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Engine RPM vs Road Speed S-IV Automatic

W

W_Pierzga

Platinum Level Sponsor
Guys:

I would be grateful if someone would provide expected engine speed (rpm) vs road speed (mph) for a Series IV Alpine with a BW Automatic transmission?

My 'beam is running close to 4000rpm at 50mph, which seems to be a very high engine speed.

Is there a simple way to determine the rear axel gear ratio - without removing the differential unit from the axel? (doesnt need to be super precise just good enough to determine if it is the standard 3.889:1 or overdrive 4.222:1)

Many thanks to the experts!

Wayne
 
alpine_64

alpine_64

Donation Time
W_Pierzga said:
Guys:

I would be grateful if someone would provide expected engine speed (rpm) vs road speed (mph) for a Series IV Alpine with a BW Automatic transmission?

My 'beam is running close to 4000rpm at 50mph, which seems to be a very high engine speed.

Is there a simple way to determine the rear axel gear ratio - without removing the differential unit from the axel? (doesnt need to be super precise just good enough to determine if it is the standard 3.889:1 or overdrive 4.222:1)

Many thanks to the experts!

Wayne
Click to expand...


The rear axel on the autos should be 3.889 whihc means you get 17.2mph per 1000 in top..

so 50mph should = 2906rpm

is the rev counter accurate?
 
T

Tom H

Platinum Level Sponsor
When was the last time the tach was calibrated? If it has not been touched since it was manufactured it is 99.9% likely to be way off.

It's easy to check the diff ratio. Jack up one rear wheel, put a chalk mark on the drive shaft. Rotate the wheel exactly two full turns and watch the drive shaft. The drive shaft should rotate "approximately" 4 times. But it will actually rotate either slightly more or slightly less than 4 times, depending on if it is a 3.89 or 4.21. If it's less than 4 times (about 1/8 turn less) , you have a 3.89, If it rotates about 1/8 turn more then it's a 4.21.

Tom
 
W

W_Pierzga

Platinum Level Sponsor
Next steps to resolution

Guys:

Thanks to all for your advice/insights.

The transmission seems to be shifting fine: I can feel it shift from 1st-to-2nd and from 2nd-to-3rd.

I dont think the tach is an issue as it is quite obvious that the engine is running very fast at 50mph.

The speedo has been serviced and is working fine.

I will pursue the suggested method for determining the rear-axel gear ratio.

Best regards,

Wayne
 
P

Paul A

Alpine Registry Curator
Platinum Level Sponsor
Hi all:

I'd be very surprised if you had a 4.21 rear end ratio, but with Sunbeams anything is possible.

With my GPS I can see my actual vs. speedometer speed. The speedo is about 10 mph off at 50 mph, showing 60 mph. The tach also reads high, showing about 3500 rpm at 50 mph, which, for Sunbeams tachs, isn't that bad.

I drove my '65 automatic to Rapid City last year for the SUNI gathering. It handled interstate speeds very well and cruised nicely at 70 mph. While I prefer the lower rpm's of my '67 with the 5 speed, the 600 miles on the interstate did not harm the engine any and it continues to run very well.
 
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