Speeds and gears are not matching-Not Happy

[Kmcallahan]

Here is the delima!
Traveling @75 mph @ 2500rpm. According to the Tremec gear calculator I should be @~92mph. I have supposedly the 3:55 IRS center section, TKX600 with .68 5th gear. Tires are 315/30/18 with a height of 25.44” according to the calculator.
Could the margin of error be ~17mph, I don’t think so!

Could I have a different gear ratio in center?
Could I have a different trans than what the SN/PN and manufacturing date states?

What can I do to trouble shoot?

[Gordon Levy]

If you have a FFR electronic speedo your calibration is off and needs recalibration, if it's GPS it may be faulty.

[NAZ]

Here is the delima!
Traveling @75 mph @ 2500rpm. According to the Tremec gear calculator I should be @~92mph. I have supposedly the 3:55 IRS center section, TKX600 with .68 5th gear. Tires are 315/30/18 with a height of 25.44” according to the calculator.
Could the margin of error be ~17mph, I don’t think so!

Could I have a different gear ratio in center?
Could I have a different trans than what the SN/PN and manufacturing date states?

What can I do to trouble shoot?

First, if you're not sure of your rear end ratio you can easily check it by lifting the rear tires off the ground and counting the number pinion shaft turns for one complete revolution of the rear tires. If you have 3.55:1 gears the pinion will turn ~3.6 turns for each revolution of the tires. Check it and you won't have to guess.

Not likely that you have a different OD ratio than what the trans tag info suggests but that can be verified too by counting the number of turns of the driveshaft per one full revolution of the engine when the trans is in 5th gear.

I suspect your actual vs calculated speed expectation varies significantly due to accumulated error. There may be some error in the tach and some in the speedometer and there is certainly error in the online calculation -- all combined will affect the actual measured results.

When I run the calculation by hand with your data I get a theoretical 2392 RPM @ 75MPH. But all these calculations and calculators are relying on inaccurate tire diameter info to begin with so your actual RPM at any given speed will vary from the calculated values. This is because the tire diameter info given from the manufacturer or calculated by the published tire size are diameters of a tire NOT mounted on a car. When a tire & wheel are mounted on a car, the tire inflation and weight it carries will cause the tire sidewall to give and the actual rolling radius will be smaller than that of a tire that is not supporting the weight of the car. What that means is the tire will turn more turns per mile than the calculated number which will give you a higher RPM at 75 MPH than the online calculator gives you.

[Kmcallahan]

If you have a FFR electronic speedo your calibration is off and needs recalibration, if it's GPS it may be faulty.

To check my speed I also used the wake navigation app and it displays the travel speed and it was acute with my GPS speedo.

I will check the tire revolutions as Naz stated.

Then check calibration of tac.

[David Williamson]

I have similar drive train except TKO600 and I have Michelin 315/30/18 tires the same size as you, I see 130 km/hr or about 80 mph at 2500 rpm. I have a GPS speedo and this is about the expected speed at RPM.
If you have not done so go to a web site like Tire Rack and get the exact diameter for the brand tires you have.
David W