Ford Forums banner

1 - 9 of 9 Posts

·
F100 351
Joined
·
74 Posts
Discussion Starter #1
hey a quick question with my 86 f100 2WD, on the open road at 100km/h she reads 3600rpm. its a 351 clevo, c4. i've had it 8 months and still learning about the truck and the history can you guys help me out and let me know what you think the diff ratio could be? i did put out a post seeking the factory ratio of this truck but got no answers and i was told to look at the tags on the 9 inch, i did and it made no sense to me

also the reverse lights dont come on, where is the swich and does anyone know what colour of the wire i'd be looking for so i can track down the problem

cheers
 

·
Registered
Joined
·
2,398 Posts
What diameter tyre are you running? or size?

Here's a link for you. You can plug in the tyre size to get the tyre diameter in inches, then go to the "FIND RPM or MPH" calculator......quick and easy to reverse engineer your diff gear :hy:

Enter the tyre diameter (inches) and transmission and transfer case ratio (both 1) and then just massage the diff ratio number to get 62.5mph (100km/h) or 3600rpm.

My guess is that it's up around 3.8 - 4.11
 

·
F100 351
Joined
·
74 Posts
Discussion Starter #4
hey cheers mate, had a play with the numbers and to have 3600rpm, and 62 mph the diff was 5.3 something. i've got 31 inch tyres.
what do you think about that, should i change it. to maybe a 4.11?
i tow probably 300-400 kg worth of work gear

i tried another rpm calculator and it gave me 5.3 also
 

·
Registered
Joined
·
2,398 Posts
F100kls said:
hey cheers mate, had a play with the numbers and to have 3600rpm, and 62 mph the diff was 5.3 something. i've got 31 inch tyres.
what do you think about that, should i change it. to maybe a 4.11?
i tow probably 300-400 kg worth of work gear

i tried another rpm calculator and it gave me 5.3 also
5.3's are quite low.....even for a rig like yours! OK for climbing up 45 degree hills etc. If you have a relatively stock motor, you wont need it to rev so high anyway.....its torque band will be low. Have you modified your engine at all? Type of gearbox, stall converter size?

4.11's would drop your revs down to 2800 @ 100km/h.....those big tyres help out :hy: You'd still pull a Mack truck too.
 

·
F100 351
Joined
·
74 Posts
Discussion Starter #6
i haven't done any mods, it's untouched since i bought it last year. C4 trans, stall?? i'm guessing stock. previous owner didn't know much about it, he had it for 18 months without changing it. he sold it cos he needed a dual cab truck. he was towing a sprint car.
 

·
Registered
Joined
·
2,398 Posts
OK, then a higher diff ratio (lower number) should be fine. 3600rpm must send you round the bend!....unless you just sit on 80.

The stall would be standard too....probably around 1000rpm. You can stand on the brakes and momentarily see how high it revs....before it either bogs down or spins your wheels.
 

·
F100 351
Joined
·
74 Posts
Discussion Starter #8
yeah i sit on 90 so i can a bit beter fuel mileage, i'll be going for a higher ratio soon, at the mo i have 1/3 turn backlash going into the diff.....it goes [/B]BANG wen i change from D to R and visa versa.
 

·
F100 351
Joined
·
74 Posts
Discussion Starter #9
ive jus rebuilt the diff with original gears. they were factory 3.5
the problem with high rpm reading has been nailed down too the tacho not
reading what it should be.
 
1 - 9 of 9 Posts
Top