|
Sponsored Links (Register now to hide all advertisements) |
|
|
Thread Tools | Display Modes |
11-10-2020, 09:23 AM | #1 |
Senior Member
Join Date: Jul 2012
Location: Central Maine
Posts: 644
|
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
|
11-10-2020, 09:42 AM | #2 |
Senior Member
Join Date: May 2010
Location: Largo Florida
Posts: 7,225
|
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.
|
Sponsored Links (Register now to hide all advertisements) |
|
11-10-2020, 10:09 AM | #3 |
Senior Member
Join Date: Jun 2010
Location: San Antonio, Texas
Posts: 16,436
|
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. Last edited by rotorwrench; 11-10-2020 at 10:21 AM. |
11-10-2020, 11:28 AM | #4 |
Senior Member
Join Date: May 2010
Location: Long Island, NY
Posts: 3,470
|
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!
__________________
Bob Bidonde |
11-10-2020, 01:16 PM | #5 |
Member
Join Date: Oct 2020
Location: East Sandwich, MA
Posts: 62
|
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 !
Sent from my iPhone using Tapatalk |
11-10-2020, 01:53 PM | #6 |
Senior Member
Join Date: May 2010
Location: Between Seattle & Tacoma
Posts: 2,354
|
Re: Rear axle ratio and car speed
Sponsored Links (Register now to hide all advertisements)
|
11-10-2020, 03:18 PM | #7 |
Senior Member
Join Date: Jul 2012
Location: Central Maine
Posts: 644
|
Re: Rear axle ratio and car speed
OK, thanks for your thoughts. Bill
|
11-10-2020, 11:18 PM | #8 |
Senior Member
Join Date: Sep 2017
Location: Mint Hill, North Carolina
Posts: 386
|
Re: Rear axle ratio and car speed
jbwthree has it right, except his post should say 3.78 / 3.54 = 1.07
|
11-11-2020, 07:07 AM | #9 |
Member
Join Date: Oct 2020
Location: East Sandwich, MA
Posts: 62
|
Re: Rear axle ratio and car speed
|
11-11-2020, 01:22 PM | #10 |
Senior Member
Join Date: May 2010
Location: Southern California
Posts: 7,032
|
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 |
11-11-2020, 09:47 PM | #11 |
Senior Member
Join Date: May 2010
Location: Englewood, Colorado
Posts: 1,372
|
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.
|
11-11-2020, 10:00 PM | #12 |
Senior Member
Join Date: Aug 2019
Location: Hagerstown MD
Posts: 224
|
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...
__________________
Building a '29 Speedster, the hard way... |
|
|
Sponsored Links (Register now to hide all advertisements) |
|