Rear axle ratio and car speed
 

Due to damage I had to replace the ring and pinion in my '29. Went from stock 3.78:1 to a 3.54:1. Did not change the speedometer gear. If my speedometer reads 40 mph what might be my actual speed ? Thanks, Bill

Re: Rear axle ratio and car speed
 

Our speedo's are notorious for being 'way out of whack'. So you'll never know. 3:54 should give about 4mph more than the 3:78 at the same engine speed. The way I check my actual speed is to compare the speedo with a GPS. When mine [ 3:78] shows 40mph the GPS says 44. At 50mph the GPS says 57.

Re: Rear axle ratio and car speed
 

The difference can be calculated but you also need to know the circumference of the rear tire. If you know someone with a hand held GPS unit, it should give you a ground speed that is relatively accurate. You can also use a timer and hold a specific speed between mile markers. The amount of time it takes to go one mile can be used to calculate the miles per hour.

I'm not sure what speedo gear housings are available for the Model A vehicles. The 1928 to 48 chassis parts catalog has a chart to calculate what gear housing assembly would be needed with the tire sizes & rear gear ratios. A person may also be able to use a ratio adapter gearbox installed in the speedo cable drive line to get a more accurate speedometer reading.

Re: Rear axle ratio and car speed
 

Put your concern to rest with this easy test. Drive by a radar speed indicator sign like those by school zones. Do it at different speeds when the school is closed on a weekend. Note the differences. Done!
https://encrypted-tbn0.gstatic.com/s...MlRLI&usqp=CAc

Re: Rear axle ratio and car speed
 

Hello. Divide the 2 ratios: 1.78/1.54 = 1.07. That number can be used to scale the speedometer reading, so 40 mph reading on the speedometer is 1.07*40 = 42.8 MPH. Your error will increase as the speed increases and vice versa. This of course assumes that the speedometer was right in the first place and some of the prior responses will help you verify that. Hope that helps !


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Re: Rear axle ratio and car speed
 

Just use a GPS app on your phone

Re: Rear axle ratio and car speed
 

OK, thanks for your thoughts. Bill

Re: Rear axle ratio and car speed
 

jbwthree has it right, except his post should say 3.78 / 3.54 = 1.07

Re: Rear axle ratio and car speed
 

GRutter - my mistake, you are correct ! Typo, thanks for picking up on that.


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Re: Rear axle ratio and car speed
 

Speedometer gears for that rear end ratio used to be difficult to find but I have noticed that Bratton's has them.

Charlie Stephens

Re: Rear axle ratio and car speed
 

Free apps are available for your cell phones that give actual speeds. Very handy. Installed on your phone in 30 seconds.

Re: Rear axle ratio and car speed
 

Bratton's has been out of the 17 tooth gears for a while. Manufacturer wanted them to do a run of several thousand before they would make the molds...