Tall tires Lower RPM?

[dinochorizo]

Ok
So If I have a 4x4 with 4.10s and want to reduce RPM wouldn't
larger tires lower it? I am new to this stuff but I'm thinking better
MPG. I will tow a 22' toy hauler loaded at 7500lb but only 6 times a year.
Thanks

[trbankii]

Yes. A larger tire has a greater circumference. So the wheel turns fewer revolutions to go the same distance as a smaller tire. Sort of like someone with shorter legs walking faster to keep up with someone with longer legs. Just don't drop it too far or you're going to worse mileage for "lugging" the engine.

[DennisG01]

Better MPG? Not entirely - larger tires are heavier and have more rolling mass, thereby reducing the benefits. But, yes, larger diameter will reduce RPM's at the same ground speed. You'll have to adjust your VSSB to keep things equal, though. Otherwise your speedometer/odometer will read lower than actal.

[dinochorizo]

MPG wont improve at slow, get them rolling speeds but once on the highyway
think it might? Also has anyone done this and how big do you need to go to actually notice lower RPM in the cab while driving say 65mph?
Thanks

[Subzilla]

Just went from 32" to 35" on the Sub and the RPM's drop. With my old 6.2, I do have to "drive" the transmission more around the hills and such. I haven't had them long enough to take a trip with them so I don't know about mileage, yet. Overall, I'm happy with them. Do a search on line for rpm calculater. You'll be able to plug in transmission type, tire size, gear ratio to get rpm's.

[Craig M]

In Subzilla's example, going from a 32" diameter tire to a 35" diameter tire he has changed 9.4% (35 divided by 32, then minus 1.0 then times 100; pie is neglected). If before he was at 2400 rpm at a certain speed, now he will be at 2194 rpm at the same speed (2400 divided by 1.094). Check the diameter of your current tire and compare to the diameter of the ones you are looking at and you can do the same thing. The internet is good for this. Goodyear, Firestone, Michelin an most biggies are all on the web.

[gerry1]

I would caution if your running duals. I had 245's on mine and the tires touched under load. Too much for my comfort. I switched to the stock 225's but did not care much for the RPM increase. Next time I'll try 235's, sort of a compromise. As far as the MPG's I can't say I saw much of a difference.

[6.5 Detroit Diesel]

I have the 4.10 rear gears and 35" tires. Made a difference in RPMS at top speed. 92KMH @1,700 RPM. And as for the bigger tires weighing more, it's a diesel, for the amount of difference in weight, the lower RPMs are worth it.

[Shikaroka]

Unsprung weight makes a big difference, no matter the size of the vehicle or the power/torque the engine makes.

I could tell a BIG difference the in the acceleration of my Sub when I went from the stock steel wheels to the newer alloy wheels, and I got the same tire size.

Going with bigger tires will slow you down a bit, both from the weight and the effective gear ratio.
You may turn less revs on the interestate (so it may be quieter), but don't expect much (if any) gains in MPG.

[bcbigfoot]

If you go to tall skiny tires like 225 or 235/85-16's (inexpensive tire)you may see a small improvement over say factory 245/75-16's may also see some going to a 265/75-16 (expensive tire) they are all approx 4% larger than the factory tire. You would probably lose mpg if you go to some mud bogging 35" tire.

[6.5 Detroit Diesel]

All I am saying is that with the bigger tires, way lower RPM than the stock tires. Lower RPM, less fuel used. Is it a great amount saved? Not that you would really notice, but there is some. I have the truck and milage to prove it. :D

[GSE2SCHMIDT]

Hey....I agree that increasing the size of the tires will lower RPM's.....not that you would see it

if you increased the tire 5% the (ground)speed would be increased 5% but looking down at the dash it would still be the same RPM per Mile per hour because that's basically turns in and turns out of the transmission....which has not changed....

so if you increased the Circumference 5% and saw no increase in Miles per gallon.....which isn't true because your mile is now 5% longer...you just couldn't see it...... you could actually be saving 5% and just not seeing it. On the other hand going 5% faster may burn up 5% more fuel......

[moondoggie]

Good Day!

My 95 pickup went from 3.80 effective ratio to 3.08 effective ratio (simultaneous change from 8% oversize tires to 4% oversize tires & Gear Vendors Aux OD). It's hard to be sure, but it looks like I picked up 2 mpg. (I have all the data, all either corrected for odo error or error measured to be ≤ 0.5%). It's pretty simple to determine actual tire circumference change vs tire size, so maybe this can help you judge whether a tire size change is worth it or not.

My limited data says there was no measureable difference in mpg between stock & 4% oversize tires (245/75R16 vs 235/85R16), which are taller & skinnier, so they probably weight about the same.

Let us know how you do.

Blessings!

[JohnC]

At one time I went from 245/75 to 255/85, a 33" tire vs. a 30.5" tire. Got about 170 RPM reduction at 65, IIRC. No measurable difference in mileage, though.

[GSE2SCHMIDT]

I'm confused....

How can anyone see an RPM change without reprogramming the speed buffer?

Speedgun?

RPM's=transmission input
MPH=transmission output

Your speedometer doesn't know it's turning bigger tires

[moondoggie]

Good Day!

Yup - I either correct my speedo in my head, or change the VSSB. I've been lucky - the few times I've changed tire sizes I was able to almost immediately get to the VSSB. On the other hand, I've found my odo & speedo to be in VERY close agreement - if one's off a certain direction, the other is too, by almost exactly the same %. I use long road trips, particularly when going through multiple states, to check odo & speedo.



Blessings!
(signature in previous post)

[GSE2SCHMIDT]

Do you do the VSSB yourself or have it done?

[JohnC]

I'm confused....

How can anyone see an RPM change without reprogramming the speed buffer?


I fixed the VSSB. At 70 mph I had a 170 RPM decrease with the bigger tires. No effect on (actual) mileage.

There have been at least 2 articles on the Dieselpage covering the VSSB. Reprints here: http://www.thedieselpage.com/65book.htm

[rjwest]

Just a comment,

I am sure you are aware that you can not change the drive shaft speed that the TCC will lock.

typically a 4.10 rear will lock the TC at 45 MPH either 3rd or 4th gear.

changing to 3.54, the tc will not lock untill ~ 53 mph...

If most of you driving is in town, ( 50 and below) you may loose Mpg

I know of no way to change this in the normal operation.
There are ways to force the lockup, but not via a reprogram,

Note: Pre obd2 vehicle are easier to force the TCC lockup.