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Posted (edited)

I just got a pair of used 245/75 R16 tires from a friend. So I mounted them up on my old aluminum mags and mount the 245's on rear axle of the truck. 

 

Selection_042.png

 

They are a bit shorter in height. The gains are great!

 

Engine load is reduced by 5% to 10% roughly. 

EGT's are about 50*F lower.

Acceleration is way better and faster. 

Cruising RPM's are right about 1,960 RPM at 65 MPH.

 

I've got to reset my cruise timing limit on the Quadzilla I need to drop from my current 25% to 20% it was holding cruise timing climbing about 2-3% grade. 

 

I'm going to document MPG usage as well in the coming days. 

Edited by Mopar1973Man

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Posted (edited)

Isn't the 245/75R16 what is spec'd on the tire data sticker in our doors? This is the size I have on my 2 wheel drive '97 and I definitely like the acceleration of it compared to my 2001 4 wheel drive with 265/75R16. 

IMG_20180624_201526.jpg.c1a7cf57924cb5f333853f16c1b17bba.jpg

Edited by 04Mach1

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Yeah, it is the door sticker size. Mine was sold with the 265's from the factory. I opted for the 235's to reduce steering wear and front axle wear. Which it did. Now I'm looking to gain a bit more by changing the ratio up. Instead of like 99% of the people here that go bigger like 285's, 35" inch and 37" inch tires I found that with 3.55 gears and 245's is an awesome setup. 

 

Here is a tweak I'm going to do to the 1996 Dodge. I found out it was manufactured with 245's as well but I'm going to switch that truck from the 235/85 R16 to 215/85 R16. That is the same as 245/75 R16 just an inch narrower. This will improve the MPG's on that truck and give better performance on snow-covered roads. That truck is about 2,000 pounds lighter than my 2002 Dodge and the wider tire tend to make it float on top of the slush and snow. The cruise RPM on that truck is too low and it struggles to pull in 4th gear. 

 

Selection_043.png

 

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Yeah. 245/75R16 is definitely the sweet spot for 3.55 ratio. The old 12 valve can get 30+ highway mpg if I behave and keep it at 65 mph. The 24 valve with 265/75R16 will get 23-24 mpg if I baby it at 65 mph and treat the throttle like there is a raw egg under it. Both have 3.55 gears.

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I paid an upgrade charge for the 265's when I bought mine. Just another option. I had the 245's on my 96. It had a V10 so no good comparison. 

 

@Mopar1973Man which engine do you have in the 96?

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Been doing some looking for a half ton. Finding a lot 318's, several 360's, even a couple 6 cylinders. The only 360 I ever had was in a 1 ton 15 passenger van. That got  almost as bad a mileage as my V10. Of course it was pulling a good bit more weight and not very aero dynamic.

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Talk about a huge difference 1 inch of tire makes.

 

EGTs are lower.

Engine load is lower.

Transmission temp is lower.

Exhaust brake performance is better.

Brake performance is way better.

Can hold 5th gear wider speed range.

Acceleration is quicker.

Less stress on the clutch during launch.

 

Just in the 120 miles I'm loving 245's over my 235's. Very impressed with the changes. Now I'm having hard time understanding people with 35 or 37 inch tires with 3.55 gears. I'm going to start fuel logs on my next tank.

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1960 RPM's at 65? Wow! The Chummy cruises 1860 RPM's at 80 with 265/70 r17. I guess the sixth gear (.61) in the Allison really does keep the RPM's down. Thank God I don't pay for my own fuel anymore! Sounds like I'll be in the market for some 3.73's when i jump up a couple sizes in the rubber.

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Realizing that now. Still, in all, I love the final ratio it way better. This truck screams now. I had a 4th gen attempt overtake me on a merge and I outran him. No problem put a 4th gen truck in his place behind me. Acceleration is wicked fast now. Then caught up with a friend for lunch down in Payette and we left together I tried to power through a left turn from a light and the tires let go really easy. Sure is amazing how much more power can be put to the ground by ditching ONE INCH of tire height. I know why the traction sucks are actually two-fold. One is the tires are extremely old and the rubber is really hard. Then the other part is I did not do the math on axle weight to tire pressure so I'm most likely overinflated. 

 

Still sit here and wonder why the 35-inch and 37-inch tires are so popular. Steal the power, put more stress on driveline and transmissions. Etc. 

