Gear ratio change

First off, what do you plan to use the car for?? If it's a daily driver with the occasional blast down the drag strip, 4.10's would be a waste, especially if you do any highway driving. I have an 86

5.0 5 speed and I installed some 3.73's and I love them. I don't drag race though, I go to Heartland Park and run the High Performance Driving Schools around the road course. It all depends on what you use the car for.

Steve

---- snip ----

Mike,

Since you and I have the same model car perhaps I can shed some light in regard to the 3.73 situation. This was the second mod that I did to my GT (the first was the obligatory TriAx shifter of course), my stock gears were 2.73. I experienced no mileage loss at all either in town or on the highway unless I just couldn't keep my foot out of the throttle. Turns out you'll end up rowing the gears more to keep the motor in the preferred operating range and thus no mileage loss around town.

For the highway part of the equation my guess is that 2500 RPM (3.73 @

75 MPH in 5th) is a more efficient operating range than the same speed with 2.73's, thus the similar mileage. Even after the headswap my mileage didn't suffer unless I pound the accelerator, then it uses a BUNCH more fuel!

Here's a great link to some calculators for figuring rpm's with respect to gears:

to "tech articles", then navigate from there. Oh yeah... don't forget the SpeedCal, The internal drive gear for the VSS in our particular T-45's will eat up the 23-tooth speedo gears.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Tom

1998 GT Coupe 5-spd. Bright Atlantic Blue K&N FIPK, Tri-Ax, 3.73's, FRPP Coated Shorties, SpeedCal, P&P 2K Heads, 2K Intake, Bassani X-Pipe and Cat-Back, Subframe Connectors, JMS Chip, Eagle Alloy Wheels

Ok thanks to you both, but Tom... and I feel stupid for asking this, but this is my first car where I did anything to it myself... what's a speedcal? :/ Thanks in advance.

-Mike

Btw... I'll be using it for both city & highway driving... more city than highway. And I'll probably pound the accelerator every now & then to show up the stupid ugly Hondas in my city that try to think they're cool with their fart sounding exhausts. I never really go to drag strips, although I'd like to see one... sounds like it'd be really fun. Anyway... Is there a noticeable change with the 3.73's? A lot of added power? *grin* hehe... thanks again fellas.

-Mike

The SpeedCal is a device that electronically alters the signal coming from the computer to the speedometer to correct for the change of gearing. The RPM sensor is on the tailshaft of the trans. With the change in gears, the RPMs of the tailshaft will not match the calibration of the computer for whichever gear you're in.. Tom mentioned the 23 tooth gear, and like he said, it will be destroyed in a short time. It won't correct your speedo anyway, but it gets it closer. The SpeedCal will make it exact for whatever combination of tire size and gears you use.

Gear selection is extremely controversial, and subject to an individuals interpretation. Which is better? Who knows, only you do. But if you're like me, I only want to do it once. It's too expensive to keep swapping around hunting for the right gear. The question usually comes down to 3.73 or 4.10. So let's ignore the other gear choices for now. There are two basic considerations: 1st gear speed and 5th gear speed. Some folks say that with a 4.10 first gear goes by so fast it's useless. But if you look at the chart at the bottom, you'll see that there is very little difference in speed at the same rpm. I chose

3,000 rpm as the shift point for this chart. So that becomes a non-issue. I think the perception is there because the car is so much quicker that it just feels like you have to shift too soon. The second issue is 5th gear speed. Do you normally drive at 55 or 85? As you can see from the chart, gear selection makes a big difference here. Try and choose the gear that puts you at your cruising speed at best cruising rpm. If you have a really built engine that makes power at 3,000 rpm and above, you don't really want to be cruising at 1800 rpm's. A stock engine will cruise very nicely at 2,000 rpm's and be very efficient. 3000 rpm 4.11 3.73 Gear FDR Speed FDR Speed 1st 3.27 13.4397 17.1 12.1971 18.8 2nd 1.98 8.1378 28.2 7.3854 31.1 3rd 1.34 5.5074 41.8 4.9982 46 4th 1 4.11 56 3.73 61.7 5th 0.68 2.7948 82.3 2.5364 90.7

2000 rpm 4.11 3.73 Gear FDR Speed FDR Speed

1st 3.27 13.4397 11.4 12.1971 12.5 2nd 1.98 8.1378 18.8 7.3854 20.7 3rd 1.34 5.5074 27.8 4.9982 30.7 4th 1 4.11 37.3 3.73 41.1 5th 0.68 2.7948 54.9 2.5364 60.5

For my Cobra project, I chose 4.10 gears. I'm building a 600hp engine won't make much power below 2500 rpm's. And most of my cruising is around town and two lane black tops. Average top speed in the mountains is 60-65 mph, and there are a lot of hills to pull at high altitude (>8,000 ft). With 4.10 gears, that should put me right at the beginning of the power band. Of course, with a 1st gear FDR of 13.44 in a 2200 pound car, getting the tires to hook up is going to take a light touch. Sounds good on paper, doesn't it? Well, we'll see.

Well, I've definitely decided on 3.73 gears... but again i ask about the speedcal thing... is it actual hardware that I leave somewhere in my car? (ie: attached to tranny or something) or is it a one-time re-calibration, or am I just a total idiot? Probably total idiot, but all feedback is much appreciated. Thanks for that, bob =). I do drive on the highway often enough that I'd want 3.73's instead of 4.10's. Thanks... any feedback is greatly appreciated. Cya folks.

-Mike