I just had the 4.10's installed today and when I took it out on the highway to compare RPM with MPH there was no difference from what I noticed with the 3.55's.

60 mph 1700 rpm
67 mph 2000 rpm
80 mph 2400 rpm

I made a special note of these RPM's to Speed before the change. I checked my old gears to make sure they didn't screw up and put my old gears back in by mistake. They were shiney in the right spots from wear and the pinion had 355 on it. That ruled that out. One thing that I noticed was the 3.55 ring didn't look very much smaller than the 4.10 in the styrofoam packing.

Is the speedo going to read the same with the RPM's even though your going slower? I thought the speedo would read high compared to the RPM's.

 

hmmm ... I would think so too ...

In my friends cobra he swapped 3.17's for 4.30's and there was a huge difference in rpm and the top speed in each gear. Maybe it isn't so evident in 4th gear as opposed to 1st?

anyone else?

 

No the speedometer will not read the same. The 4:10's will read faster than the 3:55's even though you are going slower. I believe 70 MPH for me is about 2400 RPM

 

Andre,

Was there a big difference in your ring size between the 4.10's and 3.55's? Just guesstimating, I would say there is maybe 3/4" difference from the markings in the styrofoam packing.

 

The tach runs off the ignition, the speedo runs off of the gear in the trans. did you change the gear in the trans that drives the speedo? If you have someone pace you, you are not going as fast as the speedo says you are.

 

The speedo is usually corrected with the chip. I don't know of anybody doing it with gear change in the tranny on the Gen 2 L. Those days might be gone for good. I don't know if the speedo is electronic or cable on these.

I'm going to go thrash it with the new gears to see if I can tell the difference. I had the old gears in a box in the bed on the way home so I didn't get to really mash it.

 

Ok, here's the business for all you L owners that didn't pay any attention to your MPH vs RPM before swapping gears.

I compared everything several times tonight and even cleared the PCM. The results were still the same. I read the same MPH vs RPM as I did before the gear swap.

Here is the interesting part. I paced someone doing 60 mph and my speedo read 67 - 68 mph.

I've been past a couple of those radar displays in the past around 60 mph and I was within about 1 mph of what the display posted.

It adds up now. If you correct the speedo down to 60 mph then the tach will read 2000 rpm.

 

Hey Chuck,

Ford can reprogram your PCM for the gear you have in your truck and whatever size wheel you decide to go with. I know it works on my 2001, Everytime I swapped from 18's to 20's, I would go by my local Ford dealer and have them reprogram my computer for the size tire I was using. There was a screen that also showed where you could input the size gear you were running.

 

If you are running stock F1's and 4:10's, you should be turning right at 2000 rpm at 60 MPH. If this is not what you are seeing, either you didn't get 4:10's installed or your tires are not the same diameter as the F1's. It used to be that the dealer could reprogram your PCM from the 3:55's up to 3:73's, but nothing above 4:00's. This may have changed.
My speedo was corrected in my chip program by my tuner. If you are using a chip, your tuner should be able to accomodate you.


curmudgen
The 2nd generation Lightning's speedometer is driven off of a sensor in the rear differential.
It measure the RPM of the axles, rather than that of the driveshafts as was previously done when a gear in the transmission was used. This is why the tire size also has to be programmed into the PCM as stated by DL.

Lightning Bob

 

DL,

Thanks for the info. I will check it out.

I wonder if you have you PCM recalibrated if you don't have to have you chip reburned.


Bob and Andre,

Thanks for confirming what I discovered by pacing another car. At 60 mph (by his speedo) I was turning 2000 rpm. My speedo was 7-8 mph fast.

I figure if I went up to 70 mph I would have been read about 10 mph fast and running 2400 rpm like Andre said.

I had expected a change in the tach and speedo relationship with the gear change but they stay the same even though you're not going as fast as the speedo says anymore.

The relationship between the tach and speedo only change when you recalibrate. The tach will then read higher for the speed the speedo shows.

I was thinking about it bassackwards.

 

DL,

You're right about the dealer being able to reprogram the PCM.

I need to know if I reprogram the PCM if it affects the chip or if the chip has to be reburned also.

 

LTNBOLT,

I'm pretty sure, They reprogramed my PCM with my Diablo chip in it.

 

I am also running 4.10's and all I did was have my chip reprogrammed for the 4.10's and the speedo is perfect. Gotta love those 4.10's!!!

 

Somebody out there has got to know the answer about PCM reprogramming impact on the chip question for different gears.

My chip tuner is out of town.

 

BTW, Mark / LFP the gears installed with no additional shims just like you said. They only charged me $208 tax included because the install went so smooth. They charged by the hour. The 4.10's are as quiet as the stock 3.55's which is good for that gear ratio.

Thanks for the great price and quick delivery.