Autometer speedo calibration to car

[Ant]

I have a set of Autometer gauges, the reverse wind 180mph speedometer and the right angled gearbox, I looked on the AM website and they had this information on the speedo:

The speedometer is calibrated for a 1:1 drive ratio (1,000 RPM at 60 MPH input at the speedometer head.) "this was not specific to the cobra gauges!

My question is I know my final drive ratio option , and have Transmission charts indicating what small nylon transmission gear to use with what tire size and final drive ratio.

How do I work out what parts or information I require? - not a biggy as I haven't ordered a trans yet still researching!

[Chaplin]

Ant-
look on ERA's website, and specifically in the manual that is online. There is a chart which will tell you what speedo gear you need depending on what trans and rear you are using.

[Ant]

Chaplain,

Thanks I have that chart, I guess with the speedo being 1.1 ratio 1000rpm at 60mph that is common with most speedos, maybe I am thinking there are different speedo ratios just to complicate things, and the chart has all the information required!

[Chaplin]

Ant-
I never really considered that speedos would have different ratios. However, I have the same speedo and used ERA's chart and I can tell you that it works perfectly! (Well maybe a couple miles an hour off, but close enough. )

[strictlypersonl]

Almost all aftermarket and OEM speedometers are calibrated for 1000 revs/mile (which coincidentally is 1000 revs/minute at 60 mph ) The only oddball I've seen is a small Smiths that we use in our GT, at 700 revs/mile, and a VDO electronic unit.

[machineman]

ANT;HERES WHAT YOU NEED TO KNOW;
TIRE REVS PER MILE [TIMES] REAREND RATIO [TIMES] # OF TEETH ON DRIVE GEAR [DIVIDED] BY # OF TEETH ON DRIVEN GEAR
= CABLE REVS PER MILE
If you are going to use a toploader just call David Kee transmissions they can set you up with the parts you need.


Bobby

[Ant]

Thanks guys, I was complicating the issue, as in the past I got an old Smiths speedo from the wreckers, these varied in a lot of old cars, all different turns per mile etc, imported a 180mph gauge face from the UK and then after calculating the transmission/tires and rear end, if it wasn't right I took it in to a speedo repairer and they altered the magnetism of the instrument, but with these you are unable to take the gauge apart unless you reweld the plastic case.