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. 

 

 

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 July 2, 20186 yr by Mopar1973Man

  On 7/13/2018 at 2:32 PM, Mopar1973Man said:

Remember higher the ratio number the higher in the RPM for said speed. This is why I'm running a pair of junk tires on the back of my truck for about a month to find out if I like the ratio or not. So far I love this 245's set up and think the 215's on the 1/2 ton will be awesome too.

The main reason I want bigger tires is lower rpm which means more mpg right? I’m thinking 3:69 is going to be the cats meow. I can obtain that ratio with the 285/75/17. 

Actually, you want to optimize your RPM so your highway travel is around 1,900 to 2,000 RPM.

 

Most of this comes from @cajflynn back in the day his old 2001 Dodge was a 3.73 ratio axle and towed like a dream. Then after see all the 3.55 geared truck and 37" tire troubles. Why I was not really messing with tire sizes. Now after a bit a experience and I'm jumping out to optimize the tires to gears to make the best setup. Remember bigger tires have more rolling resistance especially with 285mm wide face then with all the steel belt you going to have more rotational mass. Just be aware.

 

@Marcus2000monster

I would in your case follow my lead and find a pair of junk tires on the rear axle to play with first before committing on tire purchase. 

 

Like I could do a 215/85 R16 on my truck my problem is I will most likely overwhelm the tires with the amount of torque I produce. So, on the Cummins I'll stick with 245/75 R16 which is a 1 wider than the 215's. I had to change my fuel map for daily driving the fuel comes on too quick and the tires lose the battle pretty fast. 

Edited July 13, 20186 yr by Mopar1973Man

  On 7/13/2018 at 2:47 PM, Mopar1973Man said:

Actually, you want to optimize your RPM so your highway travel is around 1,900 to 2,000 RPM.

 

Most of this comes from @cajflynn back in the day his old 2001 Dodge was a 3.73 ratio axle and towed like a dream. Then after see all the 3.55 geared truck and 37" tire troubles. Why I was not really messing with tire sizes. Now after a bit a experience and I'm jumping out to optimize the tires to gears to make the best setup. Remember bigger tires have more rolling resistance especially with 285mm wide face then with all the steel belt you going to have more rotational mass. Just be aware.

 

@Marcus2000monster

I would in your case follow my lead and find a pair of junk tires on the rear axle to play with first before committing on tire purchase. 

 

Like I could do a 215/85 R16 on my truck my problem is I will most likely overwhelm the tires with the amount of torque I produce. So, on the Cummins I'll stick with 245/75 R16 which is a 1 wider than the 215's. I had to change my fuel map for daily driving the fuel comes on too quick and the tires lose the battle pretty fast. 

According to spicers calculator I would be at 2035 average rpms at 70 mph.

  On 7/13/2018 at 2:55 PM, Marcus2000monster said:

According to spicers calculator I would be at 2035 average rpms at 70 mph.

 

Link please I want to see this calc...

  On 7/13/2018 at 3:24 PM, Mopar1973Man said:

 

Link please I want to see this calc...

https://spicerparts.com/calculators

I may be wrong but I calculated by entering the new gear ratio hat the tires would make. I entered 3:69 for the R&P ratio and 34 inches for tire height (calculated at tire size website) and 70 mph for speed limit.

Not to throw a stick in the spokes but I am very anal when it comes to precision.  I've been in some sort if engineering my entire career and precision is required for that.  I have the hypertech programmer to adjust my tires size.  I think that 33.25" is going to be the correct size for my new tires which are advertised as 33.9".  You will lose some diameter when tires are on a vehicle.  I do this to verify that my odometer matches GPS readings.  My last set if tires, I was right around 2100 rpm's at 71 mph. I'm estimating I will be around 2000 with my new tires.

 

L8tr

D

Most tire manufactures advertise a rev/mile that is the best to use when determining rolling diameter. 

  On 7/13/2018 at 5:32 PM, SilverMoose said:

Not to throw a stick in the spokes but I am very anal when it comes to precision.  I've been in some sort if engineering my entire career and precision is required for that.  I have the hypertech programmer to adjust my tires size.  I think that 33.25" is going to be the correct size for my new tires which are advertised as 33.9".  You will lose some diameter when tires are on a vehicle.  I do this to verify that my odometer matches GPS readings.  My last set if tires, I was right around 2100 rpm's at 71 mph. I'm estimating I will be around 2000 with my new tires.

 

L8tr

D

Just what I’m lookin for. 2k @ 70

  On 7/13/2018 at 5:41 PM, AH64ID said:

Most tire manufactures advertise a rev/mile that is the best to use when determining rolling diameter. 

My hypertech use 1/4" diameter increments so I'm stuck with that.

  On 7/13/2018 at 8:29 PM, SilverMoose said:

My hypertech use 1/4" diameter increments so I'm stuck with that.

 

Just convert rev/mile to inches. 

 

5280/(rev/mile)/3.14x12= diameter in inches. 

 

The ECM uses rev/mile anyhow, the hyper tech just does the conversion. 

 

FYI Grimmjeeper.com has a much more in depth calculator that anything else I've seen. It will give you the actual final drive ratio (trans, r&p, tire size), speed at a given rpm, RPM's at a given speed, plus some other stuff if you want it.

@Scottfunk  I just went out and did some highway driving after installing a new VP44.  With my 285/75/17's I was turning 2060 rpm at 71 mph.  I was probably mistaken in my last post about turning 2100 at 71 with my 285/70/17's  It was probably 2150.

 

L8tr

d

@SilverMoose that is exactly the height of tire I'm looking at (33.8"). Which trans do you have? Curious how my final FINAL drive ratio compares to mine.

  On 7/14/2018 at 10:30 PM, Scottfunk said:

@SilverMoose that is exactly the height of tire I'm looking at (33.8"). Which trans do you have? Curious how my final FINAL drive ratio compares to mine.

I got the 47re

 

L8tr

D

Sweet thanks! That's just a smidge lower than my 5th gear which should work out fine for towing. Grimjeeper.com says I can expect right about 80 ad the same ~2060 RPM'S. 4.10 it is then.

  On 7/14/2018 at 10:18 PM, SilverMoose said:

@Scottfunk  I just went out and did some highway driving after installing a new VP44.  With my 285/75/17's I was turning 2060 rpm at 71 mph.  I was probably mistaken in my last post about turning 2100 at 71 with my 285/70/17's  It was probably 2150.

 

L8tr

d

 

I made the point of checking mine yesterday on the way to Eugene.  62 on cruise was 1810 rpm and 70 was 2030 so we are close.  That was give or take a few rpm either way since the roads are not 100% totally flat causing rpm to fluctuate a bit as you drive but these are pretty good numbers for my rig.

  On 7/16/2018 at 1:26 AM, Russ Roth said:

62 on cruise was 1810 rpm and 70 was 2030 so we are close.

 

245/75 R16 and 3.55 Gears, 0.75 (5th gear ratio).

 

65 MPH is 1960 RPM's

66 MPH is a perfect 2,000 RPM's.

80 MPH is 2,400 RPM's.

@Mopar1973Man how do i get all those fancy gadgets on my quadzilla screen like fuel usage and instant economy?

  On 7/17/2018 at 1:19 PM, Scottfunk said:

@Mopar1973Man how do i get all those fancy gadgets on my quadzilla screen like fuel usage and instant economy?

 

 

Well, I can say that tire really hasn't changed much MPG wise... Started logging at July 2nd...

 

Edited July 22, 20186 yr by Mopar1973Man