Speed Question

[indianporfavor]

I have an 87 with stock 4speed. Regeared by P.O. with 4.10s. It has ARB lockers and is currently sitting on 37s. I am getting about 55 mph at about 3200 rpm. I don't understand much about gearing but everything I read seems to say I should be able to get at least 65 with that set up. Is thwre something else I am missing. Should the gearing be redone?

Also if I go to 35s how would that affect things with the current set up?

Thanks in advance,
Patrick

 

[bhk]

55 indicated or independently measured by GPS, etc? Either way, mathematically, this doesn't seem right. The speedo is driven by the transfer case output. Indicated speed is a function of t-case output shaft rotation (in turn affected by engine RPM, transmission and t-case gear selection) and is unaffected by tire size or differential gearing. 4th gear, high t-case gear at 3200rpm should be ~73mph indicated. With 4.11/37, it should be ~85 actual.

If you're confident that you have the gear/tire size right..... and the fastest you can go is 55 and you're turning 3200 rpm, I'd guess your clutch is shot.

 

[NMfjCruiser]

If you're confident that you have the gear/tire size right..... and the fastest you can go is 55 and you're turning 3200 rpm, I'd guess your clutch is shot.

What? Are you suggesting the clutch is slipping?
The worn clutches I've experienced slip at high torque applications such as starting in first or reverse.
Also a clutch slipping at 3200 rpm driving down the road would certainly smoke and smell burnt.

I think you are right about the speedometer, it must be off. I use gps to verify mine and it reads about 5 mph slow
Stock truck I'm doing about 67 at 2700rpm with speedo indicating 60.

 

[Steve83]

Changing the rear axle gears is exactly like changing gears with the transmission - it trades speed for torque, or vice versa. The gear number is actually the ratio of driveshaft (pinion gear) revs to tire (ring gear) revs. So 4.11 gears (actually 4.11:1) make the driveshaft turn 4.11 times for each (1) tire rev.

I agree that the first step is to find the actual speed with the GPS in your phone. There are plenty of free apps.

To calculate the actual driveline speed, you need to know the tires' actual revs per mile (get it from TireRack or the mfr.'s website), the gear ratio, the trans gear ratio (usually 1:1 in the highest gear, unless it's labelled OD), and the engine RPM.

My Bronco's clutch was slipping badly in 4th gear, with no odor, and no apparent slipping when taking off. (It WAS slipping when taking off, of course; but it wasn't noticeable because it didn't keep slipping for long.) So it CAN happen.

 

[White Stripe]

I have an 87 with stock 4speed. Regeared by P.O. with 4.10s. It has ARB lockers and is currently sitting on 37s. I am getting about 55 mph at about 3200 rpm. I don't understand much about gearing but everything I read seems to say I should be able to get at least 65 with that set up. Is thwre something else I am missing. Should the gearing be redone?

Also if I go to 35s how would that affect things with the current set up?

Thanks in advance,
Patrick

Check your speed with a GPS first. With that combo you should be able to do somewhere around 80 or 85mph at 3200 rpm.

 

[Andy Thompson]

indicated speed is surely affected by tyre size because your altering the gearing after the point the speedo picks up? So the wheels are turning at a different speed relative to the transfer box output relative to if you had smaller tyres fitted?
Either way, check it on GPS

 

[indianporfavor]

Thanks guys. I'm 98% sure that I got the 55mph number from GPS because speedo has not been working for some time. The car has been sitting for over a year so I'm looking at fixing things to make it more enjoyable to drive. It's almost impossible to drive on the interstate any long distance. I'm not looking for a speed machine but need to at least be able to merge. I'll get it running again and recheck the speed and get back to you.

Thanks

 

[LAMBCRUSHER]

by the gears...my 31/10.50/15s on my fj62 with 3fe and 4 speed manual with 4.10 gears tells me I'm doing 70 when in fourth at 3400 rpm...while this number may not be 100% accurate (it is an INDICATED SPEED), it is most certainly more correct than 55 at 3200 with 4.10s and 37s. my fj55 with fj60 tcase and speedo gears, 4.10 rear end on 235/85/16s (32s) says its doing about 65 at about 3100-3200 rpms in 4th gear...your tires are MUCH bigger, which mean you'd HAVE to be going fast than I am going with the same gearing and engine speeds...I'd look into the clutch as well...but I'd also check the tcase gears and speedo gear to see if they are correct or if someone was mucking about in there with some odd ball speedo drive gears... HTH

 

[LAMBCRUSHER]

those big tires are hard to get to speed...my little tired rigs are both quite peppy...0-60 mph in under 14 seconds!

 

[OSS]

Well we know for sure that the tires are 37s which probably measure 36, and we know your speed is roughly close to 55 since you checked it with a GPS. And when the 2F is running 3200 rpm you can tell because it sounds like the end of the world is near, so that leads us to only one conclusion: those aren't 4:10s in the pumpkins (check to verify).

The chart below is at 65mph in 4th gear. Any vehicle.

 

[doug720]

Forget where this comes from and it's name...But it is one of the best gear/tire size/speed calculators around - http://www.grimmjeeper.com/gears.html

It has most vehicle brands and transmission options - Toyota and includes H41, H42 and H55!

It's easy to use and you can save vehicle info.

Now back to this situation, the gears are most likely lower - numerically HIGHER, than 4:10 you were told.

 

[LAMBCRUSHER]

Doug, don't you mean numerically higher gears? to be spinning 3200 rpm and only be going 55 with 37s, the gears would have to be, what? 5.56-1? if the gears were numerically lower than 4.1, then he'd be going a lot faster, right?

 

[doug720]

Yes....Edited!

 

[Steve83]

That is a helluva good calculator, Doug.

 

[bhk]

to be spinning 3200 rpm and only be going 55 with 37s, the gears would have to be, what? 5.56-1?

The gears would have to be 6.4:1 to match the OP's description. If tire size is actually 36in, the gears would still have to be 6.24:1. That's why I suggested clutch. There's nothing else in the drive train that can slip short of gear carnage.

 

[indianporfavor]

After talking to P.O. this morning he confirmed what many said that I had the gear ratio off. He put 4.88 gears in. Some peice of mind I'm closer to speed the chart says. Thanks guys that website is great too by the way.

Patrick

 

[lovetoski]

You need a 5 speed tranny.

 

[Shes Mad]

40 inch tires

 

[StinkyPete]

when the 2F is running 3200 rpm you can tell because it sounds like the end of the world is near

 

[OverLand Cruisers]

Beno can get a brand new H55 headed your way quickly...do it.