The Stovebolt Site Forums The Parking Lot Big Bolts speed vs RPM chart

Does anyone have a chart for rpm /speed for a 2 speed rear axle? I was sure I saw one somewhere but I have been through the 48 spec manual and the owners manual and haven't found one. With 8:20 x 20 tires

I just created one when I needed it. I don't fully understand the tire sizing, so I just looked up the revolutions per mile for my tires and went from there. I made one for a NP540GL that had both the low gear ration options, and the higher gear ratio options. I later found out that I had the lower ratios (6.4 and 8.7), and made another chart for an SM420 and the low option (deemed "LO" as opposed to "HO" on the spreadsheet).

If you found out your tire revolutions per mile and had a google account so I could share the google doc with you, then you could punch in your own numbers and play around with it.

SM420 Chart

NP540GO Chart

So I'm not sure what you meant by 8:20 x 20 tires, but I entered in the info for some 8.25-20 tires in the chart and got this:

Spreadsheet

This is assuming 542 revolutions per mile for the tires, at 3500 RPM.

Jim,

If your question has not been answered by the posts above, and you can provide the following information, the RPM/speed calculator at the link below might help you:

You will need:
1. The high-gear ratio of your transmission
2. The ratio of your rear-end (the higher of the two-speed ratios)
3. The diameter (rolling diameter, preferably) of your rear tires

http://www.csgnetwork.com/multirpmcalc.html
(use the "Transmission Final Gear Ratio Manual Entry" near the bottom of the page)

The RPM at any speed will be displayed/calculated.

PVFJR, thanks that makes things much more complicated. Yes they are 8.25/20
I had just done several 2 way runs on a marked 2 mile stretch of road for odometer calculation and got an offset of exactly 1.50 to 1. That makes the indicated 75 mph which I was running on another straight road actually 50 mph, which was the posted speed and I was running with the flow of traffic.
If I take the 67 mph I was indicating at 3500 and divide it by 1.5 I get 44.6, Which is what it felt like with the cars riding on my bumper.

Problem solved (if anyone is interested)
I had hooked up a spare tach for testing as I went out for the mileage test. Just tossed it on the seat. This morning when I started it, It said the idle was high, but it wasn't.
The cylinder selector switch had gotten bumped into the 4 cyl mode.
So the "3500" was probably 2700 or so.will recheck it and am pretty sure it will be much closer to the odometer error.

That makes sense. 46 mph at 3500 rpm would definitely have seemed a little slower than expected!

Also, for the rear end gears I was just assuming you have the same 6.40/8.72:1 ratios as me. Have you confirmed this? And all tests are being done in the high gear?