RPM = (336/Tire Diameter) x MPH x Gear ratio and this.......... Larger tires have an dramatic effect on final gear ratio. If the gear ratio and vehicle speed remain constant, then putting a larger tire on the truck will make the engine turn slower to move the same distance. If the engine turns too slow it will have poor power and perhaps even worse fuel economy than if it turned too fast. A good rule of thumb for determining which RPM your engine will run best, is to look at where the engine reaches peak torque. This can be easily found in brochures or service manuals at your dealer or by visiting your local engine builder. In most cases best economy and performance can be obtained by making sure the engine is turning in the peak torque range for your engine at an average cruising speed. If you drive on highways more often your average cruising speed will be higher than someone who drives on country roads. I got this stuff from Dynatrac's web site, So from what I can tell I was DEAD wrong in thinking bigger tires increases your RPM it looks like it DEcreases RPM!! I am way corn fuzed now........ Rock ON! 72 K5

Naaaaawwwwwww. I think they have it reversed. You are in control of your engine speed not the tires right? So if you keep your engine speed at le'ts say 2500RPM and you have 33's you can move "X" distance. If you have 35's you'll move X+whatever extra diameter is on the 35's right? So you can move the truck farther with the bigger meats at the same RPM's right? Now I don't know a flippin thing about how the power consumption from having to move bigger tires effects this whole thing, but...If you have to recalibrate your speedo because it's reading a lower speed with the bigger tires then you are going faster (or farther) than you did with the smaller tires right? Now we're both confused! <font color=red>Steve88</font color=red> 88K5 Silverado thunderdog@metallica.com

BOMB!!! Brain Fart!!!! That formula doesn't consider tranny gears eather??? Ho well I guess the best way is to buy the tires you want if you lose your power the get a gear change..... There goes another years worth of saving$$ and planning!!!! Ohwell one year at a time! Hey by the time I'm forty my rig should be preety SWEET Rock ON! 72 K5

I was informed that all factors remaining the same, if you're running 4.11's with 31" tires, then go to 33's,, you effectivly make your 4.11's, approx 3.90's,, therefor lowering your rmp at the same speed. Todd 78/87 Alaska Bound

Hmm I have seen another formula for this somewhere before. I think on a wall at a drive train shop. I remember it differently though.

Formula's pretty, but as somebody else pointed out, doesn't take tranny gearing into consideration. If you have a TH350 or TH400 with a 1:1 high gear, OK, it's fine -- but if you have a 700R4 with what, .7:1 or .9:1 or whatever overdrive is, you're SOL. I broke down and did the math once, geometry being one of my not-so-weak points in school... but it was way ugly. I can happily provide the formula in varying levels of complexity should anybody want it (and, err, when I get home tonight 'cuz I'm at work right now. ;-) -- A '74 K5 Blazer "Tank Grrl" '77 Chalet sans Blazer and more to come...

I believe at the following site you will find the formula and answers....http://www.thegrid.net/thedrivetrainpage<font color=blue> <font color=green>Eagle86K5 <font color=red>Only guy I know that can get out of line in a one car funeral

You don't need to get into formulas to understand it. If you ever rode those 12 or 18 speed bikes, see it this way: The rear gears are how tall your tire is, the front gears is your axle gear ratio. Increasing the tire size, is like shifting into a larger size gear in the rear of the bike. You will pedal slower, but each stroke is harder to push. So you reduce the gear ratio to compensate, that's like shifting to a smaller diameter gear in the front gears of the bike. That allows you to pedal faster, but each stroke is easier. Hope this makes you see everything clearer. <font color=black>////// What the heck!...Drive it like gas is $0.50/gal!!!</font color=black>

i think it was harder to pedal when the rear gear went to a smaller sprocket..... shaggyk5 aint nuthin in the world like a heavy chevy!!

Yes, the formula does not take into account transmission gearing, but the formula is correct. It is assuming the vehicle is driving in direct drive (1:1, such as the top gear in a TH350 or 400). The only thing you need to do to compensate for the transmission gearing is to multiply the answer by the trans gearing. For example a 700r4 with a .70 overdrive gear, the formula is [(336/tire dia.) x mph x ratio x trans ratio], with trans ratio = .7. Using the example with 31" tires, 3.73 rearend ratio, and 60 mph gives you about 2,400 rpm in direct drive. Multiply this by .7 for the overdrive and you have 1,700 rpm. Larger tires will lower the rpm for a given MPH speed...using the above example, if you replaced the 31" tires with 36" tires you would now run 2,100 rpm in direct drive and 1,500 rpm in OD at 60 mph. An easier way to estimate how larger tires will effect the rpm at a certain speed is to simply divide the old tire size by the new tire size (old size/new size) and then multiply this number by the original RPM. Again using the above example, the truck ran at 2,400 rpm at 60 mph with 31's. Then (31/36) = .86. Multiply 2,400 x .86 and you have right around 2,100 rpm at 60 mph with 36" tires. Clear as mud..........

Shaggy you are right. Trust me I ride 15 miles every morning. The smaller the ring in the back the harder it is to pedal, but the faster you will go. The larger the ring in the back the slower you will go, but it's easier. So you hit the big rings in back for downhills and the small rings in back for the rock climbing. Now back to bigger tires: So I'm right you go faster with bigger tires at the same RPM all else equal... <font color=red>Steve88</font color=red> 88K5 Silverado thunderdog@metallica.com

You're absolutely right... Silly me. <font color=black>////// What the heck!...Drive it like gas is $0.50/gal!!!</font color=black>