Gear ratio changed?
12-12-2014, 04:12 AM
#1
Gear ratio changed?
1989 F150 5.0L 4WD 5-speed manual, trans "M", axle "H5", 235/75R15 tires.
I've made no changes to the vehicle (it isn't even mine).
A few days ago it seemed like my gear ratios just changed, and that just makes no sense. Am I imagining it? I've never encountered something like this change in perception, but everything about it says "today you need more rpms to go 65".
I know there are no rear-axle demons, right?
Does the transfer case have a different ratio in 2H than it does in 4H, making this some sort of transfer case issue?
At 2000 RPM in 4th gear, it's at 35mph. Should it not be higher than that?
Calculating: 2000 rev/min * 91.1"/rev (tire) * 1.0/1.0 (4th gear) * 1/4.10 (rear) * 60 min/hr * 1'/12" * 1mi/5280' = 42 mph
More info: The noise triggered my questioning of my thinking, so I checked the radiator fan. Should it be locked up solid on a cold engine? Is that the whole thing, just a fan that is spinning much faster than before?
I've made no changes to the vehicle (it isn't even mine).
A few days ago it seemed like my gear ratios just changed, and that just makes no sense. Am I imagining it? I've never encountered something like this change in perception, but everything about it says "today you need more rpms to go 65".
I know there are no rear-axle demons, right?
Does the transfer case have a different ratio in 2H than it does in 4H, making this some sort of transfer case issue?
At 2000 RPM in 4th gear, it's at 35mph. Should it not be higher than that?
Calculating: 2000 rev/min * 91.1"/rev (tire) * 1.0/1.0 (4th gear) * 1/4.10 (rear) * 60 min/hr * 1'/12" * 1mi/5280' = 42 mph
More info: The noise triggered my questioning of my thinking, so I checked the radiator fan. Should it be locked up solid on a cold engine? Is that the whole thing, just a fan that is spinning much faster than before?
Last edited by ts4tomh; 12-12-2014 at 04:19 AM. Reason: added information
12-12-2014, 09:07 AM
#2
1994 F150 XLT 5.8L 2wd
The clutch fan shouldn't be locked, you should be able to spin it a few inches by hand. If it won't move at all or it spins more than a 1/4 turn then it (clutch) needs replaced.
There is nothing that would change the gear ratio magically. Though driving in 4H does put more load on the engine because now 4 wheels are being turned mechanically instead of just the 2 (2H). Also if your clutch fan is shot that will also put more drag on the engine requiring it to work harder to produce the same amount of torque/HP. Your math would be logical if it wasn't for the torque converter. There is no linear equation to determine xRPM=xMPH, because you must also factor in the load the truck is pulling, if you have a 5,000lb trailer 2,000rpm in 4th gear will not equal the speed of the same 2,000rpm in 4th gear without a load, because of the torque converter.
There is nothing that would change the gear ratio magically. Though driving in 4H does put more load on the engine because now 4 wheels are being turned mechanically instead of just the 2 (2H). Also if your clutch fan is shot that will also put more drag on the engine requiring it to work harder to produce the same amount of torque/HP. Your math would be logical if it wasn't for the torque converter. There is no linear equation to determine xRPM=xMPH, because you must also factor in the load the truck is pulling, if you have a 5,000lb trailer 2,000rpm in 4th gear will not equal the speed of the same 2,000rpm in 4th gear without a load, because of the torque converter.
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ts4tomh (12-12-2014)
12-12-2014, 11:40 AM
#3
Thanks. I'll get a new fan clutch this morning. The more I thought about it, the more I felt silly for thinking it was somehow a ratio change, like a strange gear lockup in the transfer case. But feeling silly is better than not asking a question of the experts.
5 speed manual trans, though, so I should be able to compute the rpm to mph path.
5 speed manual trans, though, so I should be able to compute the rpm to mph path.
12-12-2014, 12:08 PM
#4
Senior Member
Two possibilities here for showing more RPMs at a given speed in a given gear.
No. 1 the clutch is slipping. This would be easy to test. Get going say 60 MPH in 5th gear, push the gas pedal to the floor briefly and note the RPMs, let off the gas pedal and see if the RPMs go down significantly. If they do your clutch is slipping.
No.2 the tachometer is going bad. If the tach just shows higher than it used to it is going bad. It will probably continue to register, just not accurate. A somewhat common problem on this vintage Ford trucks.
No. 1 the clutch is slipping. This would be easy to test. Get going say 60 MPH in 5th gear, push the gas pedal to the floor briefly and note the RPMs, let off the gas pedal and see if the RPMs go down significantly. If they do your clutch is slipping.
No.2 the tachometer is going bad. If the tach just shows higher than it used to it is going bad. It will probably continue to register, just not accurate. A somewhat common problem on this vintage Ford trucks.
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ts4tomh (12-13-2014)
12-13-2014, 05:09 PM
#5
88xlt, thanks. Good things to check for. I had not heard about the tach degradation before, and I will certainly match it up against an external unit to verify operation.
In the end, perception was everything. It was a cooling fan clutch that simply seized up, making it "sound" like the engine was turning faster because the fan was generating much more noise than it usually did before it seized, at any given engine RPM.
A trip to O'Reilly's, $60, and about 10 minutes under the hood and on the bench (it really was much easier than I expected), and my perceptions returned to "normal".
Fascinating. I guess I discovered just how much I drive by ear, that the change in noise made me question my view of the engine RPM as reported by the tach, but casually ignored because I "listen", not "watch".
In the end, perception was everything. It was a cooling fan clutch that simply seized up, making it "sound" like the engine was turning faster because the fan was generating much more noise than it usually did before it seized, at any given engine RPM.
A trip to O'Reilly's, $60, and about 10 minutes under the hood and on the bench (it really was much easier than I expected), and my perceptions returned to "normal".
Fascinating. I guess I discovered just how much I drive by ear, that the change in noise made me question my view of the engine RPM as reported by the tach, but casually ignored because I "listen", not "watch".
12-16-2014, 06:27 PM
#6
Senior Member
The clutch fan shouldn't be locked, you should be able to spin it a few inches by hand. If it won't move at all or it spins more than a 1/4 turn then it (clutch) needs replaced.
There is nothing that would change the gear ratio magically. Though driving in 4H does put more load on the engine because now 4 wheels are being turned mechanically instead of just the 2 (2H). Also if your clutch fan is shot that will also put more drag on the engine requiring it to work harder to produce the same amount of torque/HP. Your math would be logical if it wasn't for the torque converter. There is no linear equation to determine xRPM=xMPH, because you must also factor in the load the truck is pulling, if you have a 5,000lb trailer 2,000rpm in 4th gear will not equal the speed of the same 2,000rpm in 4th gear without a load, because of the torque converter.
There is nothing that would change the gear ratio magically. Though driving in 4H does put more load on the engine because now 4 wheels are being turned mechanically instead of just the 2 (2H). Also if your clutch fan is shot that will also put more drag on the engine requiring it to work harder to produce the same amount of torque/HP. Your math would be logical if it wasn't for the torque converter. There is no linear equation to determine xRPM=xMPH, because you must also factor in the load the truck is pulling, if you have a 5,000lb trailer 2,000rpm in 4th gear will not equal the speed of the same 2,000rpm in 4th gear without a load, because of the torque converter.
