question about tire size and rpm etc.

sgettin · Post by **sgettin** » September 24, 2011, 8:22 am

finally got my 63 f250 4x4 4sp 292 w/4:56's on the road after about a year in the barn getting worked on. was impressed by the speed and handling, it runs and drives like a really fast tractor(about as noisy also) but thats all fine with me. only thing i'd like to improve is top speed so i'm looking at bigger tires.
anyone have experience with 33's or 34's maybe 35's on my setup? i'm thinking tall and skinny like 255/85/r16 which are 33's. i would like to be able to do 60mph without running the rpm's up too much but with a small v8 like my 292 i'm also a little worried about loosing power. given the price of new tires thought i better get some advice.
clearence? i believe it was iceman who told me 34's are as tall as my truck can handle without a lift kit and i do have the large opening fenders on it now.

thanks for any help.
scott

charliemccraney · Post by **charliemccraney** » September 24, 2011, 9:09 am

You should be around 3100RPM at 60mph right now which isn't bad. The taller tire will impact your towing ability a bit but for cruising it will be fine. A stock 292 is quite torquey for it's size. That's a part of the reason they're so awesome

sgettin · Post by **sgettin** » September 24, 2011, 9:28 am

charliemccraney wrote:You should be around 3100RPM at 60mph right now which isn't bad. The taller tire will impact your towing ability a bit but for cruising it will be fine. A stock 292 is quite torquey for it's size. That's a part of the reason they're so awesome

so currently with small tires i'm running 3100rpm at 60, my speedo is broke but drove it down the highway yesturday and i was guessing somewhere around 3000rpm at what i thought was 60mph or there abouts. sounds like 33's would be about right. any guess on rpm's at 60 with 33's?
ya i like the 292 better than i thought i would. have say it has an odd sound to it(which i like), doesn't sound like the newer small v8's i've heard. sounds more like my 2020 jd.

charliemccraney · Post by **charliemccraney** » September 24, 2011, 10:03 am

That depends on the tire you're running now. To get a good approximation take 3100 x old diameter / new diameter. So lets say the old tire is 32" tall and the new one is 33" tall, 3100 x 32 / 33 = 3006, so that will take about 100 rpm off at the same road speed.

HiBoy63 · Post by **HiBoy63** » September 24, 2011, 10:14 am

http://www.onlineconversion.com/bigger_tires.htm

This might help you out.

jamesdfo · Post by **jamesdfo** » September 24, 2011, 10:19 am

If you want to take the easy way out, there are a number of online sites that will do the calc's for you, but if you want to run the numbers yourself, here is the formula. I used to have it in the SOLVER mode on my scientific calculator, which would then let me solve for any of the variables.

MPH = (Tire Diameter x RPM) / (336 x Axle Ratio)

http://bob123e.tripod.com/askbob/Gear_R ... ulator.htm

http://www.apexgarage.com/tech/gear_ratios.shtml

http://www.wallaceracing.com/Calculators.htm

HTH
James

factorystock · Post by **factorystock** » September 24, 2011, 11:10 am

sgettin wrote: i'm thinking tall and skinny

I would be thinking the factory optional 19.5". Check out an old auction ebay 110730278766.

sgettin · Post by **sgettin** » September 24, 2011, 12:50 pm

thanks everyone for the help. checked out the calculators and looks like with 33's i'll be running about 2800rpm at 60 (although i am little confused as to if i have to take into account my transmission gear ratio and to be honest i don't know what that is). anyway i can live with 2800rpm at 60mph. 35's would lower my rpms more but are pbly a bit much for my truck and tires that tall would might eat up too much horsepower for my 292 anyway.
checked out the ebay auction factory stock, interesting and probably rare. they would definately get me some height and skinny also with a 5.25" width.

thanks
scott

charliemccraney · Post by **charliemccraney** » September 24, 2011, 12:59 pm

sgettin wrote:35's would lower my rpms more but are pbly a bit much for my truck and tires that tall would might eat up too much horsepower for my 292 anyway.

thanks
scott

Mine turns about 2500rpm @60mph in 4th gear (1:1) and about 1750rpm in 5th (.63od). Yours should not have a problem with the 35s so long as they'll fit.

sgettin · Post by **sgettin** » September 24, 2011, 3:27 pm

charliemccraney wrote:
sgettin wrote:35's would lower my rpms more but are pbly a bit much for my truck and tires that tall would might eat up too much horsepower for my 292 anyway.

thanks
scott
Mine turns about 2500rpm @60mph in 4th gear (1:1) and about 1750rpm in 5th (.63od). Yours should not have a problem with the 35s so long as they'll fit.

