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RPM question.

Nosferatu49534

Nosferatu49534

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To start I'm not sure where to post this question but I figure rpm question = engine. So my question is currently stock tires for my truck are 245/75- 16. I'm wondering if I bump up a size how much would that bring down my rpms on the highway. Right now it's around 2200 at 70mph. I've searched for a calculator but I can only find them to give me new dimensions. The truck has 3.73 gear ratio.
 
I'm not able to help you with actual experience. I have 265/75 16 tires, but my pickup has 4.11 gears. But, try this gear ratio calculator and see if it helps. It's designed for 4X4's, but you should be able to make it work for 2 wheel drive if that's what you have.

http://grimmjeeper.com/

Don
 
I would be cautious of going to a wider tire as it will increase drag and parasitic drag. You could go with a 235/85/16 which would drop your rpm's about a 100 at 70. Not really worth it unless you are in need of new tires.
 
A gasser alt pulley will drop your RPM about 200. Probably not the way you want to do it though right?
 
Are you after the taller tires for more mpg? Your starting to choke the turbo above that speed, but 4" exhaust can help a bit. Beyond that the old turbo heave ho is where it will really help.

Don't get me wrong I'm all for big tires- I like my 37's!
 
Ferm, I don't want wider I was just thinking a little taller. More mpgs really isn't my main concern it is just the lower rpms to take ease some of the stress on an old engine. If That would accomplish it without wrecking anything else That was my thinking but I could be way wrong. I have a hard time wrapping my head around all the changes from tires to gears to rpms. And so I have this rig,ht, the 245 is the width and 75 is the height? Oh and I desperately need new tires. The fronts were destroyed by me not realizing the whole front end was screwed when I bought the truck. Plus the po out cheep off brand tires on it. Exhaust is in the plans for sure, I think I'll stick with stock for now and many go a little bigger next summer if I came find the rims I want.

Thanks guys.
 
Yes, 245 is the width (in mm, divide by 25.4 to get inches), but the 75 is a percent of the width. 75% of 245 is about 184mm. This 184 is the height of the wall of the tire (from the rim to the road) and the 16 is the diam of the rim.

So the 245/75-16 tire is about 30.5 inches tall and 9.6 inches wide.
 
To help you get your head around this, consider that your driveline doesn't know you've changed tire sizes (except for a little more work with a fatter and/or taller tire). Your speedometer isn't concerned with tire size, it's concerned with RPMs measured at a certain point - ours happens to be through the alternator, I believe. All that said, if you have 2200 RPMs at 70 mph on the speedometer with little tires, you'll still have 2200 RPMs with large tires at 70 mph on the speedometer. The difference is that if you install larger tires, then the body of the truck is moving through the air faster if these two conditions still exist, thus if you have a radar check your speed, then you'll see how your speedo says 70, but the radar may say 73.

In other terms, with a tall tire, you'll be able to set the speedo at 67-68 and you'll still be doing 70 mph in real life.
 
Corrected vssb at what size tire would be the question.

If you raise the tire height enough to make the rpm 2000, it isnt going to help a tremendous IMO. If you do a lot of highway miles then yeah, just remember your working the engine a little harder to get it up to speed in order to take it a little easier at top speed. Getting a highway tread instead of offroad pattern will help also. Rolling resistance on the highway requires more happy pedal to maintain the speed.

After recouping from the $ hit for tires, if you want to take load off the engine to extend its life- new harmonic balancer & drive pulley up front are big. Focus on known weakness of the engine from catastrophic failure. That and overheating are the big 2. I'm just guessing where your at the temp isnt your panic button. Then balanced injectors so there is as equal firing load on the engine as can be, which will also allow it to produce the same amount of power with less fuel burning off through the valves.

Im not trying to side track your tire issue at the moment, just trying to say keep planning for the future and be ready to snap up a good deal when one shows up btw.
 
I'll take all the info and preventative maintenance and planning I can get. I bought this truck with the plan of not getting another one. Except for toys of course. [emoji3]
 
Best way to get the load off this engine is get rid of the Asthma Attack turbo and quit making the engine shove it's exhaust through that small GMx turbo. Next is cooling system upgrades for longer ring life depending on what exactly your truck has.

