RPM's and Gear Ratio not adding up?
#1
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Car: 1984 T-Top Z28
Engine: 350 Crate
Transmission: TH350
Axle/Gears: Posi rear with 3.23's
RPM's and Gear Ratio not adding up?
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!
Any insight would be greatly appreciated. Thanks for help with my first post on here!
#2
Supreme Member
Re: RPM's and Gear Ratio not adding up?
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
#4
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Re: RPM's and Gear Ratio not adding up?
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.
I found all the calc thread pages to not work for me on a simple 2.73 to 3.42 rear ratio conversion.
#5
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Re: RPM's and Gear Ratio not adding up?
You might want to download a speedometer app for your phone and see how accurate your speedo is.
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Car: '86 Bird, 96 ImpalaSS, 98 C1500XCab
Engine: LG4, LT1, L31
Transmission: 700R4, 4L60E, 4L60E
Axle/Gears: 3.73 Tors, 4.88 spool, 3.73 Eaton
Re: RPM's and Gear Ratio not adding up?
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.
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.
#7
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Car: 1984 T-Top Z28
Engine: 350 Crate
Transmission: TH350
Axle/Gears: Posi rear with 3.23's
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#8
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Car: 1984 T-Top Z28
Engine: 350 Crate
Transmission: TH350
Axle/Gears: Posi rear with 3.23's
Re: RPM's and Gear Ratio not adding up?
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
#9
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Car: 1984 T-Top Z28
Engine: 350 Crate
Transmission: TH350
Axle/Gears: Posi rear with 3.23's
Re: RPM's and Gear Ratio not adding up?
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?
#10
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Car: 1985 Trans Am
Engine: LG4
Transmission: 700R4
Axle/Gears: 3.08
Re: RPM's and Gear Ratio not adding up?
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.
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.
#12
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Car: 1985 Trans Am
Engine: LG4
Transmission: 700R4
Axle/Gears: 3.08
Re: RPM's and Gear Ratio not adding up?
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.