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3 hours ago, Mopar1973Man said:

Still sit here and wonder why the 35-inch and 37-inch tires are so popular. Steal the power, put more stress on driveline and transmissions. Etc. 

 

Some people just wanna watch the world burn :evilgrin:

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Posted (edited)

I surprised you can even notice any difference in the rake. The over all size being 1.2" smaller would only amount to lowering the rear only .6"

Edited by dripley

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24 minutes ago, dripley said:

surprised you can even notice any difference in the rake.

 

You do notice that the nose feels high like the hood got a bit longer visually. Nothing super noticeable but still feels different. 

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I have never messed withe rake except hauling the 5th wheel. Other the my headlights aimed a little higher I don't notice a difference then. I know I would not notice 5/8". Just me I guess.

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@Mopar1973Man all this talk of rpm's, egt's, and engine load got me to thinking...I'm still fairly new to the diesel world but I understand that more fuel means more heat in a diesel, opposite a gasoline engine. So would it be safe to say that, as a snapshot of engine efficiency, egt's could be used much the same way a vacuum gage is used in a gasoline engine to determine fuel efficiency? What is your egt cruising down the freeway at 65 with your new setup? 

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Friendly reminder.  Before you tow anything heavy... check max load of your new tires, stamped on sidewall and actual load.  Leaving at least 10 percent bewteen those numbers for safety.  Dont think you mentioned the load range of new tires? 

 

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8 minutes ago, 015point9 said:

Friendly reminder.  Before you tow anything heavy... check max load of your new tires, stamped on sidewall and actual load.  Leaving at least 10 percent bewteen those numbers for safety.  Dont think you mentioned the load range of new tires? 

4

 

Already done that too. 245's I've got right now are rated 3,042 pounds at 80 PSI Load Range E's

 

38 minutes ago, Scottfunk said:

@Mopar1973Man all this talk of rpm's, egt's, and engine load got me to thinking...I'm still fairly new to the diesel world but I understand that more fuel means more heat in a diesel, opposite a gasoline engine. So would it be safe to say that, as a snapshot of engine efficiency, egt's could be used much the same way a vacuum gage is used in a gasoline engine to determine fuel efficiency? What is your egt cruising down the freeway at 65 with your new setup? 

 

You want to use Engine Load. Engine load is the amount of fuel being injected. EGT's are not a good way being just a small hill and EGT's could climb to 800*F but the engine load could be 20%. Then flatten back out and have EGT's now down to 550*F and engine load of 17%. I'm going to have to capture a few fresh screenshots.

 

 

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Posted (edited)
On 7/3/2018 at 10:34 AM, Mopar1973Man said:

Talk about a huge difference 1 inch of tire makes.

 

EGTs are lower.

Engine load is lower.

Transmission temp is lower.

Exhaust brake performance is better.

Brake performance is way better.

Can hold 5th gear wider speed range.

Acceleration is quicker.

Less stress on the clutch during launch.

 

Just in the 120 miles I'm loving 245's over my 235's. Very impressed with the changes. Now I'm having hard time understanding people with 35 or 37 inch tires with 3.55 gears. I'm going to start fuel logs on my next tank.

 

 

It's great for improving the overall gearing, and it will tow very nice as well. You'll top out in 4th sooner, but towing in 5th is easier. 

 

I went from 285/70R17 to 265/70R17 and lost hill pulling speed, but gained ease on the interstate at 65, then I went to 245/70R19.5 and lost some ability at 60-65 in 6th, but can pull hills at ~2500 and 65 all day long in 5th. 

 

It's all a numbers game finding that optimal rpm for the speed.

 

Downshift to to 4th at 60 and you'll see the same results with EGT, load, trans temp, exhaust brake performance, etc... but you'll actually be burning more fuel. 

 

Regardless of what gear you are in it takes "x" hp to run down the road at a given speed. If you increase the drag in the motor by changing rpms you have increased the hp and fuel requirements and it's a loss. So it really is a sweet spot, but 1950 is a great cruise rpm. I suspect that for your driving routing to Ontario and back all the time you'll notice a slight increase in economy. If you spent more time at 75-85 on the interstate you would likely notice a decrease in fuel economy. 

3 hours ago, Mopar1973Man said:

 

You want to use Engine Load. 

 

Correct, but don't forget that 20% load at 1800 rpms is less total fuel than at 2200 rpms, At 1800 rpms you're burning 81.8% of the fuel as 2200 rpms even for the same load. 

 

 

 

 

Edited by AH64ID
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