RPM dash meter is incorrect. Please check my calculation.

[Hans]

Hi,
With brand New 225/60/15 tires, C4 and the 2.47 rear axle I should be at 2000 RPM at 60 miles/ hour.
Am I right?

I read 2400 RPM on my dash at 60. Verified speed with GPS.

Thanks.


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[darkd0r]

Not an expert here. By my figures, your calculations about expected rpm are pretty close, using a generic tire size which doesn't take into effect the actual tire radius when loaded.

There will be some slippage in the torque converter. TCI has a calculator at:

https://www.tciauto.com/racing-calculators

I don't know how accurate that calculator is for a C4 and your specific torque converter.

[gr79]

Am using a calculator which is preety close, using my car and truck specs.
What is the mph at 2000? About 50? 3.08

25.6 tire dia, 3.08 gear, 1:1 trans at 60 mph is about 2425.
25.6 tire dia, 2.47 gear, 1:1 trans at 60 mph is about 1950.

25.6 tire dia, 3.08 gear, 1:1 trans at 2000 is about 49-50 mph
25.6 tire dia, 2.47 gear, 1:1 trans at 2000 is about 61-62 mph.

25.6 tire dia, 3.08 gear, 1:1 trans at 2400 is 59.36 mph.
25.6 tire dia, 2.47 gear, 1:1 trans at 2400 is 74 mph.

https://www.4lo.com/4LoCalc.htm

[Hans]

The axle tag says it is a 2.47.
But sure looks more like a 3.08 from the numbers.
Before taking off the cover of the axle and immobilizing my car, I'll try to read my timing light tool from behind the steering wheel. It shows RPM too.
Ducktape it to the hood?

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[wraithracing]

The axle tag says it is a 2.47.
But sure looks more like a 3.08 from the numbers.
Before taking off the cover of the axle and immobilizing my car, I'll try to read my timing light tool from behind the steering wheel. It shows RPM too.
Ducktape it to the hood?

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Before immobilizing your car, jack up the rear of the vehicle and place on jack stands. Use a grease pencils or something that will mark the tire and driveshaft. Place a mark on the tire and driveshaft. Place the vehicle in neutral, rotate the tire two full revolutions if the rear axle is a non traction loc. If traction loc spin each tire 1 time. Count the number of revolutions the driveshaft makes and that should give you a good idea of gear ratio. @ 2.5 driveshaft rotations is a 2.47, @ 3 rotations equals the 3.08 ratio. Hope that helps.

OH! It helps if you have an extra set of eyes while doing this. Also sometimes on a one legger rear axle (non traction loc) you may need to hold the opposite tire or prevent it from rotating to get a more accurate number.

[Hans]

That is doable, thanks Trey.
Since I'm always roasting the right tire only when the street is wet, it must be a non-lock diff.
So 2 wheel turns with the opposite wheel locked it will be


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[mcb82gt]

Why 2 turns? I thought it was one full revoluton of the tire and count driveshaft turns?

3.08 :1

[wraithracing]

The two turns can be necessary due to the way a non traction loc works (spider gears and all that ). Depending on condition and which actual wheel you are turning it can vary. If you block or hold the opposite wheel it will usually only take one revolution. If you don't then often there's enough slack in the differential that you need two full revolutions for an accurate estimate of the differential gearing. It's been awhile since I have used this method on a non traction loc/limited slip setup so your experience may vary.