Trying to figure out my gears

[ireallydontcare]

Howdy everyone.
So my 1990 Bronco II has axle code D4, 3.73 limited slip. I have an FM146 and 30 inch tires. According to Novak's cruising RPM calculator, my should be 2199 in 5th gear. In reality it is at about 2700. I also did the mark the tire and driveshaft and turn test and it came out to be 3.73. Anyone have any ideas? I have been scratching my head about this since i bought the truck last year.

 

[Shran]

2700rpm at what speed?

Your number sounds about right, 2199 is very low.

My Explorer with 30" tires and 4.10 gears sits at about 2900rpm at 70mph.

 

[RonD]

+1` ^^^

Yes, AT WHAT SPEED????

Your 5th gear(OD) is .81 on that transmission
3.73
30"
.81
2,700rpm would be 80mph

2,200rpm would be 65mph

These are actual speeds
You speedometer would be low because of the 30" tires, unless you have changed the Driven gear
I think 1990 B2 came with 27" tires

So if speedometer says 65mph you are actually going 72mph, at 80mph you are going 89mph, so a pretty big difference, assuming 27" tire calibration
The door sticker will tell you the stock tire size was, and then you can re-calibrate if you want accurate speedo/odo

 

[ireallydontcare]

+1` ^^^

Yes, AT WHAT SPEED????

Your 5th gear(OD) is .81 on that transmission
3.73
30"
.81
2,700rpm would be 80mph

2,200rpm would be 65mph

These are actual speeds
You speedometer would be low because of the 30" tires, unless you have changed the Driven gear
I think 1990 B2 came with 27" tires

So if speedometer says 65mph you are actually going 72mph, at 80mph you are going 89mph, so a pretty big difference, assuming 27" tire calibration
The door sticker will tell you the stock tire size was, and then you can re-calibrate if you want accurate speedo/odo

I have accommodated for my speedo being off. I checked it with gps and at 65 on my gps in 5th gear im turning around 2700 revs.

 

[Curious Hound]

You’re not in 4lo, are you?

 

[ireallydontcare]

oh no, of course not. i'm starting to think maybe i did the turn test wrong. I need to pull the diff cover to change the fluid anyway so i guess ill find out like that. but if you have any ideas on what my ratio might be i'm all ears.

 

[RonD]

5th gear(OD) is .81 on that transmission
4.56 <<<<<
30"
.81
2,700rpm
65MPH

4.56 ratio would account for that

OR
4th gear 1:1 ratio
3.73
30"
1 <<<<
2,700rpm
65mph


Try driving in 4th gear, at 2,700rpm you should be 53mph if its 4.56 rear axle ratio

 

[ireallydontcare]

ok, will do, ill report back here and let you know how fast im going in 4th at 2700.

 

[Snoranger]

Are you going off the factory tach or a quality aftermarket? Its not unusual for them to be off.

 

[ireallydontcare]

RnoD, at 2700 rpm in 4th gear I'm going 54 mph according to my GPS.

snoranger, I've actually swapped this one in for an unrelated reason. My old original one was the same way.
Edit: it is a factory tach though not aftermarket

 

[RonD]

RnoD, at 2700 rpm in 4th gear I'm going 54 mph according to my GPS.

snoranger, I've actually swapped this one in for an unrelated reason. My old original one was the same way.
Edit: it is a factory tach though not aftermarket

You have a 4.56 axle ratio so not the stock axle

Assuming, 30" tires

Its it a 2WD ox 4WD B2?

 

[ireallydontcare]

it is a 4x4 with the dana 35 up front. i suppose my next question is do you even think it would be worth it to gear it back down to say 3.73 or 4.10?

 

[RonD]

Well you have to do both axles, of course, so cost is doubled, whether its "worth it" is up to you
No on the 3.73, 4.10 would be the lowest with 30" tires

You could go to larger diameter tires, if they will fit, that lowers RPMs, 32" would be 2,500 at 65mph, with 30" its 61mph at 2,500

 

[Shran]

Sounds like a really great time for larger tires and a lift

 

[ireallydontcare]

RonD
Well I guess that's what I'll do then, throw some bigger meats on it eventually. I'll have to lift it though.

Shran
Yeah I suppose you're right, but I don't do too much off road, so I'm thinking a body lift will suit me well enough. I have plans to put a ranger cab on my frame and either make a flatbed or shorten a ranger bed up, my body is rotted out, so I'll probably lift it when I do that.