Can I figure out my gear ratio from this?

7173Mustangs.com

Help Support 7173Mustangs.com:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
C

cazsper

Well-known member
Joined
May 30, 2012
Messages
1,066
Reaction score
2
Location
Sunnyvale, CA
My Car
1968 Coupe: 393w, TKO-600, Maier Racing springs, Global West suspension, Currie 9in with forged axles, 3.50 gears, Trutrac, Baer brakes front/rear
1973 Mach-1: 351c 4V, C-6, 3.73:1 gears and a long "To Do" list..
I know this isn't much to go on but I w wondering if I can find my gear ratio (and if my speedo is off) from the following info:

1973 Mach-1 with c-6 trans and 275/40-17 rear wheels. On the freeway driving to school today, I noticed my speedo qhowed the following speeds for the provided RPM's. I had the speedo cable replaced so I don't know if the gear is the right one or not. But I also changed the rear wheels so I am assuming it's off regardless. Thanks..

2000rpm-40mph

3500rpm-75mph

4000rpm-85mph

4500rpm- 100mph

 
I know this isn't much to go on but I w wondering if I can find my gear ratio (and if my speedo is off) from the following info:

1973 Mach-1 with c-6 trans and 275/40-17 rear wheels. On the freeway driving to school today, I noticed my speedo qhowed the following speeds for the provided RPM's. I had the speedo cable replaced so I don't know if the gear is the right one or not. But I also changed the rear wheels so I am assuming it's off regardless. Thanks..

2000rpm-40mph

3500rpm-75mph

4000rpm-85mph

4500rpm- 100mph
Click to expand...
Not really but, if you assume zero slip in the convertor and a tire height of 25.7 inches (275x40x17) with a 3.00 gear the speedometer should read 51 mph at 2000 RPM, or with a 3.25 gear 47 mph at 2000 RPM, or with a 3.50 gear 44 mph at 2000 RPM. You really need to confirm the gear ratio to determine how much the speedometer is off. The factory tolerance, at 60 mph actual was 58-65 indicated on the "speed estimator" Chuck

 
If you knew the speedometer was correct you could estimate the rear end ratio, or if you knew the number of teeth in the speedometer gear you might be able to calculate it, but not knowing any of the 3 variables makes it nearly impossible. The best way is to jack up the back of the car and turn the rear wheels one turn and count the number of revolutions of the driveshaft.

These spreadsheets will give you something to play with to see how they interact:

http://www.7173mustangs.com/thread-tire-sizes--11566

http://www.7173mustangs.com/thread-speedometer-gears

 
Do you have a smart phone? There are several GPS Speedometer apps avail to download for free. This will help determine your actual speed at what RPM. With that I could get you close.

 
If you go this route....

Easiest to calc gear ratio if you give me RPM at 50, 60, & 70 mph. Gives me three points to calc and ensures you are in 3rd gear with the trans.

 
When you get your RPMs drive on a level surface or slightly downhill so the torque converter isn't slipping and use only enough accelerator to maintain speed.

 
If you're planning to use vehicle speed and RPM to determine gear ratio, you need to use loaded rear tire radius or rolling circumference for the calculation. Rolling circumference is the more accurate of the two. Just put a chalk mark on the sidewall of tire and ground then roll forward 1 tire revolution & measure the distance. As stated, it's easier to just jack up the rear end and count driveshaft revs for 1 or 2 tire revs (depending on if it's open or limited slip diff).

 

Similar threads

HeavyCoupe
Hello from Alabama.
2
Replies
28
Views
2K
HeavyCoupe
HeavyCoupe
B
Diff Ratio Question and LSD Ford 9inch
Replies
19
Views
3K
Ballaratcastle
B
B
'73 Mach 1 project is out of time and I have to let it go...
Replies
1
Views
2K
Steve McMahan
Steve McMahan
Austin Vert
Austin Vert Chooses a New Road for Tires.
Replies
6
Views
2K
DaveMT
DaveMT
7
  • Article
How to Rebuild a Ford 9 inch.
Replies
0
Views
2K
72HCODE
7

Latest posts

Back
Top