Gear change and High RPM

[guidoverduci]

Did a gear swap awhile back and I’m trying to figure why the RPMs are so much higher than any calculator out there. I’ve got 4.56, 35” tires, and a T170 manual trans. 3rd gear is 1.0 and at 45mph it’s almost 3000 RPMs. GPS was used for speed. Most calculators are showing 2000 RPM for that configuration. I’m sure the gauge could be slightly off.. but it’s also got 40 series flowmasters and the ear test agrees with the 3000 RPMs! What am I missing?

 

[Digger556]

Did a gear swap awhile back and I’m trying to figure why the RPMs are so much higher than any calculator out there. I’ve got 4.56, 35” tires, and a T170 manual trans. 3rd gear is 1.0 and at 45mph it’s almost 3000 RPMs. GPS was used for speed. Most calculators are showing 2000 RPM for that configuration. I’m sure the gauge could be slightly off.. but it’s also got 40 series flowmasters and the ear test agrees with the 3000 RPMs! What am I missing?

ETA, something is definitely wrong. I ran the numbers manually and something is way off. Have you confirmed you got 4.56 gears? How are you measuring RPM?


Original Post:
If you punch 35" for your tire size, that is your problem. Most calculators will calculate using a true 35" diameter circle. Reality is a true 35" tire with squish down under vehicle weight, typically 1/2". Now you can't take that 1/2" off the diameter, you need to remove it from the radius of the tire, that removes 1" off the effective rolling diameter.

Now tire manufactures rarely produce a true 35" tire. BFG is well known for this. My 35" BFG's with 1/2 tread measured 32.7" effective rolling diameter. My brand new 37" BFGs measured 36.0" inches with no weight on them. They have an effective rolling diameter of 35.0" when weighted.

 

[guidoverduci]

Good to know. I figured they weren’t 35, but that was the number I used. However..even if I put 31 in the calculator it spits out 2200. Closer, but my tach hovers between 2900-3000.

 

[Rusty_S85]

ETA, something is definitely wrong. I ran the numbers manually and something is way off. Have you confirmed you got 4.56 gears? How are you measuring RPM?


Original Post:
If you punch 35" for your tire size, that is your problem. Most calculators will calculate using a true 35" diameter circle. Reality is a true 35" tire with squish down under vehicle weight, typically 1/2". Now you can't take that 1/2" off the diameter, you need to remove it from the radius of the tire, that removes 1" off the effective rolling diameter.

Now tire manufactures rarely produce a true 35" tire. BFG is well known for this. My 35" BFG's with 1/2 tread measured 32.7" effective rolling diameter. My brand new 37" BFGs measured 36.0" inches with no weight on them. They have an effective rolling diameter of 35.0" when weighted.

But the other thing is at speed the tires will get taller when your tires are spinning along at 70 mph.

For example my 31x10.50-15 tires are 30.5" diameter as per BFG`s specs. But once you go running down the road at 70-75 mph the tires would expand some and could get close if not exactly at 31" diameter. Its one interesting aspect that most people don't take into account.

 

[guidoverduci]

Fairly certain on the gears being 4.56. I had someone help with the install and we confirmed. As far as RPM, I’m just simply looking at my Tach. I suppose it could be off a bit.. but idle seems fine, 800, and was measured with a timing light awhile back.

 

[Broncobowsher]

Are you sure those are the gears you got? Friend got a set of 3.55 gears for his mustang. Opened the box marked 3.55 but the teeth said it was really 3.73
It was mis-labeled.

Your timing light has a tach on it? how does that compare to the tach on the dash at higher RPM? 2000-3000 RPM.

 

[spap]

I know this is weird but are you sure the tranny is getting into 3rd gear. I know bad question
Tire defection isn’t going to change the rpm that much
Other than that I think gears just have to be wrong

 

[guidoverduci]

Well I went from 3.50, they are definitely lower. And I’m 99.9% sure we at least verified a stamped 4.56 on them. It shifts fine. Has an OD gear but really brings it down for around town driving. Plus.. when in 4th I hear a slight ticking in tune with RPMs increasing/decreasing. Perhaps it could be tranny related, but other than what has been mentioned, it drives pretty well.

Later this week I’ll try and see how the timing light tach compares at higher RPMs. I was just curious if it was something obvious that I was missing.

 

[ame]

Im sure you verified the transfer case is in 2wd and not 4lo ?

 

[guidoverduci]

I can barley move that shifter and there’s no front drive shaft, but I’ll look into it. I almost feel that if that’s the case, it’s been that way for the last year ��

 

[spap]

Your can always estimate the gear ratio by marking the driveshaft and counting how many times it rotates 4 1/2 times it’s a 4.56 and slowly spinning the tire at lest it would eliminate that part of it.

 

[SHX669]

At those rpms you should be pushing 60 mph .
45 mph should be about 2100 rpm.
Hmm - have you verified your speedometer is correct ? There is quite a bit of difference from factory 26-28" tires to 35s {i figure actual 33" } . has the speedo gear in the TC been changed ?

 

[spap]

Checking a gear ratio calculator with your trans it looks like your not getting into 4 th gear sorry didn’t know a t 170 at first. With your set up in 3rd gear 1 to 1. At 3000 rpm you at 50 / 52 mph, close to your 46 mph at 3000 rpm
in forth 67 mph with the .78 overdrive
Sorry don’t know the trans but maybe something just isn’t engaging for forth easier than referring that’s for sure but still a pain.

 

[guidoverduci]

Just clarify, my measurements were from using GPS for mph(speedometer is way off) and 3rd gear (1 to 1). So 45 was 2900-3000 in 3rd gear.

 

[blubuckaroo]

Did a gear swap awhile back and I’m trying to figure why the RPMs are so much higher than any calculator out there. I’ve got 4.56, 35” tires, and a T170 manual trans. 3rd gear is 1.0 and at 45mph it’s almost 3000 RPMs. GPS was used for speed. Most calculators are showing 2000 RPM for that configuration. I’m sure the gauge could be slightly off.. but it’s also got 40 series flowmasters and the ear test agrees with the 3000 RPMs! What am I missing?

Looks to me that your 4.56 gears with 35 inch (static) and 33" dynamic puts you right at 3000 RPM at 65 MPH.

https://www.4wheelparts.com/a/gear-ratio-tire-diameter

If I were you I'd verify all your calculation numbers.

 

[36Fan]

IF there is a questionable doubt that the rear gears are mislabeled etc, you might want to confirm before putting the front driveline in and using 4wd. Unless you only regeared the rear for now?

 

[guidoverduci]

Only rear done at the moment. Since I’m learning all this as I go, I was just seeing if it was something obvious I was overlooking. All sounds fairly mechanical in nature leaving not many points of failure. I’ll verify the gears with the wheel spin method this weekend. In the meantime if any ideas besides wrong gears pops up, I’m all ears!

 

[Timmy390]

Clutch slipping? Just brain storming.....

Tim

 

[DirtDonk]

It was already mentioned, but you need to verify the ratio so you're sure. That's the big variable at this point.
Either roll the vehicle and count the turns, or jack up the rear and count the turns.
Gets you the information quickly and you don't have to pull it apart to look.

And it sounds like you're certain you have a T170, but what are you going by?
Out on the road, what is your rpm vs speed in 4th? Even though it's supposedly an overdrive, check those data points anyway to see if they somehow agree with the 4.56 and tire size thing.

And then finally, just so you're starting from a known point, measure the tire. Don't go by an assumption or by what's written on the tire. Measure the rolling radius and know for sure.

Paul

 

[DirtDonk]

Had intended to mention checking the trans ratios by turning the engine over by hand.

With the trans in 3rd gear and the t-case in high-range and wheels up, put a wrench on the crank bolt and turn the engine over 1 turn while either someone counts the rotations of the driveshaft, or you mark it and then verify.
If it's indeed turning over at a 1-to-1 ratio, you'll know it that way.

You can do this at the same time you're checking the rear end ratio.

Paul