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I am trying to figure out what yall's rpms are at 60 mph with what gearing. I wanted to compare it to mine to see if my torque convertor is slipping.
What rpm's do you have at 60 mph with what gearing?
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1,700 RPM on digital tach @ 60 MPH with 3.73 gears, calibrated speedo, and LT235/85R16 Michelin XPS tires. 265/75R16 tires are the "other" stock size, and have identical diameter to my tires.
If you have oversize tires, and/or something shorter than 3.73 axle ratio, give me the tire revs/mile from the tire manufacturer's website and the rear axle ratio and I'll tell you the actual RPM you should see at a calibrated speed of 60 MPH.
If you have oversize tires, and/or something shorter than 3.73 axle ratio, give me the tire revs/mile from the tire manufacturer's website and the rear axle ratio and I'll tell you the actual RPM you should see at a calibrated speed of 60 MPH.
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The math says 1,940 RPM.
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I have been running 1525rpm's for 58mph with the 265/75r16 with the 3:55 rear end in the van.
The rpms in what the scangauge would be showing, +/-.
Is the math right?
The rpms in what the scangauge would be showing, +/-.
Is the math right?
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The math shows that your speedo is off by a coupla MPH. You should see about 1,525 RPM at 60 MPH but only about 1,475 RPM at 58.
My engine when pure stock wouldn't pull the hair out of a gnat's butt at only 1,525 RPM. I had to crank it up to about 1,900 RPM to have decent power to tow an 8,000 pound trailer. Now with my mild hot-rodding, I need at least 1,700 RPM when towing without the tranny downshifting for every little bump in the road. I usually cruise at 1,800 RPM and rarely have automatic downshifting except for real hills. So I wouldn't want your 3.55 rear end.
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I am going to have to double check at to the stock tire size that is on the door for the van. I think that it is smaller than what I have on right now, I do know that they do not have the E tires as stock.... for a 1 ton van that weights in at the scale at 8500 any thing short of E is not real smart.
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This web site says that my speedo is 3.9% lower with the up sized tires.
Tire size calculator
Stock on the door is 245/75R16 while I have the 265/75R16. Now last I checked it was the 3:55 rear end.
I wounder if the scangauge is off on the RPM's for my van.
Tire size calculator
Stock on the door is 245/75R16 while I have the 265/75R16. Now last I checked it was the 3:55 rear end.
I wounder if the scangauge is off on the RPM's for my van.
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Most tire size calculators, including that one, are off by a few percent. They assume the tire really is 245mm wide, and that the sidewall is 75 percent of 245mm, and that the wheel diameter at the bead is exactly 16". But tire makers don't make their tires to the exact numbers assumed by the tire size calculators.
For example, if you put in 235/85R16 and 265/75R16, it says those are 636 and 637 revs/mile, respectively. But real tires that size are 650 to 655 revs/mile, with most being 655. That's about 3 percent difference.
If you want to do the math yourself, it's not hard.
Tire revs/mile is the key. The percentage difference in tire revs/mile will be the same percentage difference in the speedo, tripmeter, engine RPM, and "feels like" rear axle ratio.
For example:
Your stock tires were LT245/75R16 with 677 revs/mile. Your current 265/75R16 tires have 655 revs/mile.
677 minus 655 = 22, divided by 677 = 0.0325 = 3.25 percent.
So your speedo and tripmeter are 3.25% slower than stock, your RPM is 3.25% less than stock, your 3.55 rear axle ratio "feels like" 3.25% longer legs (3.55 minus 3.25% = "feels like" a 3.43 axle). Whatever RPM you were cranking at 60 MPH will be 3.25% less with the taller tires. For example, if you were cruising at 1,600 RPM with the stock-size tires, you can make the same speed at 1,548 RPM with the taller tires. Of course, without a digital tach, you can't tell 1,550 from 1,548, but you get the idea.
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Thanks Teach.
I was wrong about the rpms earlier.. was just out and at just under the 1600 at about 1590 I was doing 58 on the scangauge and 55 on the dash..
I have the scangauge adjusted +4, maybe I should be at +3.
Softeners I know, iron filters I know... and even after 6 years of driving this diesel I am still on a learning curve.
Thanks again for taking the time.
I was wrong about the rpms earlier.. was just out and at just under the 1600 at about 1590 I was doing 58 on the scangauge and 55 on the dash..
I have the scangauge adjusted +4, maybe I should be at +3.
Softeners I know, iron filters I know... and even after 6 years of driving this diesel I am still on a learning curve.
Thanks again for taking the time.
You should be able to determine if there is slip by the tach. Before the tranny locks, if you depress the go-pedal slightly the tach will go up. Once the tranny locks depressing the go pedal it will not make the tach move other than the increase as your truck speeds up.
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