The tire shop i deal with is pretty good to work with so i'll try to throw the monkey on the their back as far as fitment goes. guess i better stop by after work this week and see what they got.

jamesdfo · Post by **jamesdfo** » September 24, 2011, 4:32 pm

As long as the trans you are running does not have an overdriven top gear, you are OK (ie: your top gear is 1:1 ratio).
If your trans did have overdrive, and the online calculator you were using did not have a window to input the overdrive ratio, then you would simply take the answer (RPM) and multiply it by the overdrive ratio. (eg: 2800RPM x 0.7 = 1960 RPM), or alternatively for MPH, at the same RPM, you would divide the MPH by the overdrive ratio, ( 60 MPH/0.7) netting 85.7 MPH @ the same RPM as you would be turning @ 1:1 (direct drive).....

The trick is to find the sweet spot, as gearing too tall mans the RPM will fall too far below the torque peak, and that means at best it would feel "gutless" and at worst it would actually consume more fuel than it would at a higher RPM, but with less load........

James

sgettin wrote: i am little confused as to if i have to take into account my transmission gear ratio and to be honest i don't know what that is).

thanks
scott

ICEMAN6166 · Post by **ICEMAN6166** » September 24, 2011, 5:47 pm

fact:
my 66 f250 4x4 w 292, 4 speed and 4.56 turns 2900@ 70 mph w 255/16 tires.

292 was not offered in 66 so i have the only y powered 66 i know of.

sgettin · Post by **sgettin** » September 25, 2011, 6:23 am

[quote="jamesdfo"]As long as the trans you are running does not have an overdriven top gear, you are OK (ie: your top gear is 1:1 ratio).
If your trans did have overdrive, and the online calculator you were using did not have a window to input the overdrive ratio, then you would simply take the answer (RPM) and multiply it by the overdrive ratio. (eg: 2800RPM x 0.7 = 1960 RPM), or alternatively for MPH, at the same RPM, you would divide the MPH by the overdrive ratio, ( 60 MPH/0.7) netting 85.7 MPH @ the same RPM as you would be turning @ 1:1 (direct drive).....

The trick is to find the sweet spot, as gearing too tall mans the RPM will fall too far below the torque peak, and that means at best it would feel "gutless" and at worst it would actually consume more fuel than it would at a higher RPM, but with less load........

James

thanks james for clearing that up concerning the transmision ratio. had me stumped but at a 1 to 1 ratio apparently it cancels itself out. finding the sweet spot you're referring to is difficult because i have no experience with slicks. from what folks are telling me sounds like 33's will fit the truck and still keep my power range in the right place or the truck would pbly run good on 35's also.

qoute iceman:
fact:
my 66 f250 4x4 w 292, 4 speed and 4.56 turns 2900@ 70 mph w 255/16 tires.

got me a little stumped because the calculator says your diff should be more like a 370:1 inorder to run 2900@70 but if thats what is then thats what it is. 2900 @ 70 would work for me.
i like the looks of 35's but i think they may rub and i just get the feeling i'll loose to much power. i remember my brother upgrading his jeep cherokee to tires an inch or two taller and he wished he had never done it, lost a lot of power.
i've looked on craigslist for some used junk tires to try out but haven't seen any thing cheap yet.

thanks for the input.

charliemccraney · Post by **charliemccraney** » September 25, 2011, 10:06 am

Your stock 292 should pull the 35s just fine. You should still be a good bit over 2000rpm. The way my motor is built, it lost a noticeable amount of torque on the low end but it still manages to pull 60mph at about 1750 rpm in overdrive. A stock 292 with a stronger low end will not have trouble with that same road speed at a higher rpm.

RE the descrepancy between observed RPM and calculated RPM, the calculators do not take into account what I'll call the functional diameter of the tire, which is the diameter which varies with the load and air pressure. The calculators are best as an approximation and while they may not provide the exact RPM you'll see, the difference in RPM should be pretty close.

ICEMAN6166 · Post by **ICEMAN6166** » September 25, 2011, 10:10 am

my tires are actually 265s, not sure how much that changes the calculation but there is no fitment issues.

will be swapping to winter tires soon,
8.75 x16.5, will see how much that changes the rpms

jamesdfo · Post by **jamesdfo** » September 25, 2011, 11:49 am

Brian: you didn't mention the aspect ratio?? (ie: 265/75/16?)...75% aspect ratio is likely the safest bet, but it could be one of the others:)

James

jamon8 · Post by **jamon8** » September 25, 2011, 1:02 pm

how do you change the gears in the tailshaft to have the speedo work with bigger tires

question about tire size and rpm etc.

question about tire size and rpm etc.

Re: question about tire size and rpm etc.