just gone out. It keeps your shift forks and slider gear from popping off. It also makes it possible to shift the gear in. It's going to take a rebuild of the transmission to fix this problem.

Well every gear has its limitations. I am not sure what the mph ranges for each gear are, but i've gotten to 60+ mph in 3rd gear. The only thing about driving on the freeway in 3rd gear is that it is alot less efficient mileage wise. Your turning more RPMS while on the freeway in 3rd gear so you burn up alot more gas.
Driving in D3 is driving in 3rd gear. It could burn up the tranny but it depends on the vehicle itself. It is not really made to do highways speeds in 3rd gear. It should be in 4th or 5th (OD)

Hi
Either a broken linkage or selector problem inside g/box.

Regards
Phillip

if the hydraylic torque converter or trans is low on fluld then the bands and servos will not accuate the gears properly

Make sure you have the trany in "D" (drive) and not in a lower gear like 1,2 or 3. Some models must be in "econo" (not "power") setting for the trany to shift at a lower rpm. It will also get in overdrive sooner.

That is normal,If you do the same speed in 4th gear the rpm's will almost double.

The rpm's will repond faster in a manual than an automatic.

In a manual the engine supplies the power required directly to the gear that you have selected thus the engine rpm's will increase or decrease as needed.