Stumped on gearing

Road_Trip

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So I'm a little stumped here and looking for ideas/suggestions on this long post. I bought the car about 6 weeks ago or so and found the window sticker for it, showing 3.73 gears as an added option. On the 700 mile journey home, I felt the RPM were a little low for the shorter gears (2100-2200), but just chalked it up to possibly a taller 5th gear in the transmission. I got a tune from Lito to fix the bogging/dead spot in the 2100-2500 RPM range and told him I had 3.73's in it, assuming that was the case.

Fast forward to this past weekend on another 600 mile drive with the car and I feel like I'm just ripping past cars on I-70 despite cruise being set at 80 in a 75. After about 100 miles or so of this, I download a GPS speedometer app and it shows I'm doing 87-88mph. Thinking the app was crap, I downloaded a second to find the same thing. Ooops. Good thing there weren't any cops hiding in the dark. Once I get to my destination, I reach out and let Lito know what's going on. He sends a new tune out, still with the 3.73's just to see if there's any difference. Take it for a drive, same thing. So we send a few emails back and forth and I find a calculator showing the RPM I'm roughly spinning at 80mph (based off the original drive home when I purchased it before the tune) and it shows I have 3.31 gears. I shoot that back to Lito and tell him I'll verify it when I get home from my trip and within minutes (literally like 15), I have a new tune with 3.31's on it. I get that loaded and head home the next day.

On the way home Wednesday, I use the GPS speedometers again and I'm now off by a couple of MPH in the opposite direction, showing faster on the speedo than what I'm actually doing. We get the car jacked up today and mark the tire and differential, manually checking it. Looks pretty close to 3.31 vs 3.55 and definitely not 3.73.

Stock tires were 245/45/19 and I swapped the rears for 275/40/19 just so I didn't have to worry about the speedometer correction. My question now is, with both sets of tires being 27.7" diameter and 728 revolutions per mile, if I truly have 3.31's in there despite the window sticker, why would the speedometer still be off by a few mph? I don't know if a previous owner blew up the rear and had a new one installed, if they didn't like the 3.73's and wasted the money to go down to the 3.31's or is there something between 3.31 and 3.55 that I might have? Could they be the rare 3.27's and causing the higher speed reading with it tuned for 3.31's? Just kinda stumped and I don't want to keep bugging Lito for another tune until I can figure this out and get those few MPH accounted for.

Thanks!
 

Candy10

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I want to say the ratio is stamped on the diff somewhere, but not 100% certain where. What app did you try, can you be sure thats accurate?
 

Road_Trip

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I looked for the tag today while we were under it but didn't see one, leading me to wonder about the previous owners blowing up the rear, and the different gearing. I tried the DigiHUD and Speedometer GPS HUD apps. I'm assuming they're correct, running off GPS and being nearly identical in the speed, but your guess is as good as mine on how accurate they really are.
 

JC SSP

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There is a way of doing it by counting the driveshaft and the wheels turning.

A surefire way to find your gear ratio is to remove the differential cover and count the number of teeth on the ring gear and the pinion gear. Then, divide the ring gear's teeth by the pinion's teeth. Some ring & pinions also have the gear stamped on them.

 
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What are you using for a tuning device? I have a Steeda tune on an SCT X4 and I can modify certain parameters such as rear axle ratio, tire revs/mile, etc. Also, even though the tire size indicates a certain revs/per mile, I have found the true number to be higher. For example, my stock 255/40R19 wheels and tires (3.15 axle) based on circumference would suggest 746 revs/mile. Using my tuning device, I had to set that parameter at 770 revs/per mile. I verified that setting using my Dragy, which will give an indicated speed as you simply drive car normally. I usually start at around 30 MPH and keep checking up to around 70 - 80 MPH.

This is a pretty good site that will give a more accurate tire revs/mile parameter. Paladin Micro
According to these parameters:
Tire Size - 275/40R19 (loaded diameter of 26.83)
Tremec 3650 (.068 5th gear)
Axle Ratio - 3.73
The revs/mile should be approximately 752. (even though the 27.7-inch diameter would suggest 729)

Note (from Paladin website) this calculation is also based on:
97.0% of actual diameter is used in the RPM calculations to more accurately reflect the real world performance
of a loaded tire. This is of course only an imprecise guesstimate of the actual effective diameter--which is influenced
by vehicle weight, tire inflation pressure, and other factors--nonetheless I have found it to be reasonably accurate.

