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I have fitted the rear axle in my 25 roadster with a 3.25 : 1 gearset and would like to know the relative roadspeed for a given set of engine revs.

Allan from down under.

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If you have rear wheels of 30" OD, then they turn 5280/2.5/PI = 672 turns/mile

At 60mph (1 mile per minute) they would turn 672 turns/minute. A 3.25 axle means that the engine in high gear will turn at 2194rpm. So 1000 rpm is 27.5mph.

Low ratio is 2.75:1, so 1000rpm in low is 10mph

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Allan,

I'm sure some guys can answer your question based on experience. However, in order to answer mathematically we need to know the outside diameter of your rear tires. What size are you running?

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Both a 21-4.50" balloon and a 30-3.50" tire is supposed to have 30" outer diameter, though it'll vary some. Not much percentage wise, 31" differs about 3% from 30"

I think it's easy to remember the different rear axle ratio alternatives like this:

With the std rear axle ratio 11/40 or 3.64:1 you get about 40 rpm per mph in high. Thus the engine revs about 1400 rpms at 35 mph. With optional gears like 13/39 or 3:1 you'll get 33 rpm/mph and only about 1150 rpms at 35 mph. The 10/40 or 4:1 gives 44 rpm/mph and 1540 rpm at 35 mph while the 12/39 or 3.25:1 gives 36 rpm per mph or 1250 rpm at 35 mph.

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Hi, if it takes 2.5 turns to make the axle rotate exactly 1.0 rotation, what ring and pinion do I have and what ratio do I have? Thanks, Mike

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Mike - sounds odd, need more input.

Is it 2.5 turns of the engine that causes the whole rear axle = both rear wheels to move one turn, or is it just one wheel that turns?

Do you have a truck?

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It was 2.5 turns of the u-joint to make 1 full revolution of the axle I was watching. I think the other side was turning as well. It is a 12 rivet rearend that was rebuilt and it is not in a car at the moment.

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Still odd, there haven't been any 2.5:1 ratio gears available, as far as I know. One possibility would be that the other side axle didn't turn as easy as the side you were watching - it's the average of both axles turns that should be one while counting the turns at the U-joint. If it actually was std gears in that 12 rivet axle, then the other side axle would have had to turn just 3/8 turn while the watched side turned one full revolution. So sometimes a helper is needed when checking ratios.

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Thanks Chris and Roger. That's what I needed. Your conversions are most helpful. I am getting used to recalculating miles/hour from my speedo into Km/hour for speed limit purposes. Good job I have a calculator app. on my phone.

Allan from down under.

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Mike,

It sounds like your rear end was jacked up when you did this test? If so, I think there was some differential work at play that influenced your observations. One wheel turned a little, then the other, etc. Do it with both wheels on the ground and rolling the same.

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Mike, you might try putting one tire on the ground then counting the wheel off ground then dividing the number of turns by 2.