So....while doing the math in regards to my Rpms and gear ratio, it didn't quite add up. The car has been 350 swapped with a TH350 transmission. It also has a Posi rear with 3.23 gears. At 60 MPH, my tach says it's turning ~3000 rpms. The thing is wailing so I'm making the assumption it's not lying. Shouldn't the RPM's be lower with that gear ratio? Even while running my stock 215/65/15 tires, it shouldn't be that high.

Any insight would be greatly appreciated. Thanks for help with my first post on here!

 

IIRC the TH350 3rd gear is a 1:1 so with stock tires and a 3.23 then 60mph would be 2500 rpm and 72mph would be 3K. If your tach and speedo are accurate you would have 3.73 which was an option in 84

 

Doesnt seem right.

That would be 26" tire, so engine rpm normally should be 2510 rpm. 3000 would be over 19% converter slip which seems way high for cruising

 

Speedo gears only affect MPH reading. The best way to calc is to find out what gears you have and what percentage you are off from true roadspeed. Then change the speedo drive/driven ratio by the same percentage. In my case, I checked error at 40 MPH and 70MPH. Should be off by the same percentage.

I found all the calc thread pages to not work for me on a simple 2.73 to 3.42 rear ratio conversion.

 

You might want to download a speedometer app for your phone and see how accurate your speedo is.

 

Zero converter slip gives 72 mph.
Unless you have an extremely loose converter generating over 15% slip, your tach is probably reading high. The speedo shouldn't be off by much.

 

I have used a speedometer app and the speedo is true. I thought it may be off as well so I ended up checking

 

I've read that the 3rd gen tach has a tendency to become inaccurate over time. I wasn't sure if replacing it was the best fix

 

At 3500 RPM's I do turn just about 72 MPH. Could it be that the tach is consistently off by 500 or so rpm's? If so, how do I rectify that?

 

Did you happen to verify the math with the old OD trans? You are certain that you have a 3.23 rear ratio? I used one of the on-line ratio programs and figured out my ratio on my '85 Trans Am and the numbers came out almost perfect. At 70mph, the tach is showing slightly under 2100 rpm. With stock tires (26"), this showed I had a 3.23:1 ratio. The program gave two rpm ranges; non overdrive and with overdrive.

When I was much younger, I owned a 1970 340 'Cuda with a manual trans. That engine was screaming when I was cruising at 65-70mph. I'm pretty sure it had a 3.73 rear in it.

 

TH350 isn't an OD trans.

 

Sorry, my mistake. I thought you had replaced the trans with the TH350. I reread the original post and I assume you bought the car with the swap already done? If so, I understand your frustration.