Engine Speed on Highway

I was wondering if anyone knows how many RPM's my 2.8L is turning at 60+ mph. It is a '86 with a 904 auto and 4.10 gears. The XJ does not have a factory tach and I have tried a couple of aftermarkets and they do not play well with the GM ignition. This seems like a low gear for an automatic transmission even though it has a lock up torque convertor.

Thanks,
Jwalkman

jwalkman said:

I was wondering if anyone knows how many RPM's my 2.8L is turning at 60+ mph. It is a '86 with a 904 auto and 4.10 gears. The XJ does not have a factory tach and I have tried a couple of aftermarkets and they do not play well with the GM ignition. This seems like a low gear for an automatic transmission even though it has a lock up torque convertor.

Thanks,
Jwalkman

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How tall are your tires? That will effect the RPM's. Im assuming stock, 225-75-15's?

If so, your RPM's would be around 3100rpm's at 65 mph. 60 mph would be about 2900 rpm's. If its a lock up, the rpm's should be a little lower.

Since it is a TF904, it is a 3 spd tranny, and has a 1:1 final drive in the tranny. Thats one reason it seems like a low gear.

TF904 - 2.45 1.45 1.00 Don't know about reverse.

Get us the tire size, and it should be able to be determined better.

An aftermarket tach should drop right in, get one that has the pins in the back for changing the jumpers. I had one in my 82 2.8L S-10, a Sun super tach, worked like a champ, 3 wires, one to ground, one to the dash lights and one to the tach...I think it was like $29.

The tires are 225 70R 15.

Thanks for responding

I may have to give a Sun a try. I even borrowed a working tach and it would not register on the XJ.

Thanks for the answer.

yea I used a generic tach from walmart on my 93 TBI 2.8 S10 and it had a real simple hookup
hot ground dimmer and signal from ignition and it worked beautifully
I think mine was somewhere around 20 bux as well
but then again I am pretty sure my s10 had a different style ignition and coil setup than the older carbed 2.8's

I used a sun on my 1982 s10 2.8l, It worked great, thats what i used as a speedo for a while as i had a electric tranny in a cable style speedo.

It was a snap to hook up.

MPETESS10 said:

yea I used a generic tach from walmart on my 93 TBI 2.8 S10 and it had a real simple hookup
hot ground dimmer and signal from ignition and it worked beautifully
I think mine was somewhere around 20 bux as well
but then again I am pretty sure my s10 had a different style ignition and coil setup than the older carbed 2.8's

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Pretty much all the distributors have a hookup point for a tach, GM just happens to put a metal connector on theirs and mark it 'tach'... I have not looked for one on the XJ as I switched out the idiot light panel for a gauge panel so I never bothered. There was not any good place to mount the sun tach anyway on my 98 though I almost bought one of those hood mount tachs like GM used on the pontiac Judge GTO's...I just did not want to spend $60 at the time..JC Whitney has or had them a couple of years ago.