I am looking to make a gear ratio change (from 3.55 to ?) to optimize my towing power. With the stock 3.55s I am currently running 1750 RPM at 65MPH in 4th gear (i.e. OverDrive). 2500 RPM at 65 Mph in 3rd gear (out of OD). 1900RPM @50Mph, 4th gear. 1300RPM @ 50Mph, 3rd gear.

I assumed that towing on a grade, 50Mph would be the slowest that I would go before the auto tranny shifted into 2nd. I also considerd that 65Mph is about the max. speed that I would tow at. I wanted to maximize torque/HP across this range. Any comments on those assumptions? The idea is to get enough RPMs at the 50Mph mark to still be in the "power band" but not be running so many RPMs at 65Mph that the engine wouldn't sound like it was coming undone.

I am trying to do the calculations for my configuration of engine (5.4), tires 17", etc. to decide which gear is best. Can anyone offer up a calculator?

 

Check out this nifty calculator. Another good one, but you'll have to calculate the tire diameter on your own. There are many more if you search for gear tire engie speed calculator with Google.

FYI:
4R100 auto trans ratios (for 5.4L - 4.6L trans gears lightly different):
1st: 2.71
2nd 1.54
3rd 1.00
4th 0.71

4.10 gears might be perfect. Puts you at 2000rpm at 65mph in overdrive. 50mph would be aroud 2200rpm in 3rd gear. The only downsides are lower fuel economy and pretty a relatively high engine speed at higher, unloaded highway speeds. I generall cruise at 80mph, +/- 10mph depending on traffic. 80mph = 2450rpm with 4.10 gears. Anyway, play with the numbers yourself.

If you think that only a 3.73 would would be good for you, I'll recommend investigating other ways to increase power rather than cjange gears. For $500 (about a gear swap price), you can get an intake kit and exhaust system that provide say 10% more power while increasing gas mileage.

 

Thnx, the calculators helped. I found one (http://www.csgnetwork.com/multirpmcalc.html) that matched up pretty close (within 30 RPM) of what I measured with the stock 3.55s. There was also a metric tire conversion calculator there.

I have gotten my 4.10s installed already and it might have been a real waste of money as you suggested. Since I have a four-wheel drive it was not a $500 job but a $1500 job (front and rear diff. changes). And, I can't tell a bit of difference in the driver's seat in a non-towing situation. The interesting thing is that I still have the same exact shift points and RPM/Mph settings that I did before the change. The shop swears they changed the computer to update the speedo. I can hear the new gears whine. I can see new RTV on front and rear diff covers. The shop double-checked the box that the gears came in it does say 4.11. Shouldn't I expect a seat-of-the-pants performance improvement? I know that I should see the RPMs go up but not so. Any suggestions?

 

You should feel a diffrence between 3.55 and 4.10 as it is about 15% difference.

I would first check your engine speed at a specific vehicle seed and compare thatto whatyou get with your calculator. If thatdoes not match up, I would try other ways to check your speed vs. the speedometer. If you have a GPS, you can use that. Or one of those police radar signs, or pace someone you know in another car.

 

cmoss -

The speed is measured from the transmission tail and calculated based on rear-end gears and tire size. If you seeing the same speed at the same RPM, I'm gonna guess the computer was not changed to reflect the gear change. Like APT stated, you should check your speed-o accuracy...

You might also jack the rear wheels off the ground, put it in neutral and count the number of driveline rotaitions as you turn the rear tires one revolution (both rear tires together). If your driveline only turns 3 ½ times, you have 3.55's. If it turns a tad more than 4, then you have the 4.10's.


I'd like to hear more about the change, as I am considering a change to 4.10's for more towing power. I just can't believe you can't feel it with the seat of the pants.

What about towing - notice any difference there? Can you hold OD better than before?

And the whining - How abtrusive is it?


Thanks!

 

I changed my gears from 3:55 to 4:10's on my 04 SCrew and has been the best change I have done for towing and improved low end response. I did take it to the dealer to have the computer changed to 4:10's so speedo would be correct, however the dealer told me that update can't be made to the computer. Anyone have any experience or thoughts on that?

 

I'm not sure what you mean my computer. Changing anything so the speedo/odo are correct will also affect the engine/tranny. They use the same numbers as far as I know.

The dealer has limits on what can be changes and to what values. Aftermarket tuners (XCal for example) can be modified for your exact numbers (gearing, tire size).