Horsepower vs Range, Rear Axle Endplay

Q I’ve always heard that cars with higher horsepower engines get lower miles per tankful. The new electric cars seem quite powerful but have a limited range. Why don’t the manufacturers use less powerful motors and thus extend the range of the vehicles.


A There are quite a few variables regarding this assumption. A powerful sports car in many cases may have a small/medium size fuel tank as a space and weight saving strategy, reducing range. On the other hand modern high performance engines can return some great fuel economy numbers if driven conservatively. Looking at the Chevy Camaro, Ford Mustang, Porsche 911, Chevy Corvette, with the more powerful engine options, all can do mid 20s MPG or better on the highway thanks to lots of gears, variable valve timing, the use of turbochargers and other efficiency tricks. This is practically double that of similar cars twenty years back. City driving is a huge variable as driver habits and stop-go traffic can drop fuel economy perhaps in half.

Electric vehicles do offer amazing torque, especially at take off and low speed, resulting in great acceleration performance. As battery technology improves and cost is reduced, driving range will certainly improve. Recent entry level full-electric vehicles have jumped from 100 mile to 200+ mile range, while an upper-end electric such as Tesla’s Model X 100D comes close to 300 miles. It all depends on how much battery one desires to buy. Of course these numbers depend on driving habits and road environment.

Q I just had new tires installed on my ‘09 F-150. The guy called me back to show me a clunking and movement as he pulled in and out on each rear wheel. It did sound terrible and the wheels moved a noticeable amount. He said I should have this checked right away by a “rear end place”. What do you think is wrong?


A My guess is your truck is equipped with the 8.8” Traction Lock rear differential, which uses C-clips to retain the rear axles. A small amount of axle shaft end play is normal, about .025”. When a wheel/tire is tugged and pushed in/out, a clunking noise will be heard, which is normal. The rear brake drums act like a speaker cone, amplifying the sound.

As the miles pile up, wear occurs in the axle groove/c-clip increasing axle end play. A larger factor is wear of the Traction Lock clutch plates, which, if severe, can increase axle end play by 10X or more. These sandwich behind the differential side gears (easy to renew and about $100 for the kit) and are your likely culprit. Often times wear is severe enough one can grab the rear of the pickup bed and push/pull back and forth to cause the alarming clunk!

Excessive rear axle shaft end play is not a real big deal but does upset the contact pattern of the differential side and spider gears, increasing wear. I’d have the truck checked by a full service shop to better understand the severity of the symptom. You’re probably looking at a two hour job to inspect internals and renew the clutches and fluid.  

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