I recently installed a disc rear and went from 3.55 to 3.73 gears. I also installed the recommended 21 tooth reddish colored speedo gear. The car is reading about 7 to 8 mph too fast. The rear tires are Nitto NT555 G2's, 275-40-17. Any idea which gear for a more accurate speedo reading?
Reading was no doubt too fast with 3.55, changing ratio to 3.73 won't cause a 7-8 mph error...
Too fast means there aren't enough teeth on the driven gear, for genuine Ford, 21T is it... There is a aftermarket 23T available, due to thin teeth reported by some to have short longevity...
Best solution would be change the drive gear on output shaft from a 8T(what it likely has) to 7T, of course you have to remove the transmission tail housing to change it(no fun at all on T-5)...
https://performanceparts.ford.com/download/pdfs/SpeedometerGearUsageChart.pdf
BTW the 23T will drop reading approx 6-7 MPH...
This would work as well, since our cars have electric speedometers: https://lmr.com/item/M4209ADPTAC/94-10-Mustang-Speed-Dial-Speedometer-Adjuster-M-4209ADPT-AC
Pricey fix though.
Or get some taller tires .... I ended up putting 255/50/17 tires on (getting harder to find appropriate speed ratings unfortunately) after the old one wore out.....lived with Speedo being out till then.
Only other choice would be to send the Speedo out for a recalibration.
John
That just does same thing as reducing gear ratio... Assuming Vin likes the 3.73, using tires to equal the gear combo of 3.27 is a giant step backwards... Could need something like a G78-15 to be tall enough...
I'm 99% my fresh AOD has a 8T output shaft, I'll be using the 23T gear...
It may be a step backwards but it is very close to the stock 16" TC tire height and keeps the Speedo a little more accurate for my car.
I never bothered to calculate gear comparison to a Mustang but with a turbo....that 327 works well for me. Thanks for posting that info.
John
I put on 255/40ZR17's rear and 235/45ZR17's front after my 21T 3.73 swap. GPS verified error of 1@60, 2.5@100
Speedo was accurate with 245/45R17, 3.27 rear and 18T gear before the swap (Should point out that my AOD has all 4R70W internals, including the output shaft and its 7 tooth drive gear), in order to keep same accuracy with my 3.73, I'd need a 20.5 tooth gear. Since that's stupid, seeing as you can't have a half gear tooth, You have to make up the difference with a tire change.
I'm going to get the GPS on the phone going and do another test, then, if it's the same, I'll try the new gear. Thanks Guys. I appreciate the advice Tom. As always.
LOL yeah right... With the drive/driven gear system and mechanical speedo there was almost never a vehicle with speedo closer than 2-3 mph @ 60 mph... Just because one ordered the optional(larger) tires, the factory probably didn't use a different speedo gear... In some applications different rear ratios used same speedo gears...
My Cobra Jet is prime example, same driven gear was used with 3.25 & 3.50 rear ratios... Both were slightly off calibration in opposite directions... Now it's definitely off calibration as I swapped the 3.50 to 3.00 and the factory 16T driven gear is as short as there is... The 6T output shaft drive gear needs to be changed to a 7T... The top loaders used behind lesser engines got the 7T, but version used behind 427/428/429 with the 1 3/8" input & 31 spline output shaft only used the 6T...
I changed the drive gear on the output shaft of the T5 in my Coupe to a black 6 tooth. I did this to give me a better range to drop back in gear or move up in gear in the rear end and with tire changes. It is not hard to change and you can do it in the car if needed but its not fun...been there done that. Last time I pulled the T-5 it took a day to R&R it and change the gear.
Or this will work. More cost less effort though :hick:
Pretty sure I ran the 23t gear on my 3.73 swapped v6 car, and the speedo was a bit off but not enough to matter. I put ~30k miles on the car that way without the gear tearing itself up, but seeing the speed sensor now the teeth were starting to fracture at the root of the flank.
23 tooth gear has been ordered. Updates when I get it.
I have the 23 tooth in my car, no issues so far. It just reads a little "off".
I just got the gear today. I'll install it as soon as I can and report the results
If you're dumb when it comes to drivetrain like me, try this Speedo Gear Calculator... just for future reference.
https://lmr.com/products/Mustang-Speedometer-Gear-Calculator
I've been running the auto trans 23 tooth gear Late Model Restoration sells for my Thunderbird for several years. It seems to be working fine. I'm probably going to go with the FRPP speedo recalibrator I linked in the future so I can run a 21 tooth gear.
I'm not sure that would work for our cars?
It will. We have a VSS speedometer, like 94 and up Mustangs.
I'm running a 23t gear in mine with 3.73's. Still reads about 4-5 mph too high but that's about as close as we can get with speedo gears.