For a '59 MG Magnette. am in negotiations for purchasing a 3.909 ratio, 11 driving 43, pinion and gear set for installation in the differential "pumpkin" assembly, in order to achieve an improvement from the current 4.55 ratio. I should achieve an improvement in road speeds by a factor of about 7 / 6 for the same engine RPMs. Only problem is that the speedometer will continue to reflect the lower speeds of the engine. i.e. - when I see 60 on the speedometer and the engine is sounding and turning like the old 60, the car will actually be going 70. Perceptions, comments, opinions, most welcome & thanks ?.
Differential Ratio Upgrade
Mar 09, 2005
4 Replies
You have two options. Fit a different speedo drive gear if the correct one is available or have the speedo recalibrated by a specialist.
-- Dave Baker - Puma Race Engines
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A 3.909 is a big gear for a Magnette if you are still using a standard engine and 15" wheels.
Have you considered a 4.3 (very common Morris Oxfords, Austin A60) or a
4.1 ( harder to find)Cheers Ian F
If you have, or can borrow, a GPS handheld capable of giving MPH or something you can convert the numbers from, I think you will discover that the speedo is actually reading less than you are really doing, even with the axle swap. Even so, the difference will become obvious, either way, allowing you to supervise your opinion on the speedo.
GPS gives a minimum speed, as you travel more than a few dozen yards at a constant speed without going up or down hill or round sharp bends, the GPS eases into an accurate statement of true speed. Speedos usually overstate things and aren't at all accurate.
I use a DeLorme Blue Logger with it's Blue Tooth connection to my Palm Tungsten T2 for display, etc.
With the differential ratio change from 4.55 to 3.909, any given speedo read>Apparently >
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