I had my doubts about the tire diameter adjustment, but in my case, it was pretty much right on.

As far as stock axle ratios are concerned, I believe that the manual 05-10 GTs came with a 3.55 rear axle (Tremec 3.38 1st gear).
11-14 manual GTs came with the 3.31 rear axle (MT-82 3.66 first gear).
Of course, the 3.73 axle was an option in both cases.


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Dino Dino Bambino

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Stock tires were 245/45/19 and I swapped the rears for 275/40/19 just so I didn't have to worry about the speedometer correction. My question now is, with both sets of tires being 27.7" diameter and 728 revolutions per mile, if I truly have 3.31's in there despite the window sticker, why would the speedometer still be off by a few mph? I don't know if a previous owner blew up the rear and had a new one installed, if they didn't like the 3.73's and wasted the money to go down to the 3.31's or is there something between 3.31 and 3.55 that I might have? Could they be the rare 3.27's and causing the higher speed reading with it tuned for 3.31's? Just kinda stumped and I don't want to keep bugging Lito for another tune until I can figure this out and get those few MPH accounted for.

A common mistake is to calculate the tire circumference merely by multiplying the tire diameter by pi, whereas the correct way is to multiply the rolling radius (wheel center to ground) by 2pi (with tires inflated to correct pressure).
I find this tire size calculator to be the most reliable:


Plugging in your tire size of 275/40R19 you get 752 revs/mile. At 60mph (GPS) in 1.00:1 4th gear, the engine rpm will be 752 multiplied by the rear axle gear ratio so you can do the maths.
 

JC SSP

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Every tire manufacturer molds and cures their tires differently. There will alway be a slight variation in over all diameter.

Trust me I have tested them many times. Example: Goodyear 255-50-16 was taller than a Michelin 255-50-16. Not by much but it was taller.
 

Road_Trip

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Just an update for those of you who offered advice. Finally got around to pulling the diff cover today and checking the gears. They were stamped 31 and I counted the teeth, twice: 43/14. Just wanted to make sure since I was in there anyway. I appreciate the advice given, just disappointed it's not the 3.73's the window sticker showed as an added option.
 

Dino Dino Bambino

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Just an update for those of you who offered advice. Finally got around to pulling the diff cover today and checking the gears. They were stamped 31 and I counted the teeth, twice: 43/14.
43/14 = 3.07
Are you sure you counted the teeth correctly?
These Ford axle gear ratios and tooth counts are as follows:

4.56 = 41 tooth ring gear, 9 tooth pinion
4.30 = 43 tooth ring gear, 10 tooth pinion
4.27 = 47 tooth ring gear, 11 tooth pinion
4.10 = 41 tooth ring gear, 10 tooth pinion
3.91 = 43 tooth ring gear, 11 tooth pinion
3.90 = 39 tooth ring gear, 10 tooth pinion
3.89 = 35 tooth ring gear, 9 tooth pinion
3.73 = 41 tooth ring gear, 11 tooth pinion
3.55 = 39 tooth ring gear, 11 tooth pinion
3.31 = 43 tooth ring gear, 13 tooth pinion
3.27 = 36 tooth ring gear, 11 tooth pinion
3.08 = 40 tooth ring gear, 13 tooth pinion
2.73 = 30 tooth ring gear, 11 tooth pinion

I'd suggest that you have a 3.31 and you've miscounted the pinion gear teeth.
 
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43/14 = 3.07
Are you sure you counted the teeth correctly?
These Ford axle gear ratios and tooth counts are as follows:

4.56 = 41 tooth ring gear, 9 tooth pinion
4.10 = 41 tooth ring gear, 10 tooth pinion
3.73 = 41 tooth ring gear, 11 tooth pinion
3.55 = 39 tooth ring gear, 11 tooth pinion
3.31 = 43 tooth ring gear, 13 tooth pinion

I'd suggest that you have a 3.31 and you've miscounted the pinion gear teeth.

Thanks for posting this. I had the same question, as the 3.07 didn't seem to make sense. I give Road_Trip credit for the effort to solve this mystery...probably the 3.31 gear set, given these numbers.
 
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Road_Trip

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Are you sure you counted the teeth correctly?


I'd suggest that you have a 3.31 and you've miscounted the pinion gear teeth.
Yup, my bad, fat fingered the 4 for the 3. I wrote the numbers down on the ramps under the car both times, plus I'm assuming the stamped 31 on the gears indicated the 3.31.
 

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