Auto Racing: Rear End, rear gear ratio, fuel mileage

Question
Mr. Liddy,

I am building up a car for the gratification of having something with guts under my right foot(and perhaps a little weekend fun at the track).  I'm really tired of driving around a 4 banger with no low end anything.  Anyway, as a preference for body style I'm adding a V8 engine to a car that it wasn't initially offered in and right now the question of the rear end comes to my mind.  What rear end would be decently economical as well as produce respectable quarter and eigth mile times at the track ?  Right now there is a 4.10 rear in the car.  I don't know much about how the rear works with the transmission and how all this affects gas mileage and performance.  Any help you could offer, including pointers to other information sources, would be wonderful.  Thanks for your time and consideration of my problem.

Tim Eshleman

Answer
Hi Tim:

The most commonly used rear end in combo cars (street/strip) is probably the "Ford 9"  It was also quite popular in Winston Cup just a few years ago. It's tough, reliable, and there are many gear ratios available for it.

The hitch is that the lower your rear gear (higher the ratio number) the lower your fuel mileage will be, and it's proportionate. If you lower the rear gear ratio by 10% you will raise your fuel consumption by a similar amount.

One thing you could consider depending on your budget, would be an overdrive unit.  Last time I checked they were about $2200,00 try contacting 6 Speed Gear Vendors at 1-800-999-9555 for more complete technical info.  This system will give you up to 8 forward ratios, including an overdrive for the highway, allowing you to use a 4.10:1 rear gear, but still have an effective ratio of 3.20:1 for the highway.  In the meantime, at the track, you'll have a REAL close ratio box that will raise a few eyebrows as you shift through 5 or 6 gears, and still hit the traps at 6500 RPM..

For rear end info contact Moser Engineering at 'www.moserengineering.com' or call (260) 726-6689, ext. 813.  Basic rear ends are $995.00 and you can decide from there what else you want with it.

As far as installation goes, the companies that build these rear ends will make them to any width you need up to about 66". Be careful here, if you aren't sure what width you need, consult someone locally for help. They'll probably need to look at the project closely to give good answers. It would be a shame to spend $1200, and have the rear wheels sticking out 6" on each side.

How does it work with the transmission.  Well first you have to have the gear ratio info from the maker of the tranny.  Then say you have a 4:1 low gear ratio in the tranny, and a 4.10:1 in the rear end, add the two for a final drive ratio in low of 8.10:1.  Most trannys have a 1:1 final drive in 4th. (and about .085: 1 in 5th.) for a final drive ratio in high of 3.70 on down to about 2.25:1 for big V-8s in light cars.

For maximum highway fuel mileage, you want the engine to be turning about 2500 RPM at your cruising speed.

Finally, here's how to calculate your speed in any gear, at any RPM.

Tire diameter in inches divided by 2 multiply by  3.14 , and multiply that by RPM for inches per minute. Divide that number by 12 for feet per minute, divide that by 60 for feet per second.  Divide that number by 88, and multiply by 60 for MPH.  Study this until you get it. There are probably easier ways but I worked that method out in my head while driving down the highway.

Good luck with your project. Please don't race on the streets.

Dan Liddy
Sarasota, Florida