speedometer gear help

[dodgeit25]

so i am installing an 8 3/4 rear in my 1940 business coupe and using 3.55 gears my question is wear or how can i get gearing to make tge speedo read correctly with the 3.55 gears

[Sniper]

You need to know the tire height as well.  Parts manual will have the options available, my manuals don't go back that far

[martybose]

Almost any old-fashioned speedometer shop can get you a small gearbox that you screw into the transmission and then screw the speedometer cable into.  They have a large number of gear ratios available to correct the speedo readings.

[Loren]

The procedure I would use works like this:

You can use an old GPS unit ( if it is programed to display speed ) on a 6 volt positive ground car if you make a polarity reversing adaptor, so that you can get accurate speed readings. Now you can drive the car with a speedometer that works. Make all the tire changes you are going to make so that you can make permanent changes to the speedo.

If you are using a 1940 transmission you can remove the speedo drive gear and count the teeth. The original rear axle had possible gear ratios of 3.54 and if memory serves me that used a 17 tooth speedo drive gear. MoPar made the gears so you didn't have to change the mating gear to change the ratio. When you look at the chart it is easy ( and frustrating ) to tell they weren't too concerned with accuracy.

Next I would get the speedometer serviced. There's sometimes a number on the back which tells the technician what rate the speedo turns for calibration. Most of the time they already know what that is.

The key to getting it right is the odometer reading. You can check that with the mile markers on certain highways.

I just went through this procedure and my speedo was reading 5 miles per hour slow. After servicing it was much closer but seldom dead on to the GPS reading. SO the mechanical speedometers are just not that accurate. Best not to expect too much.  Perhaps that's why the police use an "enforcement tolerance" of 4 to 5 mph. I know of cars that brand new from the factory had a speedo that read 55 when you were going 57 mph.

Then:

With a freshly serviced speedo, an odometer error amount, a GPS speed reading of error, you can decide if you want to get a ratio adaptor box.

I made some big changes to a truck I had and found that while the speed reading was very sloppy, the odometer reading was close so I left it alone.

Hopefully that's what you will be able to do.

Your speedometer repair shop can supply the ratio adaptor box if needed and help you work out the gears. International Scouts had only one speedo drive gear and depending on the axle ratio the truck had, they either had a ratio adaptor box or not right from the factory.

[DonaldSmith]

Son, don't do the things I have done. 

When I mounted my gear ratio box at my transmission, I had to modify the floor access panel to fit over the protruding unit. 

Duh!  I could have put the ratio box anywhere else, in line with two speedo cables.