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Can I figure out my gear ratio from this?
#1
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
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#2
cazsper;283677 Wrote: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
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
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#3
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



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#4
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.

'73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

'73 F code convertible. Bright red. Needs total restore. (IE HOT MESS)

- Jason
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#5
73pony;283682 Wrote: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.

Excellent ideaBravo



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#6
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.

'73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

'73 F code convertible. Bright red. Needs total restore. (IE HOT MESS)

- Jason
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#7
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 can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#8
Don C;283687 Wrote:
73pony;283682 Wrote: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.

Excellent ideaBravo

I have this and it is excellent!

https://itunes.apple.com/au/app/speedome...71911?mt=8

Cheers...

...Mickus
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#9
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).
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#10
3.50:1 is pretty darn close based upon your variable

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"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
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