So I have a 91 runner and it has a 98 5vz in it. On the highway I run about 75-80mph at about 3100rpm. Is that normal?

 

depends on the transmission and rear axle gearing - what are the specs on those?

 

Factory 4.88 gears and I'm not sure about the tranny.

 

stick or auto?

 

Wether you used a pre or post 95 trans the gear rations shoudl be the same.

5th gear ratio in either a new or old r150 is .83 and that's a little short. The At is closer to a .7 ratio in 4th so if it's standard it doesn't sound too far off it's its an auto it might be a bit high.

Tire size can be a factor.

 

What size tires?

I was somewhere around there with 31's and 4.56, but with the 33's I have now (and the same 4.56's), the engine speed is a little lower.

And just so you know, the engine speed (rpm) for a given wheel speed (mph) has nothing to do with the motor, it's all about the various gear ratios between the transmission, axle, and tire diameter.