Skip to main content ↑ ↓
Topic: Speedo gear choices.... (Read 3464 times) previous topic - next topic

Speedo gear choices....

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?
'88 Sport--T-5,MGW shifter,Trick Flow R intake,Ed Curtis cam,Trick Flow heads,Scorpion rockers,75mm Accufab t-body,3G,mini starter,Taurus fan,BBK long tube headers,O/R H-Pipe, Flowamaster Super 44's, deep and deeper Cobra R wheels, Mass Air and 24's,8.8 with 3.73's,140 mph speedo,Mach 1 chin spoiler,SN-95 springs,CHE control arms,aluminum drive shaft and a lot more..

Speedo gear choices....

Reply #1 –
Quote from: vinnietbird;462661
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...

Speedo gear choices....

Reply #3 –
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

Speedo gear choices....

Reply #4 –
Quote from: Skunk;462669
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...

Speedo gear choices....

Reply #5 –
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

Speedo gear choices....

Reply #7 –
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.

Speedo gear choices....

Reply #8 –
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.
'88 Sport--T-5,MGW shifter,Trick Flow R intake,Ed Curtis cam,Trick Flow heads,Scorpion rockers,75mm Accufab t-body,3G,mini starter,Taurus fan,BBK long tube headers,O/R H-Pipe, Flowamaster Super 44's, deep and deeper Cobra R wheels, Mass Air and 24's,8.8 with 3.73's,140 mph speedo,Mach 1 chin spoiler,SN-95 springs,CHE control arms,aluminum drive shaft and a lot more..

Speedo gear choices....

Reply #9 –
Quote from: TheFoeYouKnow;462676
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 .

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...

Speedo gear choices....

Reply #10 –
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.

83 351W TKO'd T-Bird on the bottle


93 331 Mustang Coupe - 368 rwhp

Speedo gear choices....

Reply #11 –
Quote from: thunderjet302;462667
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 this will work. More cost less effort though :hick:
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.
 

Speedo gear choices....

Reply #12 –
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.

Speedo gear choices....

Reply #13 –
23 tooth gear has been ordered. Updates when I get it.
'88 Sport--T-5,MGW shifter,Trick Flow R intake,Ed Curtis cam,Trick Flow heads,Scorpion rockers,75mm Accufab t-body,3G,mini starter,Taurus fan,BBK long tube headers,O/R H-Pipe, Flowamaster Super 44's, deep and deeper Cobra R wheels, Mass Air and 24's,8.8 with 3.73's,140 mph speedo,Mach 1 chin spoiler,SN-95 springs,CHE control arms,aluminum drive shaft and a lot more..

Speedo gear choices....

Reply #14 –
I have the 23 tooth in my car, no issues so far.  It just reads a little "off".
Mike