# figuring out RPMs with new tires

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 85GMCJimmy, Aug 3, 2002.

1. ### 85GMCJimmyRegistered Member

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I am about to get a set of 33's, but when doing calculations when going at 45 mph, i would only be turing 1000 RPM. Did i do something wrong?? Which gear ratios do you use for your transmission? as far as first, second, third, and etc??? can someone please help me??

2. ### tackettbRegistered Member

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336 X axle ratio X mph divided by tire size.
Take a tape measure and measure the tire, don't go by what is stamped on the tire, most tires are up to 2 inches smaller than what they are stamped.
my 33's are actualy 31".
so my formula would be 336 x 4.1 x70 div by 31= 3110 rpm's at 70mph.

4. ### Pure Insanity1/2 ton status

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Or the easy way........ 4lo.com They have a gear calculator at the top of the page. /forums/images/icons/grin.gif

5. ### Grim-Reaper3/4 ton statusAuthor

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Randy's has a really nice one here.
http://www.randysringandpinion.com/calculators.shtml
I alway's put 1 inch less than the tire is marked.
For 3.08 in 3rd you would be turning 1494 and in OD 1019.
With 33's and a 700 R4 4.10 would be a good gear choice. Less stress on the tranny. 4.56 would not be a bad choice either. would let you go to 35's in the future.

6. ### Pure Insanity1/2 ton status

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Good one Grim. Its now in my favorites list. /forums/images/icons/cool.gif