Just a note to Bill Young about his work with the electronic tachometer.

Bill, your article in the recent Healey Marque Magazine about repairing and calibrating the BJ8 tachometer was enjoyable and well done. I have often wondered about the accuracy of my tach and speedo. I am very interested in keeping my rpms as low as possible while driving on the 4 lane, limited access highways where the speed limit is at times 70 mph. I use the "Super Slab" highway more often than not while running to Watkins Glen many times a year. While doing this I have checked my speedo with an onboard GPS and also with my cell phone. The speedo seems to be correct. Of particular note is that my instruments indicate that at 60 mph my rpms are 2700 or there abouts. I have 165/80 tires w/ 3.54:1 rear gears and O.D. in my BJ7.
I noticed in your "Calculated Engine RPM" chart that your indicated rpms was 2905 at 60 mph for a Healey with 3.54:1 rear gears. You show 3204 for a Healey with 3.90 gears. Then I will assume that my O.D. is providing the other 200 rpm reduction for my car to achieve 2700 rpm at 60 mph. Your chart indicates that there is only a 300 rpm reduction between 3.90 gears and 3.54 gears.
I have to admit that I was disappointed that each feature (the lower gears & the O.D.) only dropped the rpm by this stated amount. Irregardless thank you for an excellent article. Dave.

Dave.. Thank you for your review and comments on my latest article in the Healey Marque magazine relating engine RPM’s to road speed. I though this would be a simple task. Mathematical calculations seemed easy because gear ratios are mechanical. I was quite confident in the actual engine RPM’s having recently rebuilt and beach tested the tachometer.The variable was the tire circumference and rolling distance per revolution. Published data did not prove to be accurate compared to an actual rolling test measurement of 10 revolutions. Road tests were more difficult because it was not easy to read speed and RPM’s accurately due to the road conditions. Who knows what the tires were doing? That is why I qualified the results as +/_ 5%. Perhaps some of the other members of the Forum have similar examples to share with us?

Bill Young, BJ8, BN2, AN5
Columnist
Technical Service Bulletin
Healey Marque Magazine
sjahc.com

Thank you again Bill for your very comprehensive work. Dave.