4000 rpms at 70mph?

You can go bigger on the tires but only if the truck will allow it.

 

Good eye. Didn't catch that.

If you're keeping the truck for a long while, the dip in gas mileage with end up being more expensive.

 

Something is not right. Looking at marlin crawler and using gear calculators, the steepest gears they have are 5.71. Combined with a 32 inch tire in 5th gear at 70mph you're turning only 3500rpm. I guess it's possible to go with deeper gears but 5.71 would be fairly rare to find to start with. Even the manufactures recommends against them because they're weak.

With that ratio you'll be turning 4200 in 4th.

With gearing low enough to make the listed problem happen you're not really driving around much. You should have 3 useless gears and be able to launch from a dead stop in 3rd/4th gear. Starting in first you would do 3 gear changes before you crossed an intersection. And you would need 44" tires to get down to 25ish rpm in 5th. There would be no question if your truck was capable of 44s. It would be pretty obvious.



You're not driving around in low range are you?




Anyway with the assumption you have mystery gearing the only way to check is to actually check. Rear end on jackstands, crawl under and have someone turn the wheels 1 revolution. Mark the driveshaft and count how many time it turns.

If you have an open diff you can put 1 wheel up, multiply the dshaft rotations by 2. Or turn the wheel 2 revs. Either way you'll have a rough idea of the gearing.Tobe more exact pull the diff and count the teeth.

 

Based on the information that is bouncing around here, I think a reconsideration of the above post is in order............

 

Anyone concider the Tach could be off before jumping to such big conclusions as needing a regear?

 

Post 11

 

That is the factory size and factory tire for that truck. Piece of S. tire. I sell tires and just so you guys know that tire is not made in a p265/70R18 its only made in the 2 sizes below. Only way to know what gearing you have is count the pinion gear teeth and then you will know what gears are is in it. If i was to guess that truck has 4:88 or 4:56 but im sure its 4:88. Use a gps to verify your odometer a 37" tire spins at 64.63 mph on 4:88 gears. It,s perfect for turning big tires or low speed rock crawling. It will kill your gas mileage on the freeway.
P275/65R18 114 T
P275/70R18 116 T

 

that tire is exactly 30.6" not 32.5" your way off sorry. GPS will tell instantly what the problem is. check this link for rpm change with my tire size.
https://www.tacomaworld.com/tirecalc?tires=265-70r16-37-12.5r20

 

If you have 4:88 and 265\70r16 your gps should show you at like 51-55 mph while indicating 65mph. So 20-21% off

 

I essentially have the same truck.

At 70 mph in 5th gear I'm around approx 2700 rpms

Something is not right with your truck. I suspect a previous owner regeared.

 

Get in it and try starting it from a dead stop in 3rd gear on a level surface. If you can do it without the engine knocking or lugging badly then it is probably gears.

Otherwise I am betting on Clearwater Bill's suggestion.

 

Wow thanks everyone for responses I'm overwhelmed lol, okay so when I drive into work I'll take the GPS and see my mph compared to that and then ask a buddy to use the scanner tool and see if my rpms are off from the tach and I'll report back me edit-- also I will try the third gear start

 

Not exactly the 'only' way. Same idea here, but a lot less effort to accomplish.

http://www.how-to-build-hotrods.com/gear-ratio.html

 

As long as its not just a code reader scanner. That won't help.

It needs to be a real time scan. And not just reading RPM at idle, but at speed. A tach can be accurate on the low end and bonkers on the high end, so you need to see what it's doing all across the range.

 

Does it sound like it's at 4000 rpms? Or lower rpms?

 

Update! For you bill, I was able to take off from 3rd gear without lugging it at all , and for majik_man, yeh if I had to guess it would seem around 4000 rpms that's why I don't go past 70mph as I feel if I get to 75 I want to shift into gears I don't have haha , and the GPS seemed to be close to mph gauge (clocking in for work now will msg when I can )

 

Right we established this already. I thought it said 18. Not 16. But thanks again.

 

30 inch tires, 5th gear 5.71 gears, 3807rpm. Again, 5.71 really rare. Jack up the wheel and count the revs. No more mystery after 5 minutes work.

 

I was going to suggest clutch slippage but if you can take off fine in 3rd gear that's out the window

 

Going to jack rear end up tommorow and find out for you all, thanks everyone I'll report back tommorow