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post #1 of (permalink) Old 08-04-2000, 09:14 PM
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tire size?

a friend of mine want`s to change his 35x12.5 tires on 15x10 wheels back to the stock-16x7 wheel and 285? tire.what effects will the 456 gearing have as far as freeway driving/town driving etc...
will the motor wind at to high a rpm?(he has a 88 gmc with a 5.7 motor.

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post #2 of (permalink) Old 08-06-2000, 09:56 PM
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Re: tire size?

<font color=purple>Basically, he'd be switching from 35's to 33's. Assuming he has an automatic with overdrive, that wouldn't be too bad. Running in O/D, 4.56's have the effect of running 3.19's (again, assuming the automatic as O/D isn't as drastic with the 5-speed). Since the "highway" gears in the pre-overdrive days were 3.08's & those would come with 31's stock, no big deal. If he has a 5-speed, or no overdrive, 4.56's may be a bit much for 285/75's.

To do some math, let's assume that his 35's really are 35" & that his replacement 285's are a true 33". And let's assume the TH700 automatic with a .70 overdrive 4th gear. At 60 MPH with the 35's:

4.56*.70*60*336/35 = 1,838 RPM's

With the 285's:

4.56*.70*60*336/33 = 1,950 RPM's.

Not a big difference & certainly not high RPM's in either case. Assuming NO overdrive:

4.56*60*336/35 = 2,626 RPM's with 35's

4.56*60*336/33 = 2,785 RPM's with 285's.

In this case, he's already pushing it with the 35's & I wouldn't want to drop down in tire size.

Is this any help?</font color=purple>



post #3 of (permalink) Old 08-06-2000, 10:07 PM
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Re: tire size?

Well, one thing you friend can look forward to is the ease that it will take to spin the 285's at take off[img]/wwwthreads_images/icons/smile.gif[/img]. What Tex was talking about makes perfect sense and actually gave me an idea of what my truck would be reving at with 33's only my truck is a 5-speed with 4:10's. I have thought already about borrowing my dads tires (265's) off his truck for a day or so and seeing how they preform. The only thing I would be worrying about would be burning off his brand new tires seeing I can do quite the smoke show with my 35's[img]/wwwthreads_images/icons/smile.gif[/img]

Going to the 33's though should not be a problem.

Good luck...Ryan

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