OK I don't get it..... I pick up my 66 yesterday, I dropped it off on Tuesday, a few hrs later I got a call from the shop saying that I did not have 390's but 355's instead. I was having 323's installed. He said that there would not be a big difference, but I said do it anyway. The car ran 3500 @ 65 MPH on the way to the shop, and on the way home after the change to the 323's it ran 3500 @ 65 mph. No change at all!! I can not figure it out, what are you guys revving at 65, with 323's??
$789.00 and no difference.....Thanks Rick
I`m running about 2800rpm @ 65 with 15" tires on the back and 3.55 gears, manual tranny.
Remember too, you did not change the tranny speedo gear, so more then likely you were driving faster then the 65 indicated on the speedo.(or you use to drive slower then 65 when the speedo said 65) Without changeing the speedo gear it will show the same speed and RPMs you were running at before as the change you made was after the speedo gear, in order for the speedometer to be acurate you need to put the right speedo gear in the tranny too.
Here is a gear ratio calculator, if you messure your tire size you can see what speed you should be running at what rpm. Differential Gear Ratio Calculator
To verrify your gear ratio you could jack up the car and spin the wheels over one revolution and count the # of times the drive shaft turns, it should be 3 and about a quarter turns for every time the tires turn over once.
The gear is on the end of a holder that goes into the tranny at the tail housing. The speedo cable screws into it. Remove the cable, then remove the attaching nut (or bolt) that secures the gear holder to the tranny. The holder unit will pull straight out, though it may be tough to get out, try twisting or turning it to get it to let go. Once out there will be a removable plastic gear on the end, checking with your GM dealer and giving them your tire size and rear axle ratio they should be able to fix you up with a gear that will be closer then you have now. If the dealer can`t help you, check with some tranny shops in the area. You need an RPM gauge to be sure what you`re running at.
The gear is on the end of a holder that goes into the tranny at the tail housing. The speedo cable screws into it. Remove the cable, then remove the attaching nut (or bolt) that secures the gear holder to the tranny. The holder unit will pull straight out, though it may be tough to get out, try twisting or turning it to get it to let go. Once out there will be a removable plastic gear on the end, checking with your GM dealer and giving them your tire size and rear axle ratio they should be able to fix you up with a gear that will be closer then you have now. If the dealer can`t help you, check with some tranny shops in the area. You need an RPM gauge to be sure what you`re running at.
Rukee, thanks. I am going to get my gear ratio changed from the current 3:90 to one to 3:55 or 3:23 to one. Thanks to you, I will get the speedometer gear changed so that my MPH and RPM readings remain accurate. Thanks again for the good advice.
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1966 GTO tri-power 4 speed competition-plus, Tubuler Automotive competition headers. 58 thousand miles. Owned since 1969
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