vandor wrote:Depends what is highway speed, but with a 4.3 rear end 4000 rpm should be about 70 mph.

Most Diesel engines have a redline of 4500-5000 rpm. Do you know what is it on yours? You may be running at redline every time you are on the highway?

Its a higher revving Over Head Cam model so red is 5,500 to 6,000 rpm depending on what your reading so yellow is around 4,500 to 5,000.

Using simple tire speed calculations that don't account for adjusting effective overall diameter when laden due to tire squish I calculate that in 5th gear running on 165/80-13 tires with a 4.3:1 rear axle we are looking at almost 82 mph at 5,000 rpm and around 78 with the 175/70-13 tires. So yes on a 70/75 mph highway we are close especially if traffic is running fast.

Going to a 3.9:1 ratio would change that to 89 mph with the 165/80-13 and 86 mph with 175/70-13 tires.

Typically the OHC Diesel is used with around a 3.38:1 rear axle which would raise 5,000 rpm speeds to around 100 mph and redline to around 120.

On review I see that the Fiat transmission has a 0.881:1 over driven 5th gear while the Diesel potentially could be 1:1 in 5th gear unless the transmission came from a Diesel Impulse which would then be an 0.881:1 which could account for the higher tach readings I am seeing. With an 0.881:1 over driven 5th gear it would come to about 4,400 rpm at highway speeds and redline would push out near to 140 mph. Of course this is all theoretically speaking just based on tire size and gear ratios.

Still gas or diesel I am inclined to baby a home rebuild and run it in below 3,000 rpm, which is about 45 mph on mine, for a while before pushing it much higher. A few gentle rides down some shady country lanes with a little fishing or a picnic thrown in and 500 to 1,000 miles goes by in short order and life is good.

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