Speedo pinion doesn't seem right - HELP!

[cjdirtbiker]

All the charts on speedo gears say 34 tooth with 35's and 4.10's
The gear ratio charts say 2553rpm @65mph.

Mine shows 60-61mph @ 2550 +/- (best guess on the tach)
At 65mph on the speedo the tach reads closer to 2850.

What gives??? Anybody got any ideas?

Is it because the MT/r's are a fifth of an inch short and by the time they wind up to 65mph they've lost 3mph? :confused:

Any help is appreciated. My next test will be a pace car with a radio so we can compare speeds.

Thanks!!

[Dukes69]

Your best bet is probably to use a GPS to check the speed. Maybe try the next size gear?
:confused:

[nagal]

I don't think being off 3 mph at 65 is that big of a deal. Most speedo charts are built using math, best possible matches of available gears and user rounding of numbers (say it calls for a 33.4 tooth gear, did they list 33 or 34 tooth in the chart?).

Your tire size not being exactly 35" is going to cause your values to not match the chart.

[cjdirtbiker]

I guess it's close

Well I've got a few more miles on the heep and I guess the speed is pretty accurate. I'm sure not used to sitting up 3' high and with all the new running gear it felt like I was going a lot faster than I was. Everything is starting to settle in and I'm more comfortable with the way it feels now.

Yesteray I drove out to Colton, about 80 miles round-trip, and I ran 60-65 the whole way and was keeping pace with traffic (most of it anyway).

I need to have those monster tires re-balanced already as it gets pretty bumpy over 65.

Thanks for looking though. I still want to pace it with a car or borrow someone's GPS. Gee, come to think of it, that's a great excuse to buy one! "Honey, I need to buy a GPS, okay???"

Thanks!!

[igofshn]

All the charts on speedo gears say 34 tooth with 35's and 4.10's
The gear ratio charts say 2553rpm @65mph.

Mine shows 60-61mph @ 2550 +/- (best guess on the tach)
At 65mph on the speedo the tach reads closer to 2850.

What gives??? Anybody got any ideas?

Is it because the MT/r's are a fifth of an inch short and by the time they wind up to 65mph they've lost 3mph? :confused:

Any help is appreciated. My next test will be a pace car with a radio so we can compare speeds.

Thanks!!

Are you running an automatic? I have 35's, 4.10 and a 3 speed auto and my rpm's at 65 are about 2750. I also have a 34 tooth gear. Auto rpm's run a little higher the a manual.

[Dukes69]

Auto rpm's run a little higher the a manual.

Why?

[cjdirtbiker]

Igofshn

Are you running an automatic? I have 35's, 4.10 and a 3 speed auto and my rpm's at 65 are about 2750. I also have a 34 tooth gear. Auto rpm's run a little higher the a manual.

Yes, 3 spd auto.
And yes, that sounds exactly what I get. Optimal really, for freeway speeds. I'm really happy with my gearing. I'm sure 4.56 would be just fine as well, just a little higher cruise R's. Maybe some advantage in low range as well.

BTW, found a HP30 housing for $200. May take it and gear it with an ARB for the front.

I too am curious if anyone knows why the AT would run a little higher rpms. The output ratio is 1:1, same as a 5 speed in 5th gear.

I read somewhere the auto has some slip in the conversion of pumping fluid to lock up the torque converter and/or drive bands. Anyone know anything about that?

PS. Do you really go fishin'? Are you flyfisher???? That's another old hobby of mine that came from the NW. :hijack:

[rick]

Yes, 3 spd auto.

I too am curious if anyone knows why the AT would run a little higher rpms. The output ratio is 1:1, same as a 5 speed in 5th gear.

I read somewhere the auto has some slip in the conversion of pumping fluid to lock up the torque converter and/or drive bands. Anyone know anything about that?

Most modern 3 spd autos have a locking torque converter. This means that in "top gear", or the last time you feel it shift, you are locked in 1:1 in 3rd gear. Up until the point that the torque converter locks, there is some loss through the torque converter. If you pay attention the next time you run your jeep through the gears, you may notice that there are actually 4 "shifts". The last one is the torque converter locking up.

The 5 speed manual is an overdrive transmission with a 0.86:1 ratio. This is why a 5 speed will generally get better mileage for the same gear/tire combo because you run at lower rpms in top gear.

rick