Okay so I don't know the size of my rear end at the moment but I have a 305 with 5 speed manual. At highway speeds (65-70) it runs about 3500 rpm. Is this too high? Or will it put any wear on my engine? (engine has 65k miles) And If I were to switch out my rear end to run lower rpm on highway speeds will it save me any gas? thanks.

 

Well.....never mind, I better not!!
Yes the higher rpm's will put more wear on the engine, and on your wallet with lower mpg's. Are you on the highway often enough for it to make a matter to you? Lower rear gears are more fun for all around driving, but not so friendly on the highway if gas consumption is important. If you tell what size tires you are running, we can tell you what gears you have.

 

The tire size is 225/60/16

Well I Havent really drove it a lot on the freeway since I bought it about a year ago but now it is my daily driver to school which is about 40 miles in a day all highway miles.

 

I'm going to say your speedometer might be off, possibly from a gear change without updating the speedo drive/driven gears, or it's your tachometer, or both.
Based on having a .73 final drive in your T5, and a tire diameter of 26.8", at 70 mph....
you would be spinning 3,286 rpm's if you had 5.13 gears, and 3,562 rpm's if you had 5.56 gears. Either one of those would have been an insane gear swap for a street car, but not outside the realm of possibility.
Have someone with a known accurate car pace you and see what their speed is compared to what yours shows.
Also, the 3rd gen tachometers are known to go bad and can read way more than what's real...my 91 read double. However, I was fairly certain the problem was primarily with the 90-92 models.

 

I tested it today and the speedo seems to be working fine so how would I check my tach? It seems to look fine on street driving but maybe it messes up at higher speeds?

What final gears do you recommend for a lot of highway use?

 

Alright now I think it is my tach. How do I find out for sure and if it is what are some solutions to fix it?

 

An aftermarket tach is the best tach GM didn't put in the 3rd gen's.
Mine consistently read double, so I just did the math. I even went as far as changing gauge clusters, and the off reading was the same. I'm not sure if it's with the electronics on the tach, and the used one I got was bad as well, or if it's to do with the ignition module giving a false reading. But I sold the car before I took it any further.
If you have an aftermarket, or a tuning tach, you can hook it up to the (-) terminal on the coil and see what kind of results you get.

 

try the old jack the rear up and turn the wheel deal. if you have a posi then turn rear wheel 1 revolution while counting the times the driveshaft turns. a little over 4 turns would be 4.11 to 1 or 411 gears. a little over 4 and a half turns would be 4.56. trans would need to be in netural. if not posi then you will need another person to turn the other wheel at the same time you are. if you dont the other wheel will turn the oposite direction. with posi the other wheel will turn same direction.

 

If an open diff, you can either leave one tire on the ground and turn the free tire one full revolution. Or, with both tires off the ground, turn one wheel two full revolutions instead of one. The reason for two revolutions is because with an open diff where the other wheel is turning the opposite direction, the ring gear is only making a half revolution to the tire's one. I've never heard of the two man way, I would question it's accuracy. But if you've tried it and it works, then hey!

 

that doesnt sound right. if the other wheel is on the ground the shaft will turn either half or twice and many times. now im confused.