Tach accuracy and gearing

[chillman88]

Hey all,

My 91 says according to the factory door jamb sticker that I should have 4.10 gears in the rear, but the RPMs I'm turning might suggest they've been swapped to 4.56? Transmission is a ZF5.

I'm running 235/85/16 I instead of the factory 215/85/16 so my speedometer is off by a couple mph. Internet says I'm actually going 57 at 55mph. The tach says I'm turning about 2100rpm at 55 on the speedometer.

I guess mainly I'm curious how accurate the factory tach and speedometer are and what y'all with 4.10s and a ZF5 are turning at 55?

 

[chumwon]

I have 4.10s in my 92 dually and just put the exact same tires on this weekend. I run 2000ish rpms at 55. I have the e4od but they have almost the same OD gearing. It seems high according to my calculations too but ours are pretty close to the same..

 

[icanfixall]

Lift a rear tire and mark the thread and drive shaft. Turn the drive shaft and count how many turns it takes for the tire to make one revolution. that tells you what the rear gears are.

 

[IDIBRONCO]

When I used to work in a shop that remanned these engines, I drove ALOT of them after installing the new engines. Most of the ones I drove that had 4.10's and an overdrive would turn 2000-2100 rpms at 60 according to the factory tach. Which makes sense since it's been said on here that they get the best fuel mileage around 2000 rpms and our trucks were built in the "bad old days" of speed limit 55.

 

[chillman88]

Yeah I'm really only asking because I was looking at gearing and rpms online and the calculator I used said these should be right about 1850rpm at 55 with 4.10s. I guess mine's not far off if everyone else's is the same haha.

I'm not really worried about it but I figured you guys would chime in and tell me how it sounds, which you did. Thank you!

 

[pelky350]

When I talked with Justin from r and d he said he finds the taca to be off sometimes 50-100 rpms and some trucks being as old as they are i wouldn't doubt the tach to be off a little bit over time. I thought my truck was idling a littlw high after my pump instal but I think my old pump and injectors made it idle lower before. With 3.55 and zf5 315 tires at 2100 I'm doing almost 70 mph

 

[rustygold]

Stock tire
30.39 inch tire 215/85r16
2000 rpm with 4.10 57mph
2000 rpm with 4.56 51mph

Bigger tire
31.73 tire 235/85r16
2000 rpm with 4.10 59mph
2000 rpms with 4.56 54mph

This is all on overdrive.
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[laserjock]

Remember it's not just the diameter. It's actually the the circumference which can vary quite a bit brand to brand for a given tire size as well.

 

[79jasper]

Yup. Best to do a roll out test.

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[rustygold]

Remember it's not just the diameter. It's actually the the circumference which can vary quite a bit brand to brand for a given tire size as well.

I did the math as circumstance of the diameter of them tires. Now when it comes to brand they could be plus or minus 1/2 an inch or more. Tread depth, where they measure tread and aspect ratio.

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[79jasper]

Tire pressure will affect it also.

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[austin92]

Could use this as an opportunity to change the gear oil. Ring gear will have 41-10 stamped in it if it's 4.10s


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[chillman88]

Remember it's not just the diameter. It's actually the the circumference which can vary quite a bit brand to brand for a given tire size as well.

Yeah I hadn't thought about that but I've definitely noticed that. Especially between LT tires and P rated of the same size. LT tires are significantly beefier.

Yup. Best to do a roll out test.

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Roll out test... Isn't that where you roll out of your driveway and floor it and dump the clutch to "test" how far you can lay rubber? Hahahaha just kidding I know what you mean!

Could use this as an opportunity to change the gear oil. Ring gear will have 41-10 stamped in it if it's 4.10s


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Actually I'm planning on doing that this summer anyways. Seeing how the truck has been treated it's probably original I'll check the gears and probably do axle seals as a preventative measure. I'm hoping to find a cover with a drain plug that won't break the bank while I'm at it. Maybe I'll just well one into a used cover idk.

 

[austin92]

Yeah I hadn't thought about that but I've definitely noticed that. Especially between LT tires and P rated of the same size. LT tires are significantly beefier.



Roll out test... Isn't that where you roll out of your driveway and floor it and dump the clutch to "test" how far you can lay rubber? Hahahaha just kidding I know what you mean!



Actually I'm planning on doing that this summer anyways. Seeing how the truck has been treated it's probably original I'll check the gears and probably do axle seals as a preventative measure. I'm hoping to find a cover with a drain plug that won't break the bank while I'm at it. Maybe I'll just well one into a used cover idk.

I just built a 10.25 inside and out. I chose a 1/4" steel diff cover from undercover Fab works. I think it was 140$ with the drain plug. Way over built but it gave me something other than cast or stamped sheet metal to thread my diff temp gauge into and a drain plug so I'm happy


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[chillman88]

I just built a 10.25 inside and out. I chose a 1/4" steel diff cover from undercover Fab works. I think it was 140$ with the drain plug. Way over built but it gave me something other than cast or stamped sheet metal to thread my diff temp gauge into and a drain plug so I'm happy


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Exactly what I want.... I'm just too cheap to buy it!