Last, Don't be afraid to rev this "high speed diesel" up. Everyone wants to lug the hell out of this engine, but, it's not a long stroke Cummins and frankly the 6.5 hates it. The turbo is only good till 2200 RPM and the auto trans won't keep the 6.5 under that RPM anyway - just no power there either. It smokes at low RPM high throttle (and smoke dumps soot in the oil) and doesn't have enough fan/water pump RPM. Once you have a turbo that works over 2200 RPM you will have a different truck.
 
Aussie, the calculat is sweet. I was playing with it last week.

WW, in the short time I've had this truck I've definitely learned where to keep my foot so she doesn't smoke like mad. :) and also where the turbo cuts out. It seems kind of sad that it has such a narrow band to work with. Before I upgrade the turbo I have a few other things on my list, on order from Leroy at the moment is a new lift pump, filter and harness then I have trans, pyro, and boost gauges to install. Plus the cooling up grades,

Question as well, would the new rockers Leroy has be a good upgrade as well as head studs? I bought a spare set of heads a couple weeks ago and thinking about saving for the rockers and studs and swap those at the same time,
 
Buy the turbo before the rockers. The turbo will pay for itself in improved MPG esp. if used loaded or towing. I think Dennis is in your area or ask another member with an improved turbo to take a spin. Warning: this will result in your GMx turbo coming off as scrap metal...

I would then recommend a higher stall converter to bring the engine out of the low RPM dead band and take advantage of the new high RPM power if you do a lot of DD. The factory converter clutch is a weak point anyway. Not required as tuning does a lot to help nowadays.

Factory is idle to 2200 RPM on the turbo - flat lands low load low speed design. (Yeah on a high speed diesel...) And even that runs excessive boost/restriction. The factory turbo quickly goes above a 2:1 drive pressure ratio with 28 PSI in the exhaust to get 14 PSI of boost. The ATT is a drive pressure ratio of 1.1:1. This turbo improvement is where the MPG gets better, cooling load goes down, and power is freed up. But, the ATT doesn't light until 2000 RPM hence the need to slip the engine to higher RPM with a different high stall converter. When you don't need the power the high stall converter locks up.

Many turbo's out there now to choose from over the specific example I gave. Most all of them are an improvement over the GMx with the ATT being the largest turbo for the full load towing pulls like a freight train past redline. The GMX as noted is the low RPM turbo that chokes past 2200 RPM. Everything else is in between.
 
I went to the 265/75/16's on my Suburban, and am about to on my 2500 ECLB, but both have 4.10's. On yours with 3.73 you may begin to feel the loss of towing power. I'm with you on wanting to get highway rpm down a bit, I put a lot of highway miles on my Sub and running 2,400 rpm for hours gets old, just that few hundred rpm drop makes a big difference, at least for me.
 
Dave01 said:
I went to the 265/75/16's on my Suburban, and am about to on my 2500 ECLB, but both have 4.10's. On yours with 3.73 you may begin to feel the loss of towing power. I'm with you on wanting to get highway rpm down a bit, I put a lot of highway miles on my Sub and running 2,400 rpm for hours gets old, just that few hundred rpm drop makes a big difference, at least for me.
Click to expand...
My BURB had 265/75/16's when I bought it, and the milage was dismal at best. Got about 14-15 on the highway at 65-70. Switched back over to 245/75/16's when I put tires on it, and milage picked up 1-3 MPG. 235/85/16's is what I wanted to try, but I stick with 245's as they work well. Also I felt the brakes were overworked with 265's on it VS the 245's.
 
When I bought the Sub it had 235/85/16's on it briefly, they were shot. They are virtually the same size as the 265/75/16's and would clearly roll easier. I agree, the bigger tires kill mileage and I saw that on the Sub, dropped from 16-17.5 mpg to 15-16. Can't say I felt a braking difference. On the ECLB I'm going to put the 265's on just for kicks, like the look, like the lower rpm at cruise, we'll see if I feel much difference in towing/plowing. I did pass inspection but the 245's on it can't go another winter.

If I ever look for one of these trucks that I plan to put a lot of highway miles on, I'd be curious to try a 1500 with the taller gear option, can't remember, was it 3.43? Keep the stock 245's and see what mpg and how quiet a highway cruise it is at 70 plus.
 
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