The Stovebolt Site Forums The Parking Lot Tons 'o Fun Gear ratio to mph info

I got my 59 ton finished the other day finally. It runs great. But the 70's 350 turbo tranny and the original 59 speedo aren't jiving too well I guess I need to change the plastic speedo gear in the tail section of the tranny. I put a nice tach in the cab so I can check my rpm. At 2000 the speedo says 80mph I know that's wrong. What I need to know is what the true mph would be at 2000--3000 rpm. I have a semi stock 283 with stock 350 turbo tranny and stock 5.14 rear end. I'm worried about the rear end being stressed at any high mph. I'm gonna use the tach to tell me how fast I'm going to keep everything from blowing up. Cause that would be bad Does anyone know the formula for rpm to mph

Look up your tire size on the Tire Rack. The spec will give you revs per mile. Multiply that by your rearend ratio and you have your driveshaft rpm at 60 mph.

Thanks truckernix but I don't think they would have my crazy tire size on tire rack 8r17.5 maybe. I'll try

I found the info on Mickey Thompson,s tire site that said 17.5 tire would be 1222 revs per mile. If you times that by 5.14 gear ratio you get 6281.08 that can't be right 6000 rpm at 60 that's kaboom time

Try this one....

http://www.ringpinion.com/Calculators/Calc_RPM.aspx

Yea that formula shows 5573 at 55 mph that still seems so high I have been running at 2500 rpm and it showed 50 on my speedo app on my phone that's not to scientific but a lot lower rpm. I'm pretty sure I have a 5.14 rear It's a 59 3800 former tanker fire truck when it had a 4speed it could climb a mountain real slow

see this table, you need your tire diameter, unless you have 40+" dia tires you're tach is wrong

Bill

get someone to go down the hiway with you side by side,
raise your hand to mark where you want you rpm to be,
then he can look at his speedometer and give you a pretty good idea of your speed...

you have a 5:14 rear end with a 70 350 transmissison, bet your true speed an't much..

what happen to the four speed in it..

This is 2014
Have a smart phone? If not you certianly know someone that does.
Download a speedometer app, try it.

For a gear calculator I like this one
http://www.onlineconversion.com/bigger_tires.htm
For a pretty accurate tire diameter look up your actual tire specs from the manufacturer, see the effective loaded diameter, or revolutions per mile and calculate back to that diameter. Or put a mark on the floor and your tire, roll the truck forward one tire revolution, measure that length on the floor (tire circumference), divide by 3.1415 to get diameter.
I would not trust very much any "diameter" some site or rule of thumb suggest based on the generic sidewall markings, there will be variations.


Grigg

Ok red58 I checked out your site and the great info there. I measured my 8r17.5 stock tires they came out to be 31 inches timed that by .12 which gave me a rear end gear ratio of 3.73. Not a 5.13 I thought for sure I had a 5.13 but at 2500+ rpm I'm probably doing 55 maybe 60 did they make a 1-ton with a 3.73 I though 4.56, 4.88, 5.13 were what was offered on a 1ton 9ft bed single wheel rear truck If I have a 5.13 my rpm would be 3614 if I was running that fast I would now it and it wouldn't last to long I'm kinda confused I'm starting to think just having my wife drive next to me and tell me how fast I'm going like joker suggested

a 59 1T would have come with a 5.14 stock, 4.56 was the 3/4T, the 1/2T could have had a 3.9, 3.7 or 3.38, if it does have a 3.7x ratio, it's not a 1T rear, but you have to know the ratio for that chart, it's not meant to figure out the ratio

I think your tach is off and you just don't have enough faith in how a 283 winds up, they love it .... you need to confirm the rear ratio [pull the cover and count the teeth or jack up one side and turn the up tire]

Bill

I'm sure it would have a 5.14 it was a brush fire truck with large water tank pto tranny, lights , and all the fire stuff the rear is original to the truck are you telling me that a 283 will be fine running in the 3000+ rpm range? With a 5.14 rear ?

I have 4.10 gears in mine; they came in later trucks and are a direct swap. You just unbolt the third member (google) and bolt the new one in. With that 7.06:1 first gear, the PO may have very well decided that the deeper gears just weren't needed.

I don't see any problem with a 283 running 3,000RPM; that's what they did when they were new, so why not? It'll no doubt be thirstier, but I think you'll really have to wind a healthy 283 up pretty darn tight before you actually hurt it.

I run about 2,400RPM @ 55MPH and 2,700 @ 60 in my truck with 4.10 gears (and similar height tires--32"), so I'd guess that's probably what you have. FWIW, my truck also runs 70 fine @ 3,100. Not ideal, but it works. (I have a 250 inline six in it right now.)

If you go to randys ring and pinion they have a page that you can put in your numbers and it will tell you what you need.

Just get a GPS and check your MPH with it. Most will show MPH and just check your tach to know how fast you are going at a given rpm. If you still have the 5.14 gears then you will be pulling high rpm at a relatively slow speed. There are many different calculators to check this out. I personally would put an overdrive in if I was not going to change the rear gears. The .7 OD in a 700R4 would effectively change your final drive ratio from 5.14 to 3.6 which would be better than changing to 4.10 gears with the original 4 speed standard or your turbo 350 which would then be a final drive of 4.10 because they are both 1:1 final gears.