What should my speed be with overdrive?!?!?

[Chuckanut]

Hey guys, I recently installed a gearvenders over drive into my truck stock 456 gears and 235/85/r16 tires I love the set up my only issue is I found a post on here that said that with the overdrive turned on but he was doing 45 miles an hour at 1800 RPM or something like that and 75 miles an hour at 2800 RPM. And this gentleman had the exact same set up as far as gearing and tires i’m doing 45 miles an hour at around 21 to 2200 RPM and 60 at 2800 RPM. My question is is my torque converter slipping that much because of the axle gearing and how much the truck weighs? Because even with an rpm calculator from Spicer, even those calculations are correct to watch that other gentleman side and yes, my Speedometer is correct. I even double checked with my phone on how fast I was going. I really hope you guys can tell me what’s going on or if that guy was just full of c**p. Thanks guys

 

[adf5565]

Sure seems like something isn’t working right, your rpm’s should be lower than that with OD. The overdrive unit is kicking in right?

Granted I have a .70 OD ratio with the 700r4 vs the gear vendors unit at .78 but I run 60 mph at 2000 rpm

 

[Chuckanut]

Sure seems like something isn’t working right, your rpm’s should be lower than that with OD. The overdrive unit is kicking in right?

Granted I have a .70 OD ratio with the 700r4 vs the gear vendors unit at .78 but I run 60 mph at 2000 rpm

Yes, you can feel the overdrive kicking in and you can see the RPMs drops

 

[Curtisje]

Disregard. I thought maybe a tire difference was the issue but you covered that.

 

[Chuckanut]

Disregard. I thought maybe a tire difference was the issue but you covered that.

Do you have a stock setup with gearvenders?

 

[Curtisje]

Do you have a stock setup with gearvenders?

No. I ran 37 inch HMMWV tires on my M1008 with a GV Overdrive. My GV was on the truck when I bought it and had a problem with vibration while decelerating at low speed. I removed it and it sits on a shelf in my garage now. I'm rebuilding the truck and won't be using the GV Overdrive again.

 

[Recovry4x4]

How are you getting your RPM readings? Attached is the RPM readings from the grimm jeepers site. I find their calculations super close. Are you certain that your GV is a .78 unit? The chart here lists 4th gear which is actually the GV

 

[Chuckanut]

How are you getting your RPM readings? Attached is the RPM readings from the grimm jeepers site. I find their calculations super close. Are you certain that your GV is a .78 unit? The chart here lists 4th gear which is actually the GV

I installed a tack onto my alternator and as far as I can tell, it’s reading correctly at idle it’s at 650 High idle around 800 and 35 miles an hour I’m doing around 2000 RPM without overdrive with overdrive I can do 45 miles an hour at 2000 RPM

 

[Chuckanut]

How are you getting your RPM readings? Attached is the RPM readings from the grimm jeepers site. I find their calculations super close. Are you certain that your GV is a .78 unit? The chart here lists 4th gear which is actually the GV

As far as I know, I’ll get her vendors only has one gearing ratio, and that As far as I know, I’ll gear vendors only has one gearing ratio, and that’s .78

 

[Mad Texan]

Not sure your tach is reading properly at higher rpms... what tach are you running? This is from grimm jeepers. Left is stock (TH400, 4.56, 235/85/16) and right would be with GV engaged ( .78 )...

Comparing your rpms at different speeds (35 vs 45) the calculator shows similar rpms, but yours are higher.

Comparing speed vs rpms numbers from the calculator to the post you cropped, they are close (+/-100) for all speeds. There are variations in all tachs.

My truck in 5th at 66 is 2200 on the tach. The calculator says roughly 2100...

 

[chevymike]

What are you using to track your speed, a GPS or the speedo?

The numbers posted in the original post match close to what my M1010 sees BUT that is with 33" tall tires (255/85-16).

How did you calibrate you tach? I am running a tach hooked to the alternator and used a optical tach to get the actual engine RPM's and then adjusted my gauge to match.

 

[ballencd]

chevymike, Are you running the 255/85-16 on stock rims on your M1010? Do they fit OK? I'm looking for the largest tire I can get under the fenders in stock formation. An OD unit or transmission is not in the budget right now.

 

[chevymike]

chevymike, Are you running the 255/85-16 on stock rims on your M1010? Do they fit OK? I'm looking for the largest tire I can get under the fenders in stock formation. An OD unit or transmission is not in the budget right now.

Yes I am running 255/85-16 Toyo Open Country MT's. I had them on stock rims originally but found one wheel was bent. I picked up a set of aluminum wheels (from an pre 2000 Ford) and they fit fine. They are 16x7 versus the stock 16x6.5 wheel No rubbing or issues.

Stock wheel


Ford wheels

 

[ballencd]

Thanks! Just ordered five of them! They are pricey for sure.

Tried to add a picture of my current cracked 1991 (I think) tires and my iphone has deemed it necessary to start putting photos in a .heic format vs the old .jpg format so they wont drop. I did text the admin to see if there was a fix. One can hope. Did I say I hate computers yet? I do.

 

[87cr250r]

The 285/75R16 is 33 inches tall and fits stock suspension and rims.

If you see any 17 inch rims for cheap I suggest making the switch as it opens up a ton of less expensive tire options. I used steel rims from an early 2000's Dodge.

 

[ballencd]

The 285/75R16 is 33 inches tall and fits stock suspension and rims.

If you see any 17 inch rims for cheap I suggest making the switch as it opens up a ton of less expensive tire options. I used steel rims from an early 2000's Dodge.

I would love cheaper options than those Toyo tires, but I’m in a time crunch to get this thing home and the tires that it’s sitting on are not safe. As it is, I’m going to drive about 8 miles to town to get the new ones put on but that’s 50 miles from home And I need it home so I can work on it. These tires may last me the life of the vehicle as the last ones have been on since 91.
I put 17 x 8 or 8 1/2 on my suburban a few years ago, but it is six bolt. They would’ve looked good on the M1010. The tires I chose then were 34 inches tall and I think would’ve fit perfect. Baby steps.

 

[ballencd]

The tires are installed!

 

[Finnegan1008]

Looks awesome

 

[Dave Kay]

I see this thread is a bit dated, but to the OP's point regarding his speed chart RPM numbers and the GV, those are about par for your stock setup.

IMHO, I don't believe you have reason to suspect your torque conv is slipping because I too have the GV unit installed on my bone-stock M1008 with 235/85-16 tires on stock CUCV rims and can confirm your 60-65 mph RPM numbers are nearly identical to mine.

FYI: I drove my truck to work 5-days a week to a solar power plant project in Hinckley CA for a full year. Nearly one THIRD of my total travel distance was off-road. Freeway/highway travel was the other 2/3rds of my commute and MY 60-65 mph RPM numbers were again, nearly identical to yours.