Beck TD, Part 4: Fun With RPM Numbers

Cor Engelen and I have been having a conversation off-list about rear gear ratios. I know the stock TD ratio of 5.125 to 1 is too low (numerically high) for our powerful Volvo engines. Charlie Baldwin and Joe Lazenby solved the problem with overdrive transmissions, but Cor went with a 3.9 to 1 ratio out of an MGA. He reported that it works well.

I found a nicely priced used rear end on eBay today, and after showing the auction to Cor and conferring with the seller that the rear was complete, I bought it. Just one more project in a mountain of projects, but that's why I bought this car, after all.

Before buying, I made sure that my current ratio was the stock one. For a non-positraction rear like the TD, you block one wheel and turn the other wheel TWO complete revolutions, counting the number of turns at the driveshaft. I aligned it so that the grease nipple on the rear universal joint was easy to see, and then marked the wheel hub and turned two turns. The driveshaft turned about five and an eighth turns, which matches exactly with the stock 5.125 to 1 ratio. That's what I've got.

There are a bunch of online calculators to help you figure out RPM for a given gear ratio and tire size, but a lot of them make crazy assumptions about your transmission. I like this one, which lets you exactly specify all the numbers:

http://www.crawlpedia.com/rpm_gear_calculator.htm

I plan to run Vredestein classic tires in 165H15, and the Coker website tells me those tires are 25.4 inches tall. Using that and a 1.00 to 1 ratio for top gear, I came up with these numbers for RPM at 65 miles per hour:

5.125 to 1 rear: 4407 RPM
4.300 to 1 rear: 3697 RPM
3.900 to 1 rear: 3353 RPM

So this will drop my top gear engine speed over 1000 RPM at 65 MPH! That's good! The reason I also checked the 4.3 rear is because that same seller has one of those for sale also. That auction has now expired (someone bought the 4.3), but the seller has lots of other LBC (Little British Car) stuff if you're interested: http://stores.ebay.com/abingdonboy/

Continue on to Part 5...