speed calculation Q

[red66toy]

Ok so my cruiser has all stock gearing(4:11 diffs) and 32" tires and at 2500 rpms I am going only 50mph. But on the speed calculators they all say I should be going 60mph. What gives?, I don't get what can be wrong.
Thanks

 

[Paul_A]

Is 50 your speedo reading? Or a true speed by an other means of measure? I have the same specs as you and when I am at 2000 rpm's I am going 40 mph, at 2500 rpm's I am going 50 mph, 3000 rpm's going 60 mph, according to the speedo. I assume there is an equation here. Reality is I am reading 15-20% under real speed.

Paul

 

[red66toy]

My speedometer says I am almost going 60mph, but there is a 50mph zone close by and all the radar displays say I am going 50mph.

 

[70LCruiser]

I beleive a larger size tire makes u go faster than wat the speedometer is saying becasue the speedo read at the transmission or transfercase, which is wat you would be doing if your truck had stock tires, so its not ur "actual" wheel speed. Soo if ur running a taller tire U go faster than wat you are reading.

 

[red66toy]

Well, actually my speedometer is reading faster than I am going. Radar says I am only going 50mph at 2500rpms and the speed calculators say I should be going 60mph. I was just wondering why there is a difference.

 

[logic2]

Gear ratio chart -

Since both of your observations are pretty much the same (50 @ 2500) with 32" tires, reversing the calculations "indicate" that you both have a 4.76 diff...which I'm pretty sure isn't the case. The only thing I can think of is that the speedo is also off by a little.
Quesiton: Are your 32" inch tires new or worn down an inch or so ?

Diff Ratio table:

The below table can be used to get a rough idea on gear ratios. This chart represents the ideal RPM's at highway speeds (65). These calculations are assuming a manual transmission with a 1:1 ratio. If you drive an automatic your RPMs will be higher, and the opposite is true if you have overdrive (your RPMs will be lower).


Gear Ratio
3.31 3.42 3.55 3.73 3.91 4.11 4.27 4.56 4.88 5.13 5.29 5.38 5.71 6.17 7.17
----------------------------------------------------------------------------------------------------
27" 2677 2766 2872 3017 3163 3325 3454 3689 3947 4150 4279 4352 4619 4991 5800
28" 2582 2668 2769 2909 3050 3206 3331 3557 3806 4001 4126 4196 4454 4813 5593
29" 2493 2576 2674 2809 2945 3095 3216 3434 3675 3863 3984 4052 4300 4647 5400
30" 2410 2490 2584 2715 2846 2992 3109 3320 3553 3735 3851 3917 4157 4492 5220
31" 2332 2409 2501 2628 2755 2896 3008 3213 3838 3614 3727 3790 4023 4347 5051
32" 2259 2334 2423 2546 2696 2805 2914 3112 3331 3501 3610 3672 3897 4211 4894
33" 2191 2263 2349 2469 2588 2720 2826 3018 3230 3395 3501 3561 3779 4093 4745
34" 2126 2197 2280 2396 2512 2640 2743 2929 3135 3295 3398 3456 3668 3963 4606
35" 2065 2134 2215 2328 2440 2565 2664 2845 3045 3201 3301 3357 3563 3850 4474
36" 2008 2075 2154 2263 2372 2493 2590 2766 2961 3112 3209 3264 3464 3743 4350
37" 1954 2019 2095 2203 2308 2426 2520 2692 2881 3028 3123 3176 3370 3642 4243
38" 1902 1966 2040 2144 2247 2362 2454 2621 2805 2948 3040 3092 3282 3546 4121
39" 1854 1915 1988 2089 2190 2302 2391 2554 2733 2873 2962 3013 3198 3455 4015
40" 1807 1867 1938 2037 2135 2244 2331 2490 2664 2801 2888 2937 3118 3369 3915
41" 1763 1822 1891 1987 2083 2189 2275 2429 2599 2733 2818 2866 3042 3287 3819
42" 1721 1778 1846 1940 2033 2137 2220 2371 2538 2668 2751 2798 2969 3208 3728
43" 1681 1737 1803 1894 1986 2087 2169 2316 2479 2606 2687 2733 2900 3134 3642
44" 1643 1698 1762 1851 1941 2040 2119 2263 2422 2546 2626 2670 2834 3063 3559


This chart can be found here:
http://www.4lo.com/geartable.html

Cheers,

Mike

 

[fj40mayo]

Didn't SOR or someone sell a speedometer correction gear that installed inside the speedo housing on the tcase? Is it possible a PO put one of these in the case? Just a thought.

 

[red66toy]

The tires are not that old and have seen little offroad use. I am not talking about my speedometer reading wrong, I am talking about the fact that according to speed calculating tables, I should be going 60mph. But radar picks me up going 50mph which doesn't make sense to me. I just don't get why I am going slower than I should be. I highly doubt that a PO of my cruiser put higher gears in it but maybe.

 

[red66toy]

Ok, I went and checked and the drive shaft rotated 4 times for one full tire rotation. So I do have the stock gearing.

 

[toddslater]

I believe you may have 4:88 rear diff ratio...cause at 2500ish rpms all thing being the same as you stated...you'll be going 50 mph.

 

[red66toy]

Well then shouldn't the drive shaft have rotated almost 5 times for one tire revolution?

 

[dekuiper]

Not to sound like a physicist - I'm not - but I did go through physics in college years ago.

Dopler Radar - that which police and the signs on the side of the road use - read low compared to actual speed. The only way radar is accrurate is when it is directly in the path of the item it is measuring the speed of. For each degree off of the direct collision course, there is a percentage of decrease in the measured speed.

What it means when you are speeding and the officer says "you were going 50 in a 35" is that you were actually going faster than 50.

What it means for you checking your speed on one of those radar signs is; you really are going faster than it reads. Although I would not expect it to be as much as the factors you are talking about. ???

 

[IDave]

I would mistrust the doppler, too. If you can, get a GPS and go out on a straight stretch of highway with it and see what it tells you. I think that method is pretty good.

 

[red66toy]

Good call on the doppler guys. I will see if I can get my hands on a gps unit and see where it goes from there. Although I do have a feeling that I am going under 60mph. Thanks again guys.

 

[PabloCruise]

GPS is great for checking speed.
If not, and you have some long, straight, flat road with mile markers, go out with your stop watch.
Try traveling mile marker to mile marker w/ speedo at 60. Take less than a minute? - then your speedo reads slow. More than a minute? - speedo is reading fast, as 60 MPH = 1 mile per minute.
You can have fun with this formula and solve for your gear ratio mathematically:
You need to convert engine RPM to tranny RPM, to diff RPM, THEN translate that to a linear measure using the radius of your tire and the circumference of circle formed by your tire, convert inches to miles and minutes to hours.
So, PI = ~3.14, 60 min/hr, and 63360 inches/mile.
I did this when sizing tires for a street car after engine & tranny swap: 5th gear was 0.714:1, and diff was 4.266:1
Tire size was ~11" R -> 63360/(2*PI*R) = ~914.5 tire revolutions/mile

So; (4000 engine revolutions/minute)(1 driveshaft rev/0.714 engine rev)(1 tire rev/4.266 driveshaft rev)(1 mile/914.5 tire revolutions)(60 minutes/1 hour) = ~86 MPH

Knowing engine RPM, true MPH over land and loaded tire radius can allow you to solve for gear ratio.
Clear as mud?

 

[Paul_A]

Red66Toy:

Please post your findings so I will know, also.

Paul