The center section on my 40 rear has the numbers 340 or 840 stamped on the the pad below the torque tube, does this tell what ratio the ring and pinion is? If so, what would it be? Thanks.

First number should the number of teeth on the pinion the second 2 the number of teeth on the ring gear. but 3 40 and 8 40 don't come out to any ratio I've heard of. 3.54, 3.78, 4.11, 4.55 are known ratios 40 / 8 = 5.00 https://www.jalopyjournal.com/forum/threads/banjo-rear-i-d-info-needed.962677/

The banjo in my single seater was stamped twice so the numbers were really hard to read look at it again it should be as stated in the prior posts.

or it could be 349 i.e. 34/9 i.e. 3.78:1...if someone stamped the ratio back to front! If it is in the car you can remove the spark plugs , put the car in third gear / or direct drive if you have a different trans, roll the car forward one revolution of the rear wheels and observe the number of turns of the crankshaft. You will need a helper or two!

Yes , but you cannot see the driveshaft in a torque tube if it is installed in the car....BTW you need to turn both axles one revolution . One turn of one axle will halve the ratio .

I had a similar problem on a 1937. Could not read numbers clearly. Pulled the oil fill plug out, marked a ring gear tooth with a red marker, turned wheels and counted 40 teeth. The only ratio that has 40 tooth ring is a 4.44:1

Raise one wheel only and turn it TWO full turns and count the driveshaft revolutions. Yeah, ya need a helper to do the turning or watching. ONE full turn is for posi rear ends, no spider gears. Open needs two turns. Dave