deathsled

I had the 66 out on the Interstate on Sunday and she's burbling along nicely but at 65-70 mph, the revs are at a minimum of 4,000. She has a 3:50 rear end and a four speed close ratio toploader with high nickel content Richmond gears, 9 inch rear end and Detroit locker. Wondering if revs like that over a sustained period, like say a trip to the vintage races at Road America would be punishing to the engine. BTW, engine is a late model (Fox) 5.0 with roller cam and roller rockers. I believe the pistons to be forged. Your thoughts please.

Thanks,
Richard E.

67mustang302

With pushrod engines the real limiter is the valvetrain. Depends on your engine, but cruising at that rpm should be ok. Sucks for mileage though, and will make the engine wear faster.

tx65coupe

I would not want to cruise on the highway above 3000 rpm. I'd rather it be around 2500 or less.

I wouldn't think that 3.50 gears with out overdrive would equate to 4000 rpms at highway speeds.

I have 3.50 gears with a T5 and RPMs are not anywhere near that high in 4th at that speed.

palerider

That was my thougth too. I have a 3 speed with 3.44 gears - trac-loc and do about 3200 at that speed.

Iskwezm

it wont hurt anything except for your wallet (for mpg). I have buddies that drive regularly with 4.11's and 5.13's without overdrive.

Starfury

I agree. Don't worry, as long as you can afford it.

I have 3.25's and 24" (short) tires on my '67 and I cruise at ~3300rpm @ 70mph for extended periods of time. It's a bit noisy, but it doesn't really hurt anything.

lil bull

The more rpms you turn will wear out the internals of the engine faster. That is why new cars with overdrive hit 100k miles like it is nothing. In the old days 100k and the engines were wore out.

67mustang302

That has nothing to do with it actually. Old engines had old ring technology and old oil technology, so friction and wear were much higher.

Some newer cars cruise in 5th or 6th on the highway at 3k+ rpm.

Ring technology has changed a LOT in the last 20 years.

Iskwezm

I agree. I sold my 06 GT with 4.30s and 100K mikes.Turned about 3200 at 70 mph. My truck had 4.56's and 170k miles.