I have a 3spd manual '79 F100 with 2.75 gears. Currently it reads about 5 mph under when driving, ie running at 60 mph on the speedo gps' at 65mph.
I have taller tires than the stock 205/75R15, using 235/75R15.

I'm regearing the rearend to 3.50 gears and want the speedo to be accurate when finished.

Do I change a gear in the tranny or speedometer?

How do I calculate what gear I need?

Is is hard to change?

 

In the transmission.

All speedometers are the calibrated thesame.


You need a driven gear that turns slower.

They are colour coded for ease of recognition.


You can do the check here.

https://www.ford-trucks.com/calculat.../p-tire_sizes/

 

P235 R78-15" = L78-15

3 spd MT w/3.50 axle ratio = driven gear C1DD-17271-A

driven gear = Type 2A- 20 teeth - Orange in colour


You should be removing a TAN ( with oil dis-colourment) coloured one

 

Thanks, I see why you're Da Man.

 

THanks !

I'm "Da Man".

Because the guy who suggested it ...wouldn't get away with " Duh Man ! "

He would have been racked over the coals

 

or try to borrow a gps. they have a way of calculating speed by satelite.

 

hey dennis, my door doesn't have a tag and the tag on the rear end is rusted off. my speedo is off. the gear is red now. duh? i need to get my wifes gps working.

 

C1DD-17271-A speedo gear was black, it has been replaced (superceded) by Ford to: C8SZ-17271-B which is orange.
The gears (15 thru 21 teeth) are still available from Ford.

C8SZ-17271-B .. 20T Gear-Orange ~ Ford list price $8.42.

 

Ok, maybe not a question with an easy answer, but...

my truck with a C6 and 3.00 rear end and 235/75R15 tires the speedo was dead nuts on. Now with 31" (30.5 actual) tires and 3.50 rear gears the speedo reads way fast. What gear would I possibly move to? I believe the gear is red (it's been a while and it's 10 degrees outside at the moment)

Josh

 

30" and a P235 78R 15" are practically the same thing

With P235 75R 15's you are only 3% off the mark.

 

notice his rear gear ratio was changed from 3.00 to 3.50...

im having a similar problem, i have 32/12.5 r16 4.11 gears and my speedo is off by about 5 mph to fast, it says im going 65 and i'm only going 60. the current gear was white maybee tan? which once should i try now?

 

I'll get back to you.
check the caluculator I posted

 

Well 235 series tires are 28.8" and what is the ACTUAL measurement? Maybe 28? My 31" BFG TA/KO tires measure 30.5.

Also the gear change from 3.00 to 3.50. So I think my speedometer reads 5 MPH fast at indicated 40 MPH according to the Police speed signs around town (in other words I actually going 35, but the speedometer says 40).

Josh

 

235mm is the tire width

75 % of that is 176.25

divided by 25.4= 7"

7 + 7 + 15 = 29 " Mounted on wheel tire height 30+ " with inflation

 

Actual tire height is less than the written sidewall height.

And I already said a 235 tire is 28.8 inches.

Josh