Gear Selection

Selecting the right gearing for your Mustang can be confusing. Considerations such as highway drivability, RPM through the traps, effectiveness of transmission gearing, and others issues come into play. We've gone through this dilemma more than a few times for our '90GT.  I think we've been through nearly every available gear option for the Ford 8.8-inch rear.  To be honest, the gear selection process was a lot harder when our car was both a street and strip vehicle.  When it was still seeing street duty, we had to consider things such as the street-ability, gas mileage, NVH (noise, vibration & harshness), among others.  Once the car became a trailer queen and subjected to track duty only, we were down to the easy (yeah right) factors like sixty-foot performance and RPM through the traps.  

Back in those street days,  I generated a spreadsheet that showed the differences between the original 3.08, 3.55, 3.73, 4.10 and 4.30 gear sets.  The charts are very helpful when looking at the speeds that different engine RPMs will yield.

Basically, here's the equation you need to use:

MPH = (0.003 * Tire Diameter * RPM)/(Transmission Gear * Rear Gear)

So with 26.0 inch diameter slicks and 4.10 gears, we'd be going through the traps at about 110 MPH. And on the highway, we'll only be turning about 2450 RPM at 70 MPH with our 26.5 inch diameter Dunlop D40M2 street tires.

You can try this little script I've written to try some different combinations.

We initially went with 3.55's for one season, primarily because we thought anything else would be too much for the street. Boy were we wrong! The 3.55's just weren't enough change from the original 3.08's and although they did give a seat-of-the-pants improvement, they just weren't enough at the track. We then moved up to 4.10's as our modifications to the car became more aggressive and then on to 4.30's which gave a fantastic launch with the 3.35 first gear in the transmission. At the time that we were running the 4.30's, we were still a 306ci EFI combination weighing in at 3200 lbs and putting about 325 HP to the rear wheels.  Those 4.30's gave us 60' times down in the low 1.58 range on an 8.5 inch slick and had us pulling both front wheels well off the ground. Believe it or not, but I thought the 4.30 was an excellent gear for the street too; that is if you're not driving 100 miles on the freeway every day. The streetability wasn't hampered at all either IMHO. Cruising around town was still extremely manageable, especially since the overdrive gear can be used below 40 MPH and the motor turns less than 2700 RPM at 70 MPH on the highway. For any of you stick shifted NA street-type guys out there, I would definitely recommend the 4.10's, and definitely nothing less than 3.73's if you really feel the 4.10's are too much. 

The later years.....

 When we switched to the Lentech AOD transmission, I thought I'd redo the charts for the AOD gear ratios just to explore the degree of difference. Below is a simplistic estimation of the AOD characteristics since parameters such as converter slip haven't been included.  It does go to show the relative speeds in any of the gears though when the converter is locked up.

Many moons have passed since those days of street/strip gearing.  As our car passed from EFI to carbureted, we needed significantly more gearing to get the motor working up in its RPM band.  For example, peak HP with our long-runner EFI intakes was barely 6100 RPM.  When we changed to the dual-plane carbureted intake we immediately saw the peak power point move up to 7200 RPM.  Nowadays with our higher revving carbureted combination, we're seeing peak HP at 7500 RPM and shifting gears at 8000 RPM or beyond.  These kinds of revs dictate more gear.  Currently we've got a 4.88 gear in the car (after trying 4.56's) and have even considered trying a 5.13 gear.  My main concerns with the current combination are to get the rear gearing working in harmony with the torque converter to get the most torque multiplication out of the hole while keeping the engine revs in check at the big end of the track.  Its a trade off at times, but a game that you're forced to pay in heads-up competition.