I HAVE A 92 RS WITH A 350 TBI.IT HAD A 2.73 REAR 1 WHEEL PEEL.I CHANGED THE REAR TO A DISK BRAKE 3.23.I KNEW THE SPEDO WOULD BE OFF BUT IT SEEMS TO HAVE KILLED ALL MY BOTTEM END NOW IT WILL HARDLY BURN THE WHEELS OF IT. IT IS ALL TOP END POWER THE POSI IS NICE. MY GAS SEEMS TO BE WORSE.COULD THE SPEDO READING WRONG BE CAUSING THE COMPUTER TO WAIST GAS AND BE KILLING THE POWER OF THE ENGINE! OR IS IT THAT IT WAS ONLY SPINNING 1 WHEEL BEFORE AND NOW ITS SPINNING 2 TAKING MORE POWER SHOULD I CARB THE ENGINE TO GET MORE POWER?AS IM NOT EVEN SHURE THAT THEY DID ALL THE CORRECT CHANGES TO THE CAR WHEN SWAP WAS DONE. HOPE SOMEONE CAN HELP IM LOST FOR ANSWERS!!!!!!!!!!

That can't be reassuring. If you went to a larger engine and taller gears, I'd assume gas mileage would suffer some. What is your current mileage?

JamesC

Well w/a posi you are gonna get better traction instead of spinning the wheels.... It should be quicker all the way through the rpm range???

im not to sure what it was but i drive 20 miles to work and used 1/8 of a tank and than 1/8 back home now it is using alittle more than that each way my concern is why i lost so much power or is the spedo screwing up the computer? and why at 70 mph am i running around 2800 to 3000 it's a little strange.

Is your tranny hitting 4th, sounds like no OD? As far as the computer?? I can not help you there.

That would explain the high RPM and fuel consumption.

JamesC

yeh its shifts fine its not winding out or anything it just seems to want another gear
im thinking that i should just put the orignal rear back in

My 350 with 3.23's is only doing about 2300 RPM's at 70 or so. The rear is a great all-around gear. Without knowing, I'd suggest something else is going on.

JamesC

you are also running a 5 speed right? if so whats yours run at in 4th at 70

That's true. My bad. Can't say what that would be off the top of my noggin.

JamesC

next time you drive yours can you check into it ?

Hopefully you're not gauging your car's low end power based on it's ability to spin the tires.

Cars with worn ragged out suspension spin tires, fast cars don't.

When a speedo is off, so is the odometer. Until you get that fixed, it'll be hard to calculate your gas mileage.

Going to a lower gear, you are going to see an increase in RPM at cruise speed. It may be perfectly normal & you are just feeling like the RPM difference is a bad thing.

I forget whether im running 3.23's or 3.42's but either way in 4th gear at 70 mph im in the 4 k rpm range ( my 4th takes me up to between 110 and 120 mph ). I have a 305 though but it shouldn't be to much of a differrence.

4000rpm at 70mph???

I'm only running 2000rpm at 63mph! 2000 more & I'd be losing my license!

He has a T5, so his 4th is the same as your 3rd.

True, ,true....But even in OD, 70mph wouldn't net me 70 mph. 4000 rpm would still easily get me a big fast ticket, in OD!

Yeah, even in drive at 4,000 RPM with my 3.42's is a big ticket... if caught I would most likely be walking for a while.

Maybe we should be doubting his tach...

Yeah the 91-92 tachs are notorious for optimistic readings. My tach shows 2700 RPMs when the real RPMs are 2000.

Changing gears and to a posi will meke a world of difference.. to get your speedo right leave your drive gear on the output shaft.. should be red 17 tooth you need the 42 tooth drive gear for the vss assuming your runnin 245 50 16 of there about.. to get the correct shift i need to know which servo your runnin 553 554 or a vette 931 or 951.. the speedo isn't going to change shift rpm.. that will depend on servo size, gov pressure and your line bias spring.. if the 700r is the orig from he vehicle you should be able to go to the vette servo and a high output gov and be good.. this all depends on your desired shift point. if you can get me that info i can better help you..good luck

In addition you may wanna check your numbers.. to my knowledge 350 tbi wasn't available... it was 350 tpi or 305 either way regardless you should be able to get your desired shift.. IF i am wrong and the were 350 TBI cars made someone please correct me

no, the 350 was not made in a TBI version for the F-body, now if he DOES have TPI, i could see there being performance issues for a bad speedo reading, because the PCM DOES take readings from the VSS to help with fuel trims, but ti should not be THAT bad, maybe he accidentially got a 2.73 posi? thats would cause a definate power loss, and they were made, my old 88 had one of those, disc brakes and all, needless to say it went to the junkyard with the car lol, (i walked the pinion up the ring gear and blew the rear seal :P)

I lied haha. 4th is at like 3000 3rd is in the 4000 range when going 70.. sorry for the confusion.

ok and what about 5th? arent you supposed to cruise highway in 5th?

did you by any chance count out the rear gear to make sure you have the right ratio? i would start there. As mentioned above, when you move to a posi rear it is harder to spin the tires because you are sending power to two not one. I would count it out(count number of driveshaft rotations to turn the rear wheels ONCE in case you aren't familiar with the process) and then get back to us. You will then know for sure what speedo gear you need to recalibrate the speedo and, as far as the factory tach goes, well......buy a cheap sun tach to get your rpms read correctly.

Thats around 2100-2200

you are the wearkest link...goodbye!!

...fail. thats normal, your fne, dont worry about it, just get your speedo